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The Sinclair ZX81 was an extremely popular low-cost home computer, on strength of price, being a computer for less than 100 pounds. It sold over 1.5 million units, according to https://web.archive.org/web/20141223110120/http://www.sinclairzx.com/about-us.html#12

The built-in RAM was only 1K, which had to serve for all purposes including video memory. Thus, to write a program of significant complexity, you needed the 16K RAM expansion.

Are there any historical records, or even anecdotes, on how many units of the RAM expansion pack were sold, or what percentage of ZX81 owners ended up buying one?

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    Or 2 KiB in case of the US - or anyone getting hands on a 2 KiB RAM to put in (or piggypack 1Kis)
    – Raffzahn
    Aug 7 at 19:03
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    In effect, you are asking how many serious users the ZX-81 had? (Not saying that 1k chess, for example, wasn't created by a serious user, but these were rare exceptions)
    – tofro
    Aug 7 at 20:30
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    Does this question also include all non Sinclair made and sold expansions?
    – UncleBod
    Aug 7 at 20:53
  • @tofro Basically, yes. I'm trying to get an idea what percentage of users took the machine seriously, versus just playing with it for a little while and then discarding it. That question as such doesn't have an objective answer, but the adjacent question about sales of the RAM expansion does.
    – rwallace
    Aug 7 at 21:22
  • @UncleBod Yes, it does.
    – rwallace
    Aug 7 at 21:22

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I guess it will be next to impossible to get tangible sales figures for the number of RAM packs sold.

I did, however find a quote from Geoff Boyd (one of the owners of Memotech, who was a well-known vendor of expansion modules for the ZX81), that

At one stage Memotech was the largest purchaser of dynamic RAM memory in the world! In effect Memotech cornered the market for ZX81 (and Timex 2000 in USA) memory packs and were able to put the big buyers at WH Smith, Boots etc on allocation." By the end of 1984, some 250,000 Memopaks had been manufactured.

Given that Memotech was a "second source" (so, probably not the first choice by inexperienced buyers or when bought as a package), I would assume the volume of Sinclair original RAM packs about 3 to 5 times higher. (Even if Memotech's product was, at about the same price as Sinclair's or lower if on sale, the much sturdier and generally "better" product and Memotech offered expansions up to 64k.) Memotech was not the only second source for memory expansions for the ZX81, there were a lot more, mainly "cottage industry" sellers (like dk'Tronics, for example) selling memory expansions.

Overall, I would assume the percentage of memory-expanded ZX as at least 80%, very probably exceeding 90%. No one I knew who had the computer didn't also own one or the other RAM Pak. If you have a look into contemporary publications like ZX Computing (the main sources of software at the time), the vast majority of type-in programs listed needs 16k, and reader's letters very rarely complained about that.

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    As so often it might be useful to take the quote with a good amount of salt. 250,000 packs in 3 yrs would not make them #1 RAM buyer worldwide. Tandy sold already in 1980 300k computers with 16 or more KiB DRAM build in. Even more if Sinclair sold 3+ times as much as assumed above. When trying hard to give credit to the quote, one may assume they had a noticeable influence on the spot market in the UK, but for sure not worldwide.
    – Raffzahn
    Aug 8 at 9:09
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    @Raffzahn Well, it's a quote :) And, maybe even true in that one split second they sent off the order. Tandy would probably have been sleeping at that time of day ;)
    – tofro
    Aug 8 at 10:05
  • I like the idea of using the quote, that's called "hard data". But then you appear to pull the 3-5x figure out of thin air :-) Is there any support for that other than "gut feel"?
    – paxdiablo
    Aug 8 at 23:08
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    It's hard to see how Sinclair could have been selling 3x as many packs as Memotech without buying more dynamic RAM memory than Memotech. So I doubt both the quote (Tandy and Apple were selling lots of micros with significant amounts of memory, never mind the manufacturers of large-volume minicomputers like DEC) and the Memotech:Sinclair ratio.
    – John Dallman
    Aug 9 at 0:48

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