Unit conversion

Area and volume use different scale factors

An area conversion squares the corresponding length factor; a volume conversion cubes it. Do not apply a millimeter-to-inch length factor directly to square or cubic measurements. The selected quantity controls the conversion used here.

Absolute temperatures and temperature differences

Absolute Celsius and Fahrenheit temperatures require an offset as well as a scale change. A temperature difference needs only the scale change. Choose Temperature for a reading, or Difference for a rise or fall. The conversion cannot determine which interpretation your original data intended.

Distinguishing a unit from a physical quantity

A number and a unit together identify a measurement. Converting force does not convert mass into force, and converting torque does not make it an energy calculation even when the dimensions share a newton-meter expression. Select the intended physical quantity first. This prevents a numerically convenient factor from hiding a change in the meaning of the measurement.

For a simple check, 1,000 mm equals one meter, one square meter equals one million square millimeters, and one cubic meter equals one billion cubic millimeters. The different powers follow from how many length dimensions are multiplied. Temperature is another common exception to simple scaling: an absolute temperature can require an offset, while a change in temperature does not. Keep unrounded values when several conversions feed a later calculation, and round once at the reporting stage. If a unit is absent from a category, do not substitute a similarly named unit from another category; check the quantity and definition before proceeding.

Formula

Canonical quantity = value × scale + offset; convert to the requested unit using its scale and offset.