Measurement guide
Common Unit Systems
Most unit conversions come from moving between a small set of measurement systems. Knowing the system helps you choose the right converter and avoid mixing similar-looking units.
International System of Units
The International System of Units, usually called SI, is the modern metric system used in science, engineering, medicine, trade, and education. It uses base units such as meter, kilogram, second, ampere, kelvin, mole, and candela.
Metric units
Metric units scale by powers of ten. This makes conversions like meters to centimeters, kilograms to grams, and liters to milliliters predictable because prefixes show the size relationship.
US customary and imperial units
US customary and imperial units include inches, feet, miles, pounds, ounces, gallons, pints, and Fahrenheit. Some names appear in both systems but have different values, especially for volume.
Best practice
Always check whether a unit is metric, US customary, or imperial before using a result for buying materials, shipping, engineering, recipes, or travel planning.