Why the U.S. uses different units
Most of the world uses the metric system, but daily life in the United States still runs on customary units. Road signs show miles and miles per hour, the weather forecast is in degrees Fahrenheit, gas is sold by the gallon, food is weighed in pounds and ounces, and apartments are measured in square feet. Many labels show both systems, but menus, recipes, sizes and car manuals often do not. This converter helps visitors and new residents switch between the two, and it also works the other way for Americans reading metric numbers.
Exact conversion factors
Every conversion uses the exact factors published by the National Institute of Standards and Technology (NIST):
- 1 mile = 1.609344 kilometers
- 1 U.S. gallon = 3.785411784 liters
- 1 pound = 0.45359237 kilograms, and 1 ounce = 28.349523125 grams
- 1 inch = 2.54 centimeters, so 1 foot = 30.48 cm
- 1 square foot = 0.09290304 square meters
- 1 U.S. fluid ounce = 29.5735295625 milliliters
Temperature is not a simple multiplication: °C = (°F − 32) × 5 ÷ 9. For fuel economy, L/100 km = 235.2146 ÷ mpg, and the same formula converts back. The converter uses the full-precision numbers, not the rounded ones shown here.
Quick estimates in your head
For a rough idea without a calculator, multiply miles by 1.6 to get kilometers, divide pounds by 2.2 to get kilograms, and multiply gallons by about 3.8 to get liters. For temperature, subtract 30 and halve the result: 70°F becomes about 20°C, and the exact answer is 21.1°C. Use the converter whenever the exact number matters, such as cooking temperatures or medicine labels.
Watch out for gallons and ounces
The U.S. gallon is smaller than the British imperial gallon, which is about 4.546 liters, so miles per gallon figures from the U.S. and the U.K. are not directly comparable. The converter always uses the U.S. gallon. Ounces also come in two kinds: ounces of weight (oz) and fluid ounces (fl oz) for liquids. A 12 fl oz soda can holds about 355 milliliters, and it does not weigh 12 ounces.
To compare grocery prices when packages use different units, for example ounces and grams, try the unit price calculator, which converts everything to one unit for you.
U.S. shoe and clothing sizes
Size charts help, but they are only approximate. U.S. women's shoe sizes run about one and a half sizes above men's sizes for the same foot, and European sizes are numbered differently again. Clothing sizes in the U.S. are often letters, and brands cut the same size differently. Use the tables as a starting point, then check the brand's own chart or try the item on. When two sizes seem possible, check the return policy before you buy.
Frequently asked questions
How do I convert Fahrenheit to Celsius?
Subtract 32 from the Fahrenheit temperature, then multiply by 5 and divide by 9. For example, 68°F minus 32 is 36, and 36 × 5 ÷ 9 equals 20, so 68°F is 20°C. Water freezes at 32°F (0°C) and boils at 212°F (100°C).
Is a U.S. gallon the same as a U.K. gallon?
No. A U.S. gallon is 3.785 liters, while a U.K. imperial gallon is about 4.546 liters. Fuel economy in miles per gallon therefore looks higher in the U.K. for the same car. This converter uses the U.S. gallon.
Why does my shoe size not match the chart?
Sizes vary by brand, country and shoe style, and charts only give typical equivalents. Measure your foot, read the brand's own size guide and try shoes on when you can. If you are between sizes, many people choose the larger one.