Unit Converter

Why unit errors are more expensive than they look

The Mars Climate Orbiter was lost in 1999 because one team supplied thrust data in pound-seconds while the receiving software expected newton-seconds. The mismatch put the spacecraft into the atmosphere instead of orbit, at a cost of well over a hundred million dollars.

Aviation has its own catalogue: the Gimli Glider ran out of fuel mid-flight in 1983 after a conversion error between pounds and kilograms during refuelling. Medicine has repeated incidents involving milligrams and micrograms, and pounds and kilograms in paediatric dosing.

The pattern in all of them is the same — the arithmetic was easy, and the failure was in assuming which unit was meant. Where it matters, state the unit explicitly at every step rather than relying on convention.

Metric, imperial and US customary are three systems, not two

The metric system, formalised as SI, is decimal and coherent: prefixes scale by powers of ten and derived units follow from base units without conversion factors. Almost every country uses it officially.

Imperial units are the British system defined in 1824. US customary units derive from an earlier English system and are not identical to imperial, which is the trap. The US gallon is 3.785 litres against the imperial 4.546 — a 20% difference. Fluid ounces, pints and quarts all differ too.

Length and weight units happen to agree between the two, since both were tied to the metre and kilogram by international agreement in 1959. So a foot is a foot everywhere; a pint is not.

This converter uses US customary units for volume, since that is what the overwhelming majority of online recipes and product listings assume.

Temperature and data are the two special cases

Temperature scales have different zero points, so conversion needs an offset as well as a multiplier. That has a practical consequence people get wrong constantly: a temperature difference does not convert the same way as a temperature. A rise of 10 °C is a rise of 18 °F, not 50 °F.

Data storage has a different problem: two competing definitions of the same prefix. Manufacturers use decimal, where 1 GB is a billion bytes. Operating systems generally use binary, where the same label means 1,073,741,824 bytes. The 7% gap per prefix step is why a 1 TB drive shows as about 931 GB in a file manager.

The IEC introduced kibibyte, mebibyte and gibibyte in 1998 to end the ambiguity. Adoption has been partial at best, but both sets are available here so you can compare them directly.

Precision and rounding

Conversion factors between defined units are exact by international agreement: an inch is exactly 25.4 millimetres, a pound is exactly 0.45359237 kilograms. Any imprecision comes from rounding for display, not from the constants.

This tool calculates at full double precision and rounds only when showing you the answer, so copying the result gives you the unrounded value. That matters when a converted figure feeds into a further calculation — converting, rounding, then converting again accumulates error at every step.

For anything going into engineering, dispensing or a legal document, decide the significant figures your standard requires before rounding, and round once at the end rather than at each stage.