Shirt Pocket Slide Rule: History Of The HP-35

In a recently updated post, [Codex99] has a detailed history of the HP-35 pocket calculator. Unless you are a certain age, you probably don’t think much of calculators. They are cheap and not very essential in this day of cell phones and PCs. But in the 1970s they were amazing technology and the desire of every engineer and engineering student.

The story opens in 1965 when Tom Osborne — who was not an HP employee — build a floating point calculator he called the Green Machine. Apparently, he had painted the balsa wood case green. He had been showing it around but failed to get any interest until he showed it to Bill Hewlett. Hewlett wanted it to do trig functions and offered him a six-week consulting gig to work on improvements.

HP engineer Dave Cochran helped out and also helped envision making the device keystroke-programmable. By 1968, this collaboration led to a 40-pound desktop calculator — the HP 9100 — that was the size of a typewriter. It could be yours for only $4900. Keep in mind, that same amount would buy two brand new cars in 1968.

If you are wondering how you go from a 40-pound desk calculator to an HP-35, read the post. Bill Hewlett famously told the team:

I want it to be a tenth of the volume, ten times as fast, and cost a tenth as much.

That seemed like an impossible task in those days, although like watching Titanic in the theater, you already know how the story is going to end.

The post takes an interesting side trip to talk about other period calculators including those from Friden and Wang. These devices, and the 9100, all used discrete transistors and diodes, along with exotic memory techniques and big displays like CRTs or nixies.

You can probably guess what would allow Hewlett’s tenth desires to come true: integrated circuits. HP wasn’t the first to build an IC calculator. Sharp’s EL-8 was portable and Busicom’s Handy LE could actually fit in your hand. These calculators were simple affairs, though. Generally, they had four basic functions with a few embellishments. They were not scientific calculators.

[Codex99] has a lot of detail over how the design came to be ranging from the source of the math algorithms to the industrial design of the iconic calculator’s case. There were a few details we found interesting. The HP-35’s name came from it having 35 keys. At the time, HP set prices by taking the bill of materials cost and multiplying it by pi, which seems perfect for an engineering company.

By 1972, the company held its breath, hoping to sell 10,000 units at $395 to break even. GE alone wanted a quote for 20,000 units! Demand outstripped supply by 18 months even after adding a night shift. The calculator sold until 1975, although the price dropped to $295 when the HP-45 arrived.

There’s a lot more to the story. Check out the original post. If you want more technical details and pictures of several prototypes, check out the HP Museum. If you want to try an HP-35 (remember, it uses RPN) [Neil Fraser] has an online one for you to play with.

We have looked at early calculators before. If you need help with RPN, we got you on that, too.

29 thoughts on “Shirt Pocket Slide Rule: History Of The HP-35

  1. Great story. I remember the first time I saw the calculator. I was in the Air Force and an engineer walked in with one and it blew everyone’s mind. How could something that small be able to do logs and all the other functions? We had big desktop calculators that could only add, subtract, multiply and divide! The price was a bit steep for a young second lieutenant back in 1972. It was a while before I purchased a scientific calculator and it wasn’t an HP one. Years later I was fortunate to be employed by HP when it was still HP and provided customer support for their test equipment. It was the best company I ever worked for, they had the HP Way and valued their employees.

  2. I bought my HP-45 in 1974 I think, amazing machine, was stolen a few years later, was very sad over that, When you bought a calculator back then you got a newsletter thrown in free, quarterly I think for several years.

    The RPN took some getting used to but loved it, bought several other HP calculators over the years. The 11C with the keystroke programming was great.

    1. Well “back in the day™” some calculator/pocket computers did have newsletters/journals dedicated to them. Back then a tool (pricey one at that), rather than today’s disposable.

    2. Inherited my granddad’s HP-45, which aside from the battery pack needing a rebuild still works. Wish I had all of the HPs The Old Man went through over the years! :D I’ve also got the HP-11c that got me through college. There was a very cool Python app called NonPareil that emulated the hardware of most HPs, and ran the calculators’ ROM images. It fell afoul of syntax changes somewhere back around Python version 2.7 or so. And of course I run an HP-11c emulator on my phone.

    3. My granddad knew Bill Hewlett, and went through a stack of HP calculators over the years. I’ve got his HP-45, and the HP-11c that got me through college. I also run an 11c emulator on my phone. There’s a terrific emulator called Nonpareil that I ran for a long time on my linux box, but it fell victim to syntax changes between Python versions at some point, and I was far too lazy to track down the code modifications necessary to get it running again.

  3. Awesome story. I have to admit that I am a lucky person because Dave Cochran is a personal friend of mine and we have had several meals together with his lovely wife Reina. A funny anecdote is that I knew Reina for long time when she met Dave. One day while having dinner at one of their favorite places here in Los Altos (Silicon Valley) Reina mentioned that if I was aware that Dave was the guy who helped inventing the HP35 and that he knew about HP calculators. Me being a super HP calculator enthusiast, and owner of an HP-25 and having played with the HP-35 when I was little and loving the HP9200, suddenly looked at him very suspiciously and started to ask him first some basic questions trying to proof that he couldn’t have been one of the fathers of one of my childhood’s dream…. I started increasing the notch of my questions trying to discover if he was real and to my surprise he was answering the right question and even telling me stories about that. He mentioned a funny story about how Bill wanted to make that calculator (HP9100) first fit in his desk drawer and one time Bill left for few days these pranksters ended up making the drawer bigger so it could fit. And many awesome stories Dave have told me over the time. I am happy to know him and sporadically text with him and Reina and congratulate her for her birthday tomorrow (5/14) and Dave’s birthday by the end of this month. Thank you AI Williams for making me remember these stories and hw fortunate I have been about living here in Silicon Valley and meeting one of my childhood technical heroes and being friends with him. Santiago

  4. I once got a new in box HP 41-C for $4.50 at a thrift store. I could see that the battery contacts were pristine, never used. It still had the three Energizer N cells in their box, two of which still showed full voltage. The third was dead. All the original papers were with it and it had a $250 Oregon State University bookstore price tag.

    It brought considerably more than $250 on eBay.

    1. I did some stuff in BASIC with our school’s [single] TRS-80, in the late 70’s. But – it was the HP-41C (in 1982) that truly made me enjoy writing programs.

      As [AMS] noted, the keypad quality was a major feature. In later years, the calculators got more powerful but the keyboards were considered a decline in quality. To put that in perspective, there’s an FPGA version of the HP-41 main board available ( that still uses the original keypad and LCD module.

      1. Now, that is interesting! Do you know if there is anything similar for a TI-59 available? I have one here, case looking good, but apparently one of the main processors is gaga. Would like to use it, if only for the keyboard and the display.

        1. Hello Stefan,
          the display is great but the Keypad of the TI-59 is known as a bouncing one. You should verify every keystroke.
          Btw. you can buy the Ti-59 sometimes from ebay.
          I own a new in Box, new old stock where only batteries needs to be removed.

    2. Still have my HP-25, which I bought in grad school, and my 41C, which I bought after I graduated. I lusted after the 35, one of my more fortunate classmates had one, but it was way out of my budget. I had to make do with a Bowmar 4-banger and a slide rule.

      I still use a 41C emulator, as I can’t stand the default Microsoft calculator.

      RPN for life!

  5. Unfortunately in the article is not mentioned the Olivetti P101, maybe because was born as a business calculator, borrowing a lot from typewriter and mechanical adder technology (the keyboard “Matrix” was lever-based, like a teletype, and also the printer was made like a mechanical calculator one). Some technologies used on the HP 9100 were Olivetti patents.

    Olivetti calculators were a lot business-related so are used in banks

  6. I entered engineering college with my dad’s slide rule and graduated with an hp-45. You can’t begin to imagine what an enormous transition that was, and how much we envied those guys who could afford the hp-35 when it first came out.

    1. Was a bit like that with the HP-48 when it came out when I entered university. The calculator to use in the physics department. Still got it, still use it. Using units when doing calculations, computer algebra, silly games, you name it.

    2. I wonder how much easier it would have been, had we had spreadsheets, word processors and graphing calculators?

      Kids today…:-)

      I still have my Pickett slide rule. Mine was the last class to use them.

  7. Several people have mentioned here how great the keyboards were on HP calculators. I am guessing you are talking about the way they feel when you press down on them. One of the other things that made them great was that the letters and numbers for each key were injected into each key, not simply painted on. That way, the letters and numbers would never wear off. I don’t know if HP still does that for their current instruments.

    1. I just took a look at my HP Prime. I can’t say for sure, but I think the keys have some kind of print on them rather than having their values injected. I’m taking that more from the keys which have their whole surface of a different colour, but the sides are the regular white.

  8. The article describes the early pricing model for HP thusly: “At the time, HP set prices by taking the bill of materials cost and multiplying it by pi, which seems perfect for an engineering company.”

    Apparently, that model survived and flourished for years. I was the VP of marketing at Tekelec and my mentor, Allan Toomer, former EVP of the DMS 10 switching division at Nortel (Northern Telecom) required that all margins for hardware pass this test.

  9. One of my first paid programming gigs was programming a calculator related to the 9100 (the 9810) and then later an actual 9100. It was a marvelous machine. The calculator I took to college was an HP-67 and I still have it and use it occasionally. The card reader is broken (the rubber parts probably turned to goo) but the calculator is fine. My daily calculators are HP-16Cs. Seeing the story of the what prompted the development of this line for HP was wonderful. Thanks for sharing it.

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