Reference

Measurements and Unit Conversion

Sites

* Area Conversion
* Capacity and Volume Conversion
* Circular Measure Conversion
* Imperial Weights and Measures
Includes a fabulous array of old weights and measures including some particular to various trades.
* International System of Units
From the US National Institute of Standards and Technology (NIST)
* Length Conversion
* The NIST Reference on Constants, Units and Uncertainty
From the US National Institute of Standards and Technology (NIST)
* Pressure Conversion
* Speed Conversion
* Temperature Conversion
* Time
* Weight
Convert betwen Metric (Tonne, Kilogram, Milligram...), Avoirdupois (Ton, Pound, Ounce...), and Troy (Pound, Ounce, Grain...) units