Here’s the known evidence:

1 – John Blankenbaker discussed in his 2007 Felsenstein interview how he sold out to CTI and shipped all remaining parts and computers to them in South Carolina.  He said “I think I probably delivered a few computers to them because I had sold about 44 out of that 50 and so I delivered some to them” (emphasis added.)  This seems unusually vague for a meticulous engineer.  But he mentions the original run of 50 computers which he had always been planned, and says he "sold about 44” and then sent remaining parts and computers to CTI.

2 – CTI had at least 8 computers, and perhaps no more than 8.  We know this because Robert Nielsen, owner of Nielsen Electronics Institute acquired all 8 of his computers directly from CTI.  Some were acquired while CTI was still in business, some at CTI’s bankruptcy auction.  He said he purchased every Kenbak-1 related item they had, including documentation and training videos.  Four of his computers had the original Kenbak-1 faceplates (some with the Kenbak-1 name covered with a foil “CTI” label) and four had newly fabricated CTI labeled faceplates without the card-reader slot.

3 – Robert Nielsen was adamant that CTI never made their own Kenbak-1 computer from scratch, but just completed or rebranded computers that Blankenbaker had already partially assembled.  The date-codes on integrated circuits from CTI computers go to late 1973, suggesting CTI purchased components and soldered them to PC boards, but the Kenbak-1 name on all printed circuit boards suggest they didn't fabricate any new PC boards, just used the ones Blankenbaker supplied to them.   It would have been trivial to change the Kenbak name if they fabricated their own PC boards.  Nielsen also believed that CTI never sold any Kenbak-1 computers to anyone except to him, and thought he purchased every one of the Kenbak-1 computers they ever had.  Nielsen's statements would be hard to believe, but considering Robert Nielsen actually worked closely with CTI during this time, helped make training videos and brochures, and taught sales people, his words are probably true.  Blankenbaker said that he thought CTI sold one or more computers to a school in Canada, but that was probably because the school contacted Blankenbaker, who just referred them to the new maker, CTI.    

4 – There were 3 different PC board versions.  The first prototype computer used the “Rev_”  circuit board.  Then at least two “Rev A” (beta prototype) computers were made with an improved PC board.  But Blankenbaker only sold the final and perfected “Rev B” (production) computers to customers.

5 – John stamped serial numbers on the printed circuit board of most (if not all) production computers he made for sale, but not the prototype or the beta-prototypes.  These serial numbers started with 167, the last 3 digits of his address.  But one beta-prototype (Serial #216 which he sent to Boston’s The Computer Museum for their contest in 1985, had a serial number label on the back, without any serial number stamped on the printed circuit board.  It's most likely he placed this last serial number sticker just to complete the computer before he mailed it to Boston, but it wasn't a true production machine, and the serial number wasn't likely truly in an ordered sequence. 

6 - As for the 8 computers shipped to CTI, four had CTI front panels on them without card slots.  We have interior photos for 3 of these four, with one of the three having a very early Kenbak assembled board, with a Kenbak serial number.  The other two of these three were assembled late 1973 by CTI, with at least one of them having the unusual serial number 501 stamped on the PC board (the other one has a paper sticker where serial number is usually stamped, which the owner didn't want to remove.)  This shows at least once CTI put a CTI labeled front panel on an older Blankenbaker assembled computer, and it shows that CTI may have begun their own numbering scheme of the computers they assembled.  In conclusion, serial number evidence doesn't clearly determine how many computers were made.

Using the above knowledge, there are different ways to estimate the number of computers made:

Method 1: Focusing on “sold about 44 of that 50” and delivered the rest and parts to CTI: If we take that as literal and accurate, we arrive at a very high number.  If he sold 44, and kept 3 computers for himself) that adds up to 47, and then if we add in the computer that he “gifted” to his employee of 4 months, we’re up to 48 computers.  Then we add in the eight computers that CTI received or assembled, we have a total of 56 computers.  This high number seems unlikely, as the "sold about 44" seems imprecise, and Blankenbaker would have had to fabricate 53 of the production “Rev_B” circuit boards, a non-round number.  Maybe we should think of this number as an "upper-limit" for the number of computers made.

Method 2:  Perhaps 44 were made/completed, not actually sold:  As the builder, Blankenbaker may have focused more on how many he completed and had that number 44 in his head, but 35 years later during the Felsenstein interview, he was asked how many were sold and how many were sent to CTI.  This may have caught him off guard, so he said “about 44” rather than get into a lot of detail about the three he kept, and the one he gifted to his employee of 4 months.  To keep things honest, he said “about” to be clear the number was not precise.  If he made or completed 44 machines, that might include the 3 he kept for himself (prototype, and Serial 183 and Serial 216) and the one he gifted.  If we add the 8 computers we knew CTI had, we get 44 + 8 or 52 computers all together.  Still, this number isn't completely satisfying, as it assumes too much.  

Method 3: Focusing on Blankenbaker’s initial planned run of 50 machines:  John had always planned to produce an initial run of 50 production machines, and mentioned this many times, and eluded to it in his quote.  This probably does not include the prototypes (the "Rev_" or the two "Rev_A") which he never planned to sell  to customers.  Adding the run of 50 production machines, to the 3 prior prototype/developmental machines, would make 50+3 or 53 total computers made.  This is a fairly satisfying rationale, but the slight modification before I think is even better.

Method 4: Counting PC boards:  The most complicated and costliest computer part, the PC board, usually has to be ordered in quantity to get the best price.  It's really not much harder to make 50 boards than two, and the per-piece price really falls in larger quantities.  Often you order one or two of a prototype board, just to test it out, then order large quantities when it's found to be working right.  It's easy to imagine he ordered just one of the initial prototype ("Rev_") boards to test it out the new design.  Then he would also order just a small number of the improved/corrected boards ("Rev A") until he found the last of the board errors.  But then when the circuit was finalized, it would make sense to buy all 50 of the final "Rev B" production boards at once to get the best price.  If this is the case, he purchased 50 of the production “Rev B” boards in one run, and we add in the prototype board, and the two known "Rev A" boards, he would have a total of 53 total printed circuit boards to turn into computers.  However, we're not done yet.  We know he kept one bare PC board which never had any components soldered to it.  It was photographed many times during interviews after 2005, loaned to Grant Stockley for his Series 2 reproduction, and even loaned to Achim Baque to copy.  So with one unused PC board, it seems logical that 52 PC boards were made into computers, and one other board is still owned by Blankenbaker, bare without any components.

I think Method 4, which takes into account the 3 early prototypes, and then the planned run of 50 production machines, but subtracting one board PC board which was never made into a computer, used, is the most probable estimate.  So I think the total of 52 computers total, is a best estimate.

Caveat....

There are many of assumptions and conjectures in the above.  If nothing else, it explains why the frequently cited "40" or "44" probably isn't correct.  This page is a work of progress, with input from others.  Let us know if you you disagree.  See our “Contact” page.

There are several reasons for variation.  First, different instructions take different amounts of time to execute.  Most instructions are two bytes, and some instructions with indirect addressing modes require many steps and memory accesses to execute.  Since memory is implemented serially, when reading a byte from memory, there may be a shorter or longer delay until the required byte circulates to the output of the 1024-bit shift register to be read.  In fact, the vast majority of execution time on a Kenbak-1 is spent waiting for a required memory byte to be read from the serial memory.  Secondly, each Kenbak-1 has a different clock frequency.  Quartz oscillators were not used.  Simple RC circuits , with imprecise capacitors were used, and those components may change as they age.  Lastly, it seems that Blankenbaker experimented around with different capacitors in the clock circuit as he made the first three computers, but then it seems things were settled for the production machine.

Empiric measuring of a typical programs execution rate is the best way to know.  The simple "counting" program (on the first page of the kenbak.com web site) on an original computer (Nielsen #3 or "Gray Case") the high-order bit flashes on and off almost exactly once per second.  This corresponds to 768  instructions per second.  Unfortunately, this Kenbak-1 doesn't have a clock speed of 1 MHz -  it's around 1.6 MHz.  This could be due to aging of components, but more likely it's because this very early beta-prototype used different smaller capacitors in the multivibrator clock circuit.  If the clock speed was decreased to 1 MHz (by adjusting resistors or capacitors on the multivibrator clock) the instruction rate would fall to around 480 instructions per second.

A second Kenbak-1 can also be similarly measured.  A YouTube video shows John Blankenbaker running a similar counting program on the Prototype Kenbak-1 in 2018.  (https://www.youtube.com/watch?v=JNU6PFJj4tg)   John entered the counting program by memory, so it's probably the same counting program except the "A" register was incremented rather than "B" register.  This program is published in his "Laboratory Exercises" manual, as the very first program a student enters and runs.  The YouTube video shows the high-order bit turning on and off 7 times in about 11 seconds.  That would correspond to about 488 instructions per second.

One more computer was then measured:  a "Kenbak-1 Series 2" reproduction computer made in 2007 (see kenbakkit.com.)  While this is a modern reproduction, it uses the same circuitry and capacitors as the production "Rev B" computers (but slightly different transistors.)  It seems to be operating at 428 instructions per second.  I have not yet measured it's clock rate, but suspect it may be closer to 0.9 MHz than 1 MHz.

High resolution photographs of known original Kenbak-1 computers show the first prototype computer used two different capacitors, a big brown mica capacitor, and a large ceramic disc capacitor.  The Nielsen3 Revision A computer seems to use two big brown mica capacitors (which apparently made the clock a bit too fast.)  The Revision A John2 computer in the Computer History Museum uses two large ceramic disc capacitors.  But then all other production Revision B computers seem to show standardized very small ceramic disc capacitors.  John seems to have been experimenting with different capacitor values in the first 3 prototypes, but then settled on values for the production machines.

My opinion differs from those of John Blankenbaker.

I think schools were resistant to purchase the Kenbak-1 because machine code programming is not as good for introductory programming as higher-level languages that could be obtained with a time sharing system.  John thought the problem was the slow and complex school budgeting process, and perhaps that's the excuse the schools gave him.  John did a great job of marketing to schools.  He traveled to educator conferences to demonstrate his computer.  He put advertisements in popular education magazines and journals.  John correctly recognized that timesharing systems were his main competition in schools.  Timesharing systems allowed schools to lease terminals and phone lines to connect to a large central computer which served many schools and businesses.  He acknowledged this competition in an advertisement in November 1971  Nations Schools which compared the cost of buying a Kenbak-1 to the higher cost of renting time sharing terminals.  But the problem wasn't the cost, it was the capability.  Timesharing systems offered high level languages, such as Dartmouth BASIC, or COBOL which are much better for teaching high school students than machine code.  A student can make much more useful programs with BASIC than they could with machine code.  Schools usually teach machine or assembly code programming much later in computer science or engineering curriculum.  So while John recognized his competition, I don't think he understood why schools were choosing his competition.  

Why the individual customer (the hobby, or amateur customer) didn't buy is more complex.  I suspect the level of detail in advertisements or sales brochures was probably too limited for individuals to shell out such a huge amount of money (equivalent to $5,500 in 2025.)  John thought he didn't focus on this hobby market enough, but he did reasonably well.  He got an early article written in the Amateur Computer Society M John's advertisements focused on how "fun and educational" the Kenbak-1 was, but lacked enough details for customers feel confident they knew what to expect.  I suspect customers were weary of these mail order advertisements.  It's not at all surprising that only about 6 individuals bought his computer, and that only included about 3 engineers or programmers (numbers from the Amateur Computer Society newsletter.)

Had John sold 100 or 200 computers that first year, history might have been much different.  He could have hired more help to assemble the computers and handle administrative details.  Then John could have done what he did best: focus on designing and improvements.  Maybe he could have even made a "Kenbak-2" when microprocessors and cost effective RAM chips came out.

For the 1986 Boston Computer Museum contest the industry expert judges realized their first job was to decide what qualifies as a "Personal Computer."  Even defining what a "computer" is was not obvious.  They had to decide if they should include programmable calculators, analog computers, and even the cardboard or plastic "toy computers" like the "Geniac" which were commonly sold from the 1950's through the early 70's.  The judges decided not to include programable calculators, and the mechanical, or analog computers, focusing only on digital computers with a stored program.  They even made the arbitrary decision to disqualify any "kit" computers, and focus on items which were commercially available.  Many people have offered opinions of what a "personal computer" should be.  The Deutsches Museum in Munich Germany saw the ambiguity in definition, so placed a note on their Kenbak-1 saying it is the "first personal computer for educational purposes."  But that does little to clarify the ambiguity. 

In November 1999, an early and thoughtful treatise tried to name the "First Personal Computer."  The "Blinkinglights.org" site (permanently archived here) asked "what was the first personal computer?"  They do an excellent job of explaining the difficulty in defining a PC, and listed several contenders, usually ruling them out.  Unfortunately, they go down the rabbit hole of including the cardboard and wire "toy computers" like the Geniac, the paper-clip computer, and others, before finally settling on the 1950 "Simon" which most people would not consider was a true computer, and definitely not a stored program computer that people think of today.  Instead of convincing readers that "Simon" was the first personal computer, many readers were left feeling the Kenbak-1 matched their own notion of a first personal computer.    

While only around 52 were made, almost all Kenbak-1s were sold to high schools, colleges, and technical schools.  Hundreds of students from 1971 to 1976 and beyond used these to get their first exposure to computer programming.  This was a pivotal time when every high school wanted to start their own computer classes.  But even people who never saw a Kenbak-1 in person were likely inspired by the idea of a $750 computer.  Thousands of people and educators saw the many advertisements and it got them imaging a world where they could have their own computer.  The Kenbak-1 left a legacy of interest and imagination which eclipsed it's very dismal sales numbers.  Now 50+ years later, hundreds of enthusiasts still buy and make Kenbak-1 reproductions and learn to program them, share programs, and share the joy of experiencing this ancestor of modern computers.

John and his wife aren't as active as they used to be.  They recently moved to a senior community, so he doesn't have to do all the hard home maintenance.  But he remains excited to do new experiences, and even traveled to see his first solar eclipse in the spring of 2024!