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How to use an RPN calculator

RPN has no equals key and no parentheses. You key the numbers first, then the operation, and intermediate results wait on the stack until you need them.

Problem

Work out (3 + 4) × (5 + 6) without writing anything down.

Keystrokes

  1. 3ENTER4+
    7.00

    ENTER separates the two numbers. + adds them right away.

  2. 5ENTER6+
    11.00

    The 7 waits on the stack while you work out 5 + 6.

  3. ×
    77.00

    Multiplies the two results.

Answer

The answer is 77.00, in three short steps.

The stack

The stack is four registers, X, Y, Z and T. X is what the display shows. ENTER copies X into Y so you can key a second number. A two-number operation like + combines Y and X and drops the stack down.

In the problem above, after the second + the stack holds 11 in X and 7 in Y, so × multiplies them. No parentheses needed, because the order you key things is the order they’re done.

Handy keys

x≷y swaps X and Y, so you can fix a subtraction or division in the wrong order. R↓ rolls the stack to look at what’s underneath. CLx clears just the number in the display. gLSTxENTER brings back the last X used by an operation, to undo a mistake.

Questions

Why do people prefer RPN?

Fewer keystrokes, no parentheses to balance, and you see every intermediate result. Most people who learn it don’t go back.

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