Metric vs U.S. Customary Measurement
Most international work uses the International System of Units (SI). Everyday measurements in the United States often use U.S. customary units. British Imperial units are historically related, but they are not identical to the U.S. system.
The Metric System
The metric system (International System of Units, SI) is used by most countries worldwide. It's based on powers of 10, making conversions straightforward.
- Length: meter (m), centimeter (cm), millimeter (mm), kilometer (km)
- Weight: gram (g), kilogram (kg)
- Volume: liter (L), milliliter (mL)
- Temperature: Celsius (°C)
The U.S. Customary System
U.S. customary measurement uses units such as inches, feet, yards, miles, ounces, pounds, and U.S. gallons. NIST advises against calling these units "British" or "Imperial" because the systems differ—especially for volume and some mass units.
- Length: inch (in), foot (ft), yard (yd), mile (mi)
- Mass/weight in everyday use: ounce (oz), pound (lb)
- Volume: U.S. fluid ounce, cup, pint, quart, and gallon
- Temperature: Fahrenheit (°F)
Key Conversions
- 1 inch = 2.54 centimeters
- 1 foot = 30.48 centimeters
- 1 mile = 1.609 kilometers
- 1 pound = 0.4536 kilograms
- 1 U.S. gallon ≈ 3.785 liters
- °F = (°C × 9/5) + 32
Which System Should You Use?
Use SI for scientific and international work unless a required standard specifies otherwise. For everyday U.S. measurements, use U.S. customary units. This online ruler supports length in centimetres, millimetres, and inches after screen calibration.