About MeasurementDB

MeasurementDB is a reference guide to 112 units of measurement. It covers the seven SI base units and all 22 SI derived units with special names, the metric units accepted for use with the SI, the imperial and US customary systems, troy and apothecaries' weights, CGS units, astronomical and scientific units, and a selection of traditional units from Japan, China, South Asia, Russia and the ancient world.

What each entry contains

Every unit page gives the unit's symbol, the measurement system it belongs to, the physical quantity it measures, its value in SI units, the date or instrument that defined or adopted it where that is well established, and a short account of its history and current use. Where a conversion is exact by definition the value is marked exact; 61 entries are. Values written with ≈ are rounded from a longer or measured figure. Where a reliable date is not known the field is left out rather than guessed.

Sources

Definitions of SI units and the 2019 defining constants follow the BIPM SI Brochure. Conversion factors for non-SI units follow NIST Special Publication 811 and NIST Handbook 44, imperial volume and mass values follow the UK Weights and Measures Act 1985, and astronomical units follow IAU resolutions such as Resolution B2 of 2012. Traditional and ancient units were checked against standard references and national legislation; ancient values are approximate because they are reconstructed from surviving artefacts.

Scope

The collection favours units that people actually meet, in science, trade, cooking, navigation or history, rather than trying to list every local measure ever used. Prefixed forms such as the kilometre or milligram are not listed separately because they follow directly from the SI prefixes.

Units by system

Units by quantity