Metric vs Imperial: The Complete Conversion Guide

Most of the world uses metric. The United States, Myanmar, and Liberia officially use imperial. And everyone else is stuck converting between the two on a regular basis. This guide explains the difference, the history, and the most common conversions you'll actually need.

Why do two systems exist?

The Imperial system evolved organically over centuries in Britain, based on practical measurements — a foot was literally the length of a foot, a yard was a stride, an acre was how much land an ox could plow in a day. These units spread through British colonialism and became entrenched in countries like the United States.

The metric system was designed from scratch during the French Revolution in the 1790s, with a deliberate goal: a universal, rational system based on powers of ten. The fundamental unit of length (the metre) was defined as one ten-millionth of the distance from the equator to the North Pole. Everything else derived from there.

The metric system won globally because it's simply easier to work with — converting between millimetres and kilometres requires moving a decimal point, not memorizing that there are 5,280 feet in a mile.

Length conversions

• 1 inch = 2.54 centimetres
• 1 foot = 30.48 centimetres
• 1 yard = 0.9144 metres
• 1 mile = 1.609 kilometres
• 1 kilometre = 0.621 miles
• 1 metre = 3.281 feet

Weight conversions

• 1 ounce = 28.35 grams
• 1 pound = 453.59 grams (0.4536 kg)
• 1 stone = 6.35 kilograms
• 1 kilogram = 2.205 pounds
• 1 metric tonne = 2,205 pounds (1,000 kg)

Temperature conversions

Temperature is where the two systems feel most different day-to-day, because there's no simple multiplication factor — you need a formula.

• Celsius to Fahrenheit: multiply by 9/5, then add 32. Or: (°C × 1.8) + 32 = °F
• Fahrenheit to Celsius: subtract 32, then multiply by 5/9. Or: (°F − 32) ÷ 1.8 = °C

Key reference points to memorize:

• 0°C = 32°F (water freezes)
• 20°C = 68°F (room temperature)
• 37°C = 98.6°F (body temperature)
• 100°C = 212°F (water boils)

Volume conversions

• 1 teaspoon = 4.93 millilitres
• 1 tablespoon = 14.79 millilitres
• 1 fluid ounce = 29.57 millilitres
• 1 cup = 236.6 millilitres
• 1 US gallon = 3.785 litres
• 1 litre = 0.264 US gallons

Speed conversions

• 1 mph = 1.609 km/h
• 60 mph = 96.6 km/h
• 100 km/h = 62.1 mph
• 1 knot = 1.852 km/h = 1.151 mph

Convert any unit instantly

Our free unit converter handles length, weight, temperature, volume, speed, area, time, and data — showing all conversions simultaneously so you don't have to pick a target unit.