If you read about the history of computing, you'll hear how the first computers were "huge". You will often come across assertions that in the early days of commercial computing, a single computer would be "so big that it filled an entire building".

Now, poking around Wikipedia, I can find plenty of photos of old computers the size of an entire server rack, or several server racks. But I can't seem to find any pictures of a computer filling an entire room, much less a whole multi-story building.

Are these claims of a computer "filling an entire building" actually accurate, or is that a wild exaggeration? I can well imagine if you just paid a few million USD for a computer, you probably put it in its own special room with locked doors. But do any of these systems really fill a whole building? Do any of them really "fill" a whole room? Most pictures seem to just show a mostly empty room with cabinets across one wall.

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    This is peripherally (ha ha) related to your question, but I want to mention it because it's awesome: megaprocessor.com Commented Jan 24, 2019 at 21:09
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    This reminds me of a quip by Fred Cisin on cctalk: “You can lose a screw in a microcomputer. You can lose a screwdriver in a minicomputer. You can lose a scope in a mainframe. (It is an exaggeration to say that a person could get lost in one. I think.)” Commented Jan 24, 2019 at 22:32
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    A room crammed full of racks and computer equipment is pretty hard to photograph, because it's, well full.
    – tofro
    Commented Jan 24, 2019 at 22:51
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    Note on the title question: "mainframes" have varied in size over the years, with the most recent examples sometimes being only as large as a small refrigerator. Commented Jan 25, 2019 at 21:33
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    Not quite an answer to the question, but as late as 1990 my father's employer maintained a large analog computer (basically an enormous plug-board system for setting up coupled differential equations). It occupied a bay about 20 meters long, 4 meters deep and two stories tall (the extra overhead was filled with cable trays and ventilation ductwork). Commented Jan 26, 2019 at 6:55

13 Answers 13


In the 1980's a certain bank with its headquarters in Edinburgh has a problem with (IBM) disc storage that had to be kept online for live customer account information for branch and ATM machine operation that it ran out of city buildings to put the disc drives in.

Yes: Not just a building, but buildings. Luckily, just after that time radical developments were made in disc storage densities and the need for more real estate diminished, but computer floor space was a big issue at the time.

Here, also, is a picture of the machine room at Manchester Computer Science, containing on single machine, the MU5. This is just the processor, the peripherals and disc storage are in another adjacent room:

MU5 Source: http://www.cs.manchester.ac.uk/about-us/history/mu5/

It was rather large, but the lower floor computer room that contained an ICL 1906A was even bigger; and then there was the CDC 7600 and the Cyber 106 too.

An earlier machine was Atlas. Here is a picture of the large room containing only the processor of the London University Atlas; several other large halls contained the peripherals and storage:

Atlas Source: http://www.chilton-computing.org.uk/acl/technology/atlas/p010.htm

They were all very big power hungry beasts that took some real estate.

There are plenty of examples on the internet in the computer history archives.

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    Chilton Atlas: upstairs - the I/O equipment and downstairs - the serious stuff
    – dave
    Commented Jan 24, 2019 at 23:40
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    Serious computing will always fill a room. It did then, and it does now.
    – pipe
    Commented Jan 25, 2019 at 12:41
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    To put stuff in perspective - whatever smartphone you have in your pocket now, it is a few orders of magnitude more powerful than those gigantic beasts. At the time. the idea that we would eventually develop gigahertz chips the size of a dime in less than a century would be discarded with all the "THE END IS NEAR" and lizardpeople in the sewer theories.
    – T. Sar
    Commented Jan 25, 2019 at 16:20
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    One day someone will say these never existed, it is a conspiracy and those photos are just CGI made in a joint-venture Hollywood/NSA/Disney basement in Langley. Also: Commercial LEO in 1951: ta.mdx.ac.uk/leo/leo-computers basically a telecom installation... Commented Jan 27, 2019 at 12:40
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    Fascinating, the first commercial quantum computer (and I think the first one to properly work together with a normal computer) does not fill an entire room. It's huge, of course, but not this huge: techcrunch.com/2019/01/08/… Commented Jan 28, 2019 at 16:10

But I can't seem to find any pictures of a computer filling an entire room, much less a whole multi-story building.

enter image description here

(Image taken form Centre for Computing History)

Well, for example look at this picture of a 4341 setup. This is a small entry-level mainframe of ~1980. With a believable setup for such small machine with:

  • CPU (in the middle against the 'wall')
  • Networking,
  • 4 Tapes with Controller,
  • 4 Fixed Disks
  • 6 Removable Disks,
  • Card Reader
  • Card Punch
  • One High Speed Printer
  • One Small Printer
  • One Operator Console
  • One Terminal

It's a reasonable setup for this machine, and I'd certainly call that a room full.

enter image description here (Image taken from Science and Technology Facilities Council)

A large 3090 of the same time will, as seen here, fill about the same space with just the CPU. And it'll definitely need more peripherals than just a handful of disks and tapes.

Are these claims of a computer "filling an entire building" actually accurate, or is that a wild exaggeration?

As usual it depends on the size of building you look at. A fully configured mainframe of the '70s or '80s, with adequate peripherals, can easy fill 1000 m² (~11,000 sq ft). Then again, companies using those kind of commercial computers usually had more than one machine.

Let's take a nice example of a mid to upper size bank system like I had a job with in 1981. They had a building the size of a Tesco Superstore (or one of these large DIY stores) to house 6 computers with all I/O and offices for machine operators and IT management. No user or any other department was located there. About ⅔ of that building was the machine room. 5 of them were used for daily business, while the 6th was a developer system. One of these five had the single job of operating a high speed optical reader, an awesome device ... anyway.

You see, they could get pretty big. A CPU (that's the mainframe term for the computer itself, processor, memory, memory interface, I/O controller and I/O interface - so without any peripheral device, not even a boot disk) occupied four to six 21" full-size racks (or more, for high-end systems), depending on the memory installed. These 1980s machines were already considered small, as the previous generation could have up to 10 racks just for the CPU. Later, around 1990, everything fitted in just 1-2 racks.

It was mentioned by AnoN that this (the 4331 setup) doesn't look like much and one could just as well stack everything against the wall. Maybe, if these were standard 19" mini computer components. But they aren't. They are not made up from small 1...4 HE components, to be operated from one side.

They are mainFRAMEs - Each rack (like) unit had two frames installed on each side, which could be swung out like a door (after opening the doors), Depending on the machine and usage, this frame was a single component - or in case of CPUs maybe only a part thereof. For more compact items, like memory, usually a half-frame featured one compact component. For maintenance both sides had to be accessible, that's why you'll (almost) never see a mainframe pushed against a wall, but with at least 95 cm walkway between units (*1). The closest to compare may be two 19" racks pushed back to back.

Similarly, peripherals also needed access from both sides - in addition to front access for operators, someone needs to change disks and tapes, refill paper or take the printouts. Disks at that time (*2) were top loaders. So no stacking either.

While these large metal cabinets may look somewhat like today's 19" racks, they are not. It's a completely different design. Much like Mainframes are.

Mainframes are in many ways more like an industrial production line, than today's computers. Today's machines have their origin in minicomputers which are more like a multi-tool for a handyman - extremely versatile but slow. Useful for everything, but excelling at nothing.

*1 - I started out in Mainframe service, and we really appreciated any additional space above the bare minimum.

*2 - near the very end of removable disks IBM introduced a line of full-height racks with 3 disk units in each to be pulled out and loaded.

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    See, to my untrained eyes, this just looks like a mostly empty room with lots of bits of computer in it. It looks like you could probably move the cabinets a bit closer together and it wouldn't take up nearly as much space. (Although of course you need to be able to get at them I guess...) Commented Jan 25, 2019 at 9:22
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    @MathematicalOrchid I think correct heat dissipation requires some gaps between the "boxes" so its not just to get to them ... but yes that photo is mostly empty ... Would be interesting to know the temperatures ... In one of mine previous jobs we had a small Internet backbone hub in a basement and that beast was at 48C (degrees Celsia) with open windows and -6C outdoor temperature and cooling fans spinning laudlly... and that was around 2003 now imagine old TTL or even tubes ...
    – Spektre
    Commented Jan 25, 2019 at 9:43
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    @AnoE Except, they can't be stacked, as many need top operation, but as well for weight. Some (like printers) need front or back access as well. This especially count for all CPU or controller frames, as they are not your average small scale 19" racks, but mainFRAME rack - 21" wide and a frame to be swung out like a gate in each side holding boards. They are not puny 1-4 HE elements that can be pulled out, but usually a contained group (like a memory block) is 1/2 of such a frame. Everything more dense packed than on a mini computer.
    – Raffzahn
    Commented Jan 25, 2019 at 12:31
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    Re: space between boxes. Sure, I could probably fit my houseful of furniture in one room, but I wouldn't be able to use it. That's why I say I've got enough furniture to fill a house, and don't say enough to fill a room.
    – dave
    Commented Jan 25, 2019 at 18:26
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    @MathematicalOrchid To add to the other comments, keep in mind that the interconnection wiring, power cables, an in some cases water cooling were all beneath the false floor that you see, and when engineering work was in progress you still had to be able to move around the room when half the floor panels between the boxes had been removed. You didn't shut down the whole mainframe system for a day just because somebody had to remove some floor panels to repair something, or install new kit!
    – alephzero
    Commented Jan 25, 2019 at 22:40

Here's a picture of the "Strela" (arrow) computer (1954) Strela main Vacuum tubes, 2000 op/sec (on 43-bit fixed point), 150kWt, 300 m³

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    Quite a beautiful machine room, really. Commented Jan 28, 2019 at 8:57
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    @MathematicalOrchid True. Presumably, that's what those odd benches sitting against the cabinets are for.
    – Eric
    Commented Jan 28, 2019 at 16:14
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    Still more power efficient than a Pentium 4.
    – John
    Commented Jan 29, 2019 at 4:13
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    @John ... and 100.3415% more accurate than an original Pentium.
    – occipita
    Commented Feb 23, 2023 at 16:14
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    @John It's the 1994 Pentium FDIV bug, where Intel's floating point unit sometimes gave slightly wrong results. Intel tried to avoid replacing the chips by claiming it wasn't a big error.
    – Armand
    Commented Feb 26, 2023 at 22:06

The excellent history book "AN/FSQ-7: the computer that shaped the Cold War" describes the size of a 1950's era radar monitoring air defence computer.

A single installation (a single computer) was built in a specially designed four-storey "blockhouse" - 74 feet high and providing 90,000 square feet of floorspace. A separate building housed the generators and cooling towers. The "computer" itself took 2 whole floors. One floor housed telephone equipment and the top floor was all the operator consoles.

Some stats: 10 technicians just to look after the computer. More than 100 display consoles. Three-shift staffing meant it needed around 627 people.

To quote from the book:

A typical installation required about 1 megawatt of electrical power to run the central computer with its about 55,000 vacuum tubes, 175,000 diodes and eventually even 13,000 transistors (all of which were spread over more than 7,000 pluggable units), weighing in excess of 275 tons, and occupying about 21,000 square feet of floor space.

Edit: if you want to be really picky, each AN/FSQ-7 installation was actually two individual computers, duplexed for fault-tolerance. But it was still a monster.


Here's the computer room of the Shuttle Mission Simulator (SMS), building 5 of NASA's Johnson Space Center, in the early 1980s. Most of the boxes are parts of a Sperry-Univac 1100 mainframe, but some are "intelligent controller" satellite computers, Perkin-Elmer 8/32s. There are also some Singer-Link Flight Simulation proprietary visual system cabinets and a unique interface device for the IBM flight computers.

There were two simulators in the building, so there are two of everything.

I was privileged to be a simulation software engineer on this beast from the early 80s to the late 90s. The Univac mainframes lasted until, I think, the early 2000s, when they were replaced with Silicon Graphics machines.

enter image description here


The main computer hall of the company I worked for in the 1970s and 1980s was about half the size of a soccer pitch - about 200 feet by 150 feet. That contained three IBM S/370 mainframes at one end, and the rest of the room was packed full of disk drives, stacked up to 6 or 7 feet high with narrow walkways between, with the outside walls lined with tape drives.

The power supplies and cooling systems filled the whole of the ground level, and the computer hall was the next floor up, built on a false floor to accommodate the wiring and plumbing for the water cooling.

To be fair, that was only half the complete building - one floor of the other half was an open plan area filled with punched card operators, and the other floor was occupied by programming teams.

The magnetic tape library occupied about half the machine hall area, on the third floor - basically, wall-to-wall racks of 12-inch tape reels, and a staff of tape librarians to make sure things didn't get lost!

At a later time there was also a Cray supercomputer in the main hall - though unless you knew where it was, it was almost hidden from view by all the rest of the kit.

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    The power supply system is for me the most impressive thing on the IBM mainframe, enabling one to replace the power supply while in use.
    – Tomachi
    Commented Jan 27, 2019 at 20:01

Such rooms were not FULL in the sense of not being able to get any more stuff in them, like a storage locker might be, because you had to be able to get to the various pieces. There was room to walk between the various items, and open panels for servicing, although sometimes not very much. But they were full in the sense that almost all available, useable space was taken up by the machinery. You could not put in any more without blocking access to stuff you needed.

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    There is also the consideration of heat dissipation and cooling capacity. Equipment has to be spaced sufficiently to allow for adequate cooling.
    – Anthony X
    Commented Jan 27, 2019 at 20:35

If you want to know exactly how big the classic 1960s and 1970s IBM mainframes were, see the System/360 Physical Planning and System/370 Physical Planning guides. They give complete measurements of every part of the system.

Consider the IBM System/360 Model 85, a large (but not the largest) mainframe from 1964. The processor itself took up 12 refrigerator-sized cabinets, weighing a total of 5.5 tons and occupying an area 28 feet by 24 feet.

The entire computer installation was much larger than the CPU itself, since you'd have card readers, disk storage units, numerous tape drives, I/O controllers, stand-alone cabinets of memory, and so forth. Adding 2 megabytes of memory to the system used the IBM 2385 Processor Storage unit, a set of 11 cabinets that weighed 7 tons and took up 23 feet by 23 feet.

To answer the original question, mainframes could fill anything from a large office to an entire floor. Filling an entire building was pretty unusual except for the SAGE air defense system or maybe an NSA installation.

  • In the mid-90s I visited an IBM Federal Systems datacenter. The mainframes in this giant raised-floor room were as I expected from years of seeing their glossy marketing brochures - and as described in your answer and others here....
    – davidbak
    Commented Feb 3, 2021 at 3:45
  • ...But there were these odd blue&white boxes hung from the walls in an array. Each was the size of a small freezer - maybe 5-6' wide, 3' high, and coming out about 3' feet from the wall. Maybe a dozen of them hanging there on the wall....
    – davidbak
    Commented Feb 3, 2021 at 3:46
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    ...I asked what they were. I forget the model number but they were communication units. Each one handled 8 <== count'em: EIGHT - synchronous terminal lines, I forget the baud rate, something low, 2Kbits/s? 4Kbits/s? 8 slow lines in a freezer sized box. That was the mainframe world.
    – davidbak
    Commented Feb 3, 2021 at 3:48
  • @davidbak reminds me of the NASA shuttle simulator Univac mainframe remote terminals which had "cluster controllers" the size of washing machines. They were booted up by 8" floppy discs, the only examples of that media I ever saw. Commented Feb 16, 2023 at 13:45

One of the largest computers ever built was the SAGE system, built to gather information about surprise attack on the US. It filled a building. One might argue that SAGE was more than just one computer. But if you accept it as just one computer, it meets your criterion. Wikipedia Article

Edit: A closer look at the Wikipedia Article shows that SAGE was a collection of computers, and a network. Each computer was an AN/FSQ-7, which is mentioned in another answer. That computer was the largest ever built, taking up about 21,000 square feet of floor space. That's big.


Consider ENIAC. From wikipedia:

It weighed more than 30 short tons (27 t), was roughly 2.4 m × 0.9 m × 30 m (8 ft × 3 ft × 98 ft) in size, occupied 167 m2 (1,800 sq ft) and consumed 150 kW of electricity.

That's roughly building-sized.

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    It's nearly 100 feet long, but it's only 8 foot heigh and 3 foot deep. Sounds like it occupies one wall of a long hall. (Unless you're saying it would be 100 feet long if you lined all the cabinets up together or something?) Commented Jan 24, 2019 at 21:54
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    @MathematicalOrchid does it matter terribly? It probably could have been arranged differently, and it occupied enough floor space to fill a medium-sized house.
    – hobbs
    Commented Jan 24, 2019 at 23:46
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    The "occupied 1800 sq ft" was the selling point. :-) There's a difference between a building-sized computer cabinet and a computer that requires it's own building. You need space for cables and HVAC, plus front-panel access for multiple operators -- you couldn't just ssh in from a nearby workstation -- so working areas count against your square footage. You might be able to cram all the pieces into a couple of storage containers, but it wouldn't be usable in that space.
    – fadden
    Commented Jan 25, 2019 at 5:52
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    Exactly. The computer takes up a certain amount of space within the building, but something of that tech level can't do anything unless you also leave a good bit of space around it for several people to sit (at an attached, and also quite bulky teletype console or card reader and punch machine) and circulate (operating the front panels, feeding in cards, collecting printout, and also doing work around the back... reloading printer paper, opening panels to tinker with the circuit boards on a broken unit or tweak the settings on an underperforming one)... etc. That takes up at LEAST as much.
    – tahrey
    Commented Jan 25, 2019 at 7:01
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    ENIAC was modular. There were originally something like two dozen modules, each of which was roughly 8 feet high and 3 feet deep. The modules were then wired together with plugged cables; the cable configuration determined the program that was executed. In principle the modules could have been arranged in a line 100 feet long, but I doubt they ever were, because the room (just one room) was not that shape. Wikipedia has a picture. Commented Jan 27, 2019 at 6:37

A mainframe in the 1970's was not too different from today's desktop computers, except for physical size.

They had a CPU, which was about the size of a refrigerator.

They had disk drives, each of which were about the size of washing machines.

They had I/O devices, which often ran the size of a desk.

They had cooling, which was the size of a whole-house AC unit.

They had wiring to connect everything up. Just like the tangle under your desk except using higher voltage driving higher current.

Unlike your PC, they also had multiple tape drives each again the size of a refrigerator, and those tapes needed to be stored on racks somewhere.

So, if you scale today's CPU/memory/mobo to something the size of a refrigerator, you're looking at maybe 1000x. So, following that, visualize the space take up by 1000 PC cases, then double it to account for the tape drives, thicker wires (for higher currents and voltages) and extra AC airflow. Then double it again because people had to have access to all these components all the time.

Don't you think that would fill a very large room? Even if you're using a laptop as your basic measure, it's going to be bigger than a typical office cubicle.


I can't remember where I saw or put it, but sometime in the last couple of months I came across an installation for an early IBM system somewhere online, in scanned PDF form (maybe in the Bitsavers documentation archive?), most likely their first "RAMAC" hard-drive using rig plus all the associated access and computing hardware for using it at the heart of an electronic accounting system.

It's some pretty heavy engineering. The first parts of it lay out the minimum allowable, and recommended dimensions for the room that the drive, the computer, the operator console, their various power supplies and glue logic racks will be installed into, in plan form, mapping out their positions (a multi-cabinet U-shape with the free-standing operator console nestled within) plus the floor-level cabling runs and the space needed for operators to get around the back of the machines to access service panels etc that would otherwise be sandwiched against the walls. I can't recall the minimum dimensions, but the recommended space wasn't far off the entire floor area of my apartment (which is about the same size as several previous workplaces, encompassing different offices and even medical examination rooms), pegging it around a square 25ft / 7-and-a-bit metres on each side. The access space around the "back" of the cabinets was no more than 4ft even so.

And that's for a relatively minimal system. Bear in mind that the term "mainframe" comes from an old term for the central processor of a computer - each of the system's primary components (CPU, memory, I/O handler, power regulator, etc) would be built into one or more of those large "frames", of which the processor (made up of a great many individual, interconnected rackmount cards stuffed full of discrete components or, much like HAL 9000, small-scale integrated circuit packages) was of course the "main" example. The singular encompassing the plural, it came to imply any complete computer system built along similar lines, regardless of size...

And of course as well as the plans there were photos and an artist's-impression birds-eye view of the machine in use. The two or three humans in the picture didn't look ever so big compared to the computer, but it was still sort of human-scale, instead of being a monstrous factory-filling item.

Of course, that's not literally filling the room, but it is taking up enough of it, in awkward enough positions, that you can't really make practical use of the space that's left unless you install the machine in the corner of a considerably larger room and employed the alternative L-shaped layout instead.

A "minicomputer" setup, by comparison, would have been one small enough to only occupy one or two such frames, or within a wider but lower piece of furniture analogous to a mainframe's operator console, with the processor, memory, offline storage devices, power supplies, and maybe even the user interface (where it wasn't a separate compact terminal or a teletype) all fitting within that limit.

Incidentally, you make reference to mainframes being "like" rackmount servers... actually the comparison is more direct than you might first imagine. The format of 19-inch communications and server (etc) rackmount frames is pretty much exactly the same as that of, particularly, IBM mainframes and/or DEC minicomputers. I can't remember if it's a direct heritage or just convergent evolution, but in either case it's the same environmental limitations that moulded them - to whit, they have to be able to fit through a standard doorway without getting stuck or damage being caused to equipment or building fabric, even if that may mean temporarily taking the door off its hinges. And it also needs to get around tight corners in corridors, stairwells, etc whilst in transit to the target room and being turned to fit through the door. But at the same time you want to have the maximum amount of useful space within each frame to minimise the floor area taken up by it, to maximise the amount of tech you can cram inside each box, and minimise the number of boxes and thus the building and shipping costs. So the format quite rapidly converges on the largest rectangular box that will fit through doorways and can be moved easily through buildings made to a common human scale. I think at one point it was even commonly called IBM Frame Size. But in general it's about 24 inches wide, a shade under 7ft high, and about 30 inches deep...

(I've even seen distinctly non-mainframey equipment clearly built to similar guidelines - like the back-projection mobile interactive whiteboards made by Smartboard back before short-throw lenses were good enough to make slimline front-projection models practical. They were quite a bit wider across the beam of course, but measured just less than a doorway's width front to back - a little less than a mainframe in fact, so you didn't have to take the doors off - were just short enough to fit under the lintel when lowered to their minimum height, and the castors and V-shaped integrated trolley compensated for the width by making it somewhat easier to "lever" around difficult corners and wiggle into rooms from narrow corridors... I expect all-up size was something similar to the largest part of the RAMAC setup, ie the massive, ~36-inch, horizontal-spindle hard disc unit proper, which needed a crane to lift on and off of trucks and planes and sometimes demanded remodelling of buildings to get it installed... like, holes would have to be made in walls and then bricked back up again afterwards, floors preemptively reinforced, etc. With the console being almost as bad, but at least being lower-rise and a sensible weight. Though the Smartboard was way lighter and moveable by one person, whereas even the pure logic parts of a mainframe usually needed a team of two or more...)


If you read about the history of computing, you'll hear how the first computers were "huge".

This depends on what you call a "computer" and what you call "huge".

The Zuse Z3 (see Wikipedia article) is seen as the first binary computer; it was built 5 years before the ENIAC and looking at the photo I think it was not larger than a large wardrobe.

So it definitely did not need a whole building.

However before the Z3 there were non-digital machines:

The "Analytical Engine" (see Wikipedia article) was designed in the 1830s but never built. That computer would have worked with mechanic parts only (such as gears); the device would have filled a whole building while the memory (the equivalent of "RAM" in modern computers) would have needed most of the space.

I heard about a mechanical computer using ternary instead of binary arithmetic that has actually been built in the 1890s. Unfortunately, I don't know how large this device was.

... commercial computing ...

Commercial computers came later. According to Wikipedia, the first commercial computer (UINVAC I) needed about 36 square meters and weighted about 8 tons.

36 square meters is much less than a whole building.

  • "36 square meters is much less than a whole building" ... but of course, that depends on the building. I'm currently living in a two bedroom house that's only a little larger than that. If I knocked one of the bedrooms off, I think it would be less.
    – occipita
    Commented Feb 23, 2023 at 16:20

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