Metric units scale by powers of ten. Imperial and US customary systems use several different relationships between units.
The formula
How to use it
Metric prefixes make conversions systematic: kilo means one thousand, centi means one hundredth, and milli means one thousandth. Traditional units require the definition for each measurement. US and imperial gallons differ: the imperial gallon is 4.54609 liters. This site uses US gallons, cups, quarts and fluid ounces, and international feet and miles. Always check a label when copying a measurement from another country.
Related measurement tools
Check the result
Keep intermediate digits until the end, then round to the precision appropriate for the original measurement. Converting to a different unit does not make an imprecise measurement more accurate. Try the linked tool for a reference table and a comparison with related units.
Open converter →Definition references: NIST Guide to the SI and BIPM SI Brochure.