Millimeters, centimeters, meters, kilometers, inches, feet, yards and miles — one box, every unit.
Every unit is converted through one exact base unit, so any pair stays consistent — converting A to B and back returns your original number. All factors are the standard international definitions.
Which units are included? The units in the two dropdowns — the everyday set for this measurement, using US liquid measures where gallons/pints/quarts are concerned.
How accurate is it? Factors are the exact defined values (1 inch = 25.4 mm and so on); display rounds to 6 decimal places.