Almost every country in the world uses the metric system as its official standard — with a small number of prominent exceptions, most notably the United States, which still uses imperial (or US customary) units for most everyday purposes. The split isn't arbitrary; it comes down to history, cost, and momentum.
Why metric won almost everywhere
The metric system was designed deliberately in the late 18th century around clean, base-10 relationships — 100 centimeters to a meter, 1,000 meters to a kilometer — which makes conversions and calculations far simpler than the irregular divisions of imperial units (12 inches to a foot, 3 feet to a yard, 1,760 yards to a mile). That simplicity made it attractive for international trade, science, and standardization efforts through the 19th and 20th centuries.
Why some countries kept imperial
In the US specifically, imperial units are deeply embedded in infrastructure, manufacturing tooling, construction standards, and everyday habit — the cost and disruption of a full conversion has repeatedly been judged to outweigh the benefits, even though US scientific and some industrial sectors already use metric internally. It's a case where switching costs, not the merits of either system, explain the status quo.
The conversions worth actually memorizing
A handful of approximate conversions cover most everyday situations: 1 inch is about 2.54 centimeters, 1 mile is about 1.6 kilometers, 1 kilogram is about 2.2 pounds, and 1 US gallon is about 3.8 liters. For anything more precise, or less common unit pairs — area, speed, or temperature — our unit converter handles the exact math instantly across all the common categories.