Offers a wide variety of unit conversion tools and conversion charts for fast lookup.

Physics laboratory of the National Institute of Standards and Technology (NIST) sponsors and maintains this reference resource of CODATA-recommended values of fundamental physical restraints.

History and usage of English/Imperial/customary weights and measures, with conversion factors, and pictures. Minims, drachms, pecks, bushels, rods, poles, perches, furlongs, ounces, fathoms, pints, gallons, nails, chains, chaldrons, tods, crowns, florins, groats, farthings and guineas.

Convert units from one measure to another (e.g. meters to yards, liters to gallons). A very extensive collection of units and categories. Includes usage and definitions descriptions of units and categories.

Definitions of SI units and SI prefixes, units outside the SI, rules and style conventions, historical context for SI units, international aspects of the SI, and unit conversions.

A site promoting the metric system which argues why metric is better and English is worse. Provides an English-metric online calculator.

Standard-to-metric conversion factor tables with definitions, derived units and prefixes.

Includes information about the National Measurement System, legislation and guidance, services to industry, and its enforcement role.

Pressure group campaigning for the completion of metrication in the UK. History, legislation, and practical help.

Conversions tables and equivalences of the principal units of measure of the SI and Anglo-Saxon systems.