Calc
A calculator with exact decimal arithmetic, unit conversion and a paper tape — where 0.1 plus 0.2 is 0.3.
Running here in the page — the same app the desktop opens.
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Almost every calculator on a computer uses binary floating point, so 0.1 plus 0.2 comes to 0.30000000000000004. For money that is the wrong answer. This runs addition, subtraction, multiplication and division on a decimal type built over BigInt, so the result is exact.
Expressions are parsed properly rather than evaluated as code, which means functions, variables, percentages that behave the way a till does, and hex or binary literals all work. Unit conversion covers length, mass, temperature, data, time, area, volume and speed — temperature as a real affine conversion, not a ratio.
What people open it for
- Doing money arithmetic without floating-point drift
- Converting units, including temperature, correctly
- Keeping a running tape you can click to reuse earlier lines