RPN Financial Calculator

A free browser-based RPN financial calculator in the classic HP 12C style: four-level stack, TVM solving, cash-flow NPV and IRR, statistics, and date math.

Key Terms (Click to expand)

RPN (Reverse Polish Notation)

Postfix entry: type the numbers first, then the operator. There is no equals key and no parentheses, and values live on a four-level stack.

Stack (X, Y, Z, T)

The four-level register stack. X is the displayed value, and each operator consumes the top values and leaves the result in X.

TVM (Time Value of Money)

The relationship between n, i, PV, PMT, and FV. Supply any four registers and the calculator solves the fifth.

NPV / IRR

Net present value discounts a cash-flow stream to today at a chosen rate. Internal rate of return is the rate at which NPV equals zero.

Annuity (begin / end)

Whether each payment falls at the start of the period (begin, "annuity due") or the end (end, "ordinary annuity").

LASTx

Recalls the value that was in X just before the last operation, useful for recovering from a mis-key or reusing a constant.

Tool

0.00

Methodology

The calculator is a deterministic reverse Polish notation engine. Numbers are pushed onto a four-level stack (X, Y, Z, T) and each operator consumes the top values:

Entry: type a number, press ENTER to lift it onto the stack, then the next number and the operator.
TVM: n, i, PV, PMT, and FV describe one cash-flow equation. Enter any four and press the fifth key to solve for it. The periodic rate i is per period, not annual.
Cash flows: CFo and CFj record an uneven stream. NPV discounts it at the i register and IRR solves for the rate where NPV is zero.
Sign convention: money received is positive (PV) and money paid out is negative (PMT, FV).

An algebraic (ALG) entry mode is available via the RPN/ALG toggle for left-to-right infix evaluation with an equals key.

The keyboard follows the layout conventions popularized by classic RPN financial calculators such as the HP 12C, so keystroke sequences from HP 12C tutorials and exam-prep materials work here unchanged.

Using This Tool Well

When to use this tool

Re-solve a payment after a rate quote changes: key 360 n, 0.5 i, 200000 PV, 0 FV, then PMT for about -1,199.10, change i to 0.5417 and press PMT again without re-entering the loan.
Rank two projects: enter CFo and each CFj for the stream, set the i register, then compare NPV against IRR before clearing your hurdle rate.
Chain a calculation: run a 3-step tax, margin, then markup sequence while RPN keeps every intermediate value live on the four-level stack instead of forcing a re-entry.

How to read the result

The display is the X register, the bottom of the stack and the result of the last operation. After a TVM solve, X holds the value you solved for. Press the swap key or roll the stack down to see the other levels. An "Error" message means an operation has no valid result, and the next keystroke clears it.

Common mistakes

Sign convention: PV is usually positive (cash in) while PMT and FV are negative (cash out). Enter a $200,000 loan with all-positive PV, PMT, and FV and the solver returns no solution.
Periodic vs annual rate: i is the rate per period, so on a monthly loan enter 0.5 (6% / 12) rather than 6, because entering the annual rate overstates the cost roughly twelvefold.
Stack lift after ENTER: ENTER duplicates X and the next digit replaces it, so typing 5 ENTER 3 leaves 3 in X and 5 in Y, not 53.

Reviewed by Leonardo, Software Engineer

Last reviewed June 23, 2026

Premises and Limitations

The interest register i is a per-period rate, so convert annual rates to the compounding period before entering them.
Cash-flow direction follows the sign convention: inflows are positive and outflows are negative, and a TVM system with no consistent solution returns an error.
Results are deterministic planning figures and do not reproduce the rounding or display conventions of any specific physical device.

FAQ

Is this an HP 12C emulator?

No. It is an independent RPN implementation whose keyboard layout and keystroke conventions will feel immediately familiar to anyone who has used an HP 12C: the same f and g shift keys, TVM registers, cash-flow keys, and sign convention. It does not reproduce the rounding or display behavior of any physical device, and Cleartali is not affiliated with HP. If you are practicing for an exam that allows the HP 12C, the keystrokes here transfer directly.

What is RPN?

Reverse Polish Notation is postfix entry: you enter the numbers first and the operator last, with no equals key and no parentheses. Values live on a four-level stack.

How do I switch between RPN and algebraic entry?

Press f then EEX for algebraic mode, or f then CHS to return to RPN. The active mode shows in the display badge. Algebraic mode evaluates left to right with an equals key, while RPN mode uses the stack and ENTER.

Why is my payment negative?

The sign convention treats money you receive as positive and money you pay out as negative, so a solved payment on a loan you received comes back negative.

HP and HP 12C are trademarks of HP Inc. Cleartali is an independent project and is not affiliated with or endorsed by HP.