Factors to Convert Reported Units of Quantity to Harmonized System Units of Quantity. This is the official US Government list of conversions to the metric system.

Converts almost any unit of measurement in the world to almost any other unit.

Large set of weights, measures and units conversion/calculation modules, including unusual and ancient units.

Web site that performs online conversions (e.g. metric conversions) for many measurement systems both commonly used like metric and U.S. Avoirdupois and quite exotic like Ancient Greek and Roman.

Calculators and conversion databases for most standards, dates and other calculations.

Converts most units of measure between diverse systems.

Website that converts between metric and imperial units of measurement for temperature, length, area, volume and weight with a series of online calculators. In addition to the calculators, a user can create and print a conversion chart for a conversion between any two units or add one of the converters to their own pages.