Feb 1, 1972 — The HP-35 Was Released

Exactly 54 years ago today, on February 1, 1972, the world of math changed forever. Hewlett-Packard released the HP-35, and it wasn’t just a new gadget — it was the beginning of the end for the slide rule.

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Before the HP-35, if you were an engineer or a scientist, you carried a wooden or plastic ruler with scales on it to do complex math. If you wanted a digital calculator, you had to use a heavy machine that sat on a desk and plugged into a wall. Bill Hewlett, the co-founder of HP, wanted something different. He told his engineers he wanted a “shirt-pocket-sized” version of their big desktop calculator.

The “Slide Rule Killer”
The engineers pulled it off. The HP-35 was the world’s first scientific pocket calculator. It could do more than just add and subtract; it handled logarithms and trigonometry with the press of a button.

It was famous for a few things:

The Red LEDs: It had a bright red display that showed up to 10 digits.

RPN Logic: It didn’t have an “equals” ($=$) button. Instead, it used “Reverse Polish Notation.” To do $2 + 2$, you pressed 2, then ENTER, then 2, then +. It felt strange at first, but it was much faster for long equations.

The Price: It cost $395. In today’s money, that is over $2,800.

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Why It Mattered
Even with that high price tag, HP sold 100,000 of them in the first year alone. It was a status symbol for NASA engineers and college students alike. Within just a few years, the slide rule—a tool used for centuries—became a museum piece.

The HP-35 proved that high-performance computing didn’t need a giant room or a desk; it could fit right in your pocket. Every time we open a calculator app on our phones today, we are using the legacy of that little plastic box from 1972?