Exact unit definitions and conversion standards
Most conversion factors you find online are rounded. These are the defined, exact values published by the standards bodies themselves — the numbers our converters actually use.
The headline exact values
- 1 yard = 0.9144 metre exactly, and 1 inch = 25.4 mm exactly — both fixed by the 1959 international yard and pound agreement.
- 1 avoirdupois pound = 0.453 592 37 kilogram exactly — fixed by the 1959 international yard and pound agreement.
- 1 US liquid gallon = 3.785 411 784 litres exactly, because it is defined as exactly 231 cubic inches.
- 1 acre = 4046.856 422 4 square metres exactly, because an acre is 4840 square yards and a yard is exactly 0.9144 m.
- 0 °C = 273.15 K exactly, and °F = °C × 9/5 + 32 exactly — these are definitions, not approximations.
- 1 mph = 0.447 04 m/s exactly and 1 knot = 1852/3600 m/s, both following from exact length definitions.
- 1 GB = 1 000 000 000 bytes under SI, but 1 GiB = 1 073 741 824 bytes — a 7.4% gap that explains 'missing' drive space.
Browse by measurement family
Length & distance
Base unit: metre (m)
6 defined values · 3 sources
Mass & weight
Base unit: kilogram (kg)
5 defined values · 2 sources
Volume & capacity
Base unit: cubic metre (m³); litre (L) = 0.001 m³
6 defined values · 3 sources
Area & land measurement
Base unit: square metre (m²)
5 defined values · 2 sources
Temperature
Base unit: kelvin (K)
5 defined values · 2 sources
Speed & velocity
Base unit: metre per second (m/s)
4 defined values · 2 sources
Digital storage & data
Base unit: byte (B)
6 defined values · 2 sources
Why exact values matter
A conversion factor is either defined or derived. A defined factor — the inch at 25.4 mm, the pound at 0.45359237 kg — carries no uncertainty at all, because a standards body simply declared it. A derived factor, like kilograms to pounds, is the reciprocal of a defined one and often does not terminate, so it must be rounded for display.
Knowing which is which tells you when a difference in the sixth decimal place is a real error and when it is only display rounding. Every conversion page on this site links back to the family page that holds its defining constant, and our methodology page lists the rounding policy we apply.