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Unit Converter

Built on the exact 1959 definitions rather than rounded factors, with the conversion shown alongside the answer.

The factors that are definitions

Most conversions people distrust are actually exact. The inch, the pound and the mile were pinned to metric values by international agreement in 1959 — they are not approximations that drift, and any conversion built on them is exact too.

Exact conversion factors and the ones that catch people out 1 inch= exactly 2.54 cmDefined, not measured — agreed internationally in 1959.1 mile= exactly 1.609344 kmFollows from the inch: 63,360 inches.1 pound= exactly 0.45359237 kgAlso a definition, exact to the last digit.1 US gallon= 3.785 litresThe imperial gallon is 4.546 L — 20% larger.1 fluid ounce= 29.57 ml (US)A volume. The weight ounce is 28.35 g — different thing.1 GB= 10⁹ bytes, or 2³⁰Drive makers use 10⁹; Windows shows 2³⁰. Factors marked "exactly" are definitions — there is no rounding error in them at all.
The bottom three rows are where mistakes cluster: two different gallons, two different ounces measuring different quantities, and two different gigabytes.
Keep the precision you were given, not the precision the screen offers. Converting a recipe's "1 cup" to 236.588 ml implies a measurement nobody made. Round to the accuracy of the input — usually two or three significant figures for everyday use.

The gigabyte that isn't

Storage is the one category where two rival definitions are both in daily use. Drive makers count in powers of ten; operating systems count in powers of two. Neither is wrong, and the gap between them widens with every step up the scale.

How far the binary and decimal storage prefixes drift apart The binary unit exceeds the decimal one by 2.4 per cent at kilobyte scale, 4.9 at megabyte, 7.4 at gigabyte and 10.0 at terabyte. How much bigger the binary unit is than the decimal one 2.4%KB4.9%MB7.4%GB10.0%TB A 1 TB drive holds exactly what it says — Windows just calls it 931 GB.
Both units are offered in the converter above, labelled GB and GiB, so you can see which number a device is quoting you.

Questions

Are these conversions exact?

The core ones are definitions rather than measurements: an inch is exactly 2.54 cm and a pound is exactly 0.45359237 kg, fixed by international agreement in 1959. Conversions built on them inherit that exactness. Temperature is also exact (°F = °C × 9/5 + 32). What is not exact is anything involving mass and volume together — converting litres to kilograms depends on what the substance is.

Why is my new hard drive smaller than advertised?

Because two different definitions of a gigabyte are in use. Drive manufacturers use the decimal one, 10⁹ bytes. Windows reports using the binary one, 2³⁰ bytes, which is about 7% larger. A 1 TB drive is 1,000,000,000,000 bytes, which Windows displays as roughly 931 GB. Nothing is missing — the two are just counting differently.

Is a fluid ounce the same as an ounce?

No, and this is the most expensive unit confusion in cooking. A fluid ounce measures volume (about 29.6 ml in the US); an ounce measures weight (28.35 g). They coincide only for substances with water's density. Eight fluid ounces of flour weighs far less than eight ounces of flour.

References

  1. NIST — International System of Units conversion factors
  2. BIPM — The International System of Units (SI Brochure)
  3. IEC 80000-13 — binary vs decimal prefixes for information quantities

Reviewed September 2026.