Published 2026-07-22
Compare metric and US customary units for length, weight, volume, and temperature. Learn conversion factors and when approximate mental math is enough.
Two measurement systems
Most countries use the metric system (meters, kilograms, liters, Celsius). The United States uses US customary units for everyday life (miles, pounds, Fahrenheit, fluid ounces) while science and medicine often use metric.
Travel, ecommerce, and international recipes constantly mix systems. A clear conversion avoids expensive ordering mistakes and recipe disasters.
Length and distance
1 inch = 2.54 centimeters exactly. 1 mile ≈ 1.609 kilometers. A 5K race is 5 kilometers, not 5 miles - roughly 3.1 miles. Road signs abroad in km/h confuse drivers used to mph.
Use Unit Converter for exact values; for mental math, remember 1 meter ≈ 3.28 feet and 1 foot ≈ 30 cm.
Weight and mass
Pounds (lb) vs kilograms (kg): 1 kg ≈ 2.205 lb. Shipping labels and gym plates often show both. Ounces (oz) are 1/16 of a pound; 16 oz = 1 lb. Metric grams and kilograms scale by powers of ten, which simplifies science.
Recipes may list ounces or grams; baking benefits from precise grams on a kitchen scale.
Volume and cooking
US cups are not identical to metric cups in all countries. 1 US liquid gallon ≈ 3.785 liters. Milliliters (ml) align cleanly with liters (1000 ml = 1 L). Tablespoons and teaspoons differ slightly between US and UK measures - check recipe origin.
When precision matters, convert to ml or grams rather than chaining multiple imperial units.
Temperature is not a ratio
Celsius and Fahrenheit do not share a zero point. You must use a formula, not multiply by a single factor. See our Celsius to Fahrenheit guide for the exact equation and common reference points like 0°C = 32°F and 100°C = 212°F.
Unit Converter handles temperature with proper offsets, not approximate scaling.
Everyday habits
When shopping internationally, convert dimensions and weights before checkout - return shipping for wrong-size items costs more than a minute with a converter.
Teach kids anchor points (1 meter ≈ a yardstick, 1 kg ≈ a liter of water) so estimates feel natural abroad.
Label your kitchen scale toggle clearly; switching oz/g mid-recipe without noticing ruins baked goods.
Sports events abroad often post distances in kilometers while your watch defaults to miles - convert before pacing a race.
Summary
Metric scales by tens; imperial units mix irregular ratios - exact tools beat memorizing every factor.
Pick one system per task, convert once, and write down the result to avoid double-conversion errors.
Keep Unit Converter bookmarked when reading international news, sports, or weather reports.
Engineering drawings and hardware specs may list both systems side by side - always note which column you copied.
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