Conversion standards and accuracy
Our calculators use documented unit relationships and show six significant digits by default so results remain useful without implying false precision.
SI definitions are checked against the Bureau International des Poids et Mesures (BIPM) SI Brochure. Conversions between SI and U.S. customary or other non-SI units are checked against NIST Special Publication 811.
Some relationships are exact, such as 1 inch = 25.4 millimeters. Others are conventional or rounded for practical display. Calendar months and years are averages because their real duration varies.
Fuel-economy conversions use U.S. gallons. Volume pages label U.S. and Imperial gallons separately to prevent a common and consequential mix-up.
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