If you used a scientific calculator in the 1970s or 1980s, there was a fair chance that it worked differently from almost every calculator you see today. Instead of typing:

2 + 3 =

you entered:

2 ENTER 3 +

There wasn’t even an equals key. Hewlett-Packard made this system — Reverse Polish Notation, or RPN — practically synonymous with serious scientific calculators until other players like TI and Casio got serious. Once you got used to it, ordinary algebraic calculators could feel annoyingly clumsy.

Today, RPN calculators look like a nearly extinct species. HP left the calculator market, licensing the HP calculator line to Moravia Consulting. Old HP-15Cs, 16Cs, 32Ss, 42Ss, and 48s have become collectibles. But RPN isn’t dead. You can still buy new hardware, build your own, or turn almost any computer or phone into a very capable RPN machine. There are reasons some of us still want to.

But Why Polish?

The name goes back to Polish logician [Jan Łukasiewicz], who devised a notation in which operators precede their operands. Instead of writing:

A + B

you can write:

+ A B

The big advantage is that parentheses aren’t required. The structure of the expression tells you exactly what operates on what. Reverse Polish notation simply puts the operator at the other end:

A B +

[Łukasiewicz] wasn’t designing calculators, of course, but the same idea turned out to be extremely convenient for computers and calculators. Your software doesn’t have to remember what operation is in progress. Each operator is ready to go and can simply work on the operands that you’ve already read.

RPN isn’t exactly the way people calculate with pencil and paper, and it certainly wasn’t derived from the slide rule, but there is a similarity in the way you work. With a slide rule, you generally establish some value, operate on it, and continue from the result. When doing a long-hand calculation, you often calculate a subexpression, write down the answer, and use that answer in the next step. You will probably start with the inner parenthesis and work outward, just like someone with an RPN calculator does. RPN formalizes that process with a stack.

Suppose you want:

(3 + 4) × (5 + 6)

On a conventional calculator, you either need parentheses, or you have to calculate one result and remember it. On an RPN calculator:

3 ENTER

4 +

5 ENTER

6 +

×

The first + leaves 7 on the stack. The second leaves 11 above it. The multiply consumes both and leaves 77.

Notice what’s missing: parentheses, an equals key, and any need to tell the calculator about precedence. This isn’t much of a win for a five-key calculation. It becomes more apparent with something like computing the value of a bunch of parallel resistors:

R=1/(1/R1+1/R2+1/R3…)

An RPN user can calculate each reciprocal, add it to the running result on the stack, and finally take the reciprocal. Intermediate answers stay in the calculator naturally instead of being stuffed into memory registers or enclosed in increasingly impressive collections of parentheses.

Is RPN better? Calculator users have been arguing about that for half a century. But once RPN gets wired into your fingers, it can be surprisingly hard to give up.

RPN Before HP

Although RPN and HP are nearly inseparable in popular memory, HP didn’t invent the RPN calculator. That distinction generally goes to the Friden EC-130, introduced in 1964. It was a 44-pound transistorized desktop calculator costing $2,150, and its CRT actually displayed all four levels of its RPN stack at once. More than 18,000 were eventually made. You can even simulate it in Verilog, if you like.

HP entered the business in 1968 with the HP 9100A, a programmable desktop scientific calculator. It weighed about 40 pounds and sold for $4,900, but it provided logarithms, trigonometry, hyperbolic functions, coordinate conversion, programming, and RPN operation. HP famously called it a calculator rather than a computer partly because customers could often buy calculators without getting corporate computer departments involved.

Then came the machine that changed everything: 1972’s HP-35, the first HP pocket scientific calculator. From there, RPN became a defining characteristic of HP calculators for decades.

There were financial calculators, programmer’s calculators, scientific calculators, graphing calculators, and eventually the RPL machines such as the HP-28 and HP-48, which generalized the stack idea into an entire programming environment.

Eventually, though, algebraic entry won the mass market. Even HP began producing machines with algebraic modes, and some later calculators let you choose either system.

So did RPN die out? Are there any RPN machines left in 2026? Quite a few, as it turns out. The real question is do you want a calculator that pretends, in some way, to be your favorite old-fashioned RPN calculator, or do you want a more modern device that happens to use (or, at least, is able to use) RPN?

RPN You Can Still Hold In Your Hand

You can still get an HP12C (public domain).

The strange survivor is the HP-12C. Introduced in 1981 as a financial calculator, it remains available new more than four decades later. Depending on the retailer, expect something around $50 to $70. The 12C Platinu...

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