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Metric vs Imperial: A Quick Guide
The world primarily uses two measurement systems. The metric system (SI) is used by nearly every country on earth and is based on powers of 10 — making conversions straightforward. The imperial system is primarily used in the United States, with some continued use in the UK and Canada for certain measurements.
Most common conversions to memorize
1 mile = 1.609 km (US road distances vs European)
1 kg = 2.205 lbs (weight of people and objects)
1 inch = 2.54 cm (exact, by definition)
°C to °F: multiply by 9/5 then add 32
1 gallon = 3.785 liters (US gallon)
1 foot = 30.48 cm
Why the US still uses imperial units
The US is one of only three countries (alongside Myanmar and Liberia) that hasn't officially adopted the metric system. Efforts to metricate in the 1970s largely failed due to public resistance and the cost of converting infrastructure. Science and medicine in the US use metric — only everyday commerce and informal measurement remain imperial.
Frequently Asked Questions
How many centimeters in an inch?
Exactly 2.54 centimeters per inch. This is a defined constant — not an approximation. So 1 foot (12 inches) = 30.48 cm, and 1 yard (36 inches) = 91.44 cm.
What is the difference between a US gallon and a UK gallon?
A US gallon is 3.785 liters. A UK (imperial) gallon is 4.546 liters — about 20% larger. This matters when comparing fuel economy: a car getting "30 mpg" in the UK uses significantly less fuel per mile than a US car rated "30 mpg" since UK miles per gallon uses the larger imperial gallon.