Introduction
Unit conversion expresses the same quantity in a different measurement unit. The physical quantity does not change; only its numerical representation and unit label change.
Reliable conversions use a known conversion factor and keep track of units throughout the calculation.
Key concept
A conversion factor is a ratio equal to one, such as 100 centimeters ÷ 1 meter. Multiplying by the correctly oriented ratio cancels the original unit and leaves the target unit.
Example
To convert 2.5 kilometers to meters, multiply by 1,000 meters per kilometer: 2.5 km × 1,000 m/km = 2,500 m.
Temperature is different because Celsius and Fahrenheit have different zero points. These conversions require multiplication and an offset, not only a ratio.
Common mistakes
Check whether the conversion factor should multiply or divide. Area and volume conversions must apply the linear factor two or three times respectively.
Data storage may use decimal multiples of 1,000 or binary multiples of 1,024, so read the converter's stated convention.
Checking an answer
Estimate whether the target number should be larger or smaller. Converting meters to centimeters should increase the numerical value, while converting centimeters to meters should reduce it.