Last updated: September 2026
Recipes, road trips, gym routines, and online shopping all run into the same problem: different parts of the world measure things differently. The United States mostly uses US customary units (inches, pounds, cups), while most of the rest of the world uses the metric system (centimeters, kilograms, liters). Converting between them is just multiplication by the right factor — once you know the factors, the mystery disappears.
The metric system is built on tens: 1,000 millimeters in a meter, 100 centimeters in a meter, 1,000 meters in a kilometer. That makes conversions within metric trivial — you just move the decimal point. US customary units grew out of historical English measures and have no such tidy pattern: 12 inches in a foot, 3 feet in a yard, 1,760 yards in a mile. Neither system is "wrong," but metric's decimal structure is why science and most countries standardized on it. For daily life, you mostly need conversions between the two systems, and those come down to a handful of exact factors.
| From | Multiply by | To get |
|---|---|---|
| 1 inch | 2.54 (exact) | centimeters |
| 1 foot | 30.48 (exact) | centimeters |
| 1 yard | 0.9144 (exact) | meters |
| 1 mile | 1.60934 | kilometers |
| 1 centimeter | 0.393701 | inches |
| 1 meter | 3.28084 | feet |
| 1 kilometer | 0.621371 | miles |
Handy rule of thumb: a meter is a little longer than a yard, a kilometer is about six-tenths of a mile, and an inch is roughly two and a half centimeters.
| From | Multiply by | To get |
|---|---|---|
| 1 ounce | 28.3495 | grams |
| 1 pound | 453.592 | grams |
| 1 kilogram | 2.20462 | pounds |
| 1 stone (UK) | 6.35029 | kilograms |
Rule of thumb: a kilogram is about 2.2 pounds, so to convert pounds to kilograms in your head, just halve the number and subtract a tenth (200 lb → roughly 90 kg).
| From | Multiply by | To get |
|---|---|---|
| 1 US fluid ounce | 29.5735 | milliliters |
| 1 US cup | 236.588 | milliliters |
| 1 US pint | 473.176 | milliliters |
| 1 US quart | 0.946353 | liters |
| 1 US gallon | 3.78541 | liters |
| 1 UK (imperial) gallon | 4.54609 | liters |
Kitchen measures are where conversions matter most, because a wrong factor can ruin dinner. In US cooking: 1 tablespoon = 3 teaspoons = about 15 ml; 1 cup = 16 tablespoons = 48 teaspoons = about 237 ml; 1 stick of butter = 8 tablespoons = about 113 grams. For dry ingredients, weight is more reliable than volume: 1 ounce of flour weighs about 28 grams, so 8 ounces is roughly 227 grams. If a recipe says "1 cup of flour," that's about 120–130 grams depending on how it's scooped — which is exactly why serious bakers weigh ingredients instead.
Length and weight scales both start at zero, so converting is simple multiplication. Temperature scales start at different zeros — 0°F is not the same cold as 0°C — so you must shift the scale as well as scale it:
Quick mental anchors: −40° is the same in both scales; 0°C = 32°F (freezing); 100°C = 212°F (boiling); normal body temperature is 37°C = 98.6°F.
Keep this guide bookmarked and unit conversions become a thirty-second task instead of a guessing game. When in doubt, convert with the exact factor, double-check whether the source is US or UK, and never convert temperature with a plain multiplier.