Free42 Decimal is a free, open-source Mac application that faithfully recreates the HP-42S programmable scientific calculator in software, offering the same decimal-precision arithmetic and keystroke programming model that engineers and scientists relied on for decades.
What is Free42 Decimal?
Free42 Decimal is a software emulator of the HP-42S — one of Hewlett-Packard's most beloved RPN (Reverse Polish Notation) scientific calculators — running natively on macOS. Unlike a generic calculator app, it reproduces the HP-42S's exact instruction set, memory model, and display behavior down to the way the original hardware would have rounded and flagged edge cases. The "Decimal" in the name refers to its internal arithmetic engine: it uses IEEE 754 decimal floating-point, which means it handles numbers exactly the way the original HP-42S did, without the binary rounding surprises you get from most software.
The project is maintained by Thomas Okken and has been actively developed and refined over many years. If you have ever muttered under your breath at a touchscreen calculator app that fumbles 0.1 + 0.2, Free42's decimal engine will feel like a revelation.
What does Free42 Decimal do best?
Free42 Decimal excels at keystroke-programmable RPN calculation with bit-perfect numeric accuracy — the combination that made the HP-42S the calculator of choice for aerospace engineers, physicists, and land surveyors long after HP discontinued it. You get the full HP-42S instruction set: matrix operations, numerical integration, root-finding, complex-number arithmetic, statistical registers, and a surprisingly capable programming environment in a two-inch stack of keys.
I've used it alongside Python and Wolfram Alpha for quick sanity checks, and there is something deeply satisfying about the way RPN forces you to think through the order of operations before you enter a single digit. The learning curve is real — if you have never used an RPN calculator, budget an afternoon — but once the muscle memory clicks, you will reach for it reflexively. The Mac app also supports custom skins, so you can match the exact bezel of the physical calculator you remember, or swap in a high-resolution modern skin that scales beautifully on a Retina display.
- Full HP-42S keystroke programming with named labels and subroutines
- Decimal floating-point arithmetic (no binary rounding artifacts)
- Matrix editor with real and complex matrix support
- Built-in integration and solver functions
- Importable and exportable program files (.raw format — compatible with the original hardware and with physical HP-42S owners)
- Customizable skins, including a bundled high-DPI skin for modern displays
Is Free42 Decimal free?
Yes — Free42 Decimal is completely free to download and use. Thomas Okken releases it under an open-source license and accepts no payment for the Mac desktop version. If you want to run it on iOS, there is a small paid version on the App Store, but on macOS it costs nothing at all, which makes it one of the best-value apps in any engineer's toolkit.
Who should use Free42 Decimal?
Anyone who already knows and loves the HP-42S is the obvious answer. If a physical HP-42S is sitting in a drawer right now, Free42 lets you continue that workflow on your Mac without babying irreplaceable hardware. Beyond the HP faithful, Free42 is compelling for any engineer, scientist, or programmer who wants a programmable RPN calculator that prioritizes correctness over convenience.
I would not recommend it to someone who just wants a quick desktop calculator — for that, macOS's built-in Calculator app or Soulver are friendlier. And if you want a broader scientific calculator experience that does not demand RPN fluency, PCalc is polished and approachable. Free42 is specifically for the user who wants the HP-42S and nothing else.
What are the best Free42 Decimal alternatives?
The closest direct competitor is Plus42, also by Thomas Okken — it extends Free42 with an algebraic entry mode, a larger display, and unit conversions, making it a strict superset. If you want the classic HP-42S behavior untouched, stick with Free42; if you want modern conveniences bolted on, Plus42 is the upgrade path.
For a broader Mac calculator experience: PCalc is the longtime Mac power-user favorite with RPN support, widgets, and a beautiful native interface. Soulver takes a completely different approach — natural-language arithmetic in a notepad — and is fantastic for financial rough-work. Neither replaces what Free42 does for programmable scientific calculation.
How does Free42 Decimal compare to Plus42?
Free42 Decimal is the strict emulation: it reproduces HP-42S behavior exactly, warts and all, which matters when you need results to match legacy programs or physical hardware. Plus42 trades that purity for a significantly improved user experience — bigger stack display, algebraic mode, and extra math functions. If you are running inherited programs from a colleague's physical HP-42S, Free42's fidelity is worth preserving; if you are starting fresh, Plus42 is arguably the better daily driver.
