Guide
How to Convert Between Metric and Imperial Units
Whether you're traveling internationally, following a recipe from another country, or working on a global project, unit conversion comes up constantly. This guide covers the most common conversions and when to use our free converter tools.
Last updated: April 10, 2026
Why Two Systems Exist
The metric system (meters, kilograms, liters) is used by most of the world. The imperial system (feet, pounds, gallons) is primarily used in the United States, and partially in the UK and a few other countries.
Neither system is inherently better — they just use different base units. The metric system is based on powers of 10, making mental math easier, while the imperial system uses historical units that can feel more intuitive for everyday measurements in countries that use them.
Most Common Conversions You Need
- Length: 1 inch = 2.54 cm | 1 foot = 30.48 cm | 1 mile = 1.609 km | 1 meter = 3.281 feet
- Weight: 1 pound = 453.6 grams | 1 kg = 2.205 pounds | 1 ounce = 28.35 grams | 1 stone = 6.35 kg
- Temperature: °F to °C: subtract 32, multiply by 5/9 | °C to °F: multiply by 9/5, add 32 | Key points: 0°C = 32°F, 100°C = 212°F
- Volume: 1 US gallon = 3.785 liters | 1 liter = 0.264 US gallons | 1 cup = 236.6 mL | 1 tablespoon = 14.8 mL
- Speed: 1 mph = 1.609 km/h | 1 km/h = 0.621 mph | 60 mph = 96.6 km/h | 100 km/h = 62.1 mph
Tips for Quick Mental Conversions
- Kilometers to miles: multiply by 0.6 (100 km ≈ 60 miles).
- Kilograms to pounds: double and add 10% (70 kg ≈ 154 lbs).
- Celsius to Fahrenheit: double and add 30 for a rough estimate (20°C ≈ 70°F).
- Liters to US gallons: divide by 4 (8 liters ≈ 2 gallons).
- Centimeters to inches: divide by 2.5 (10 cm ≈ 4 inches).
When Precision Matters
Mental approximations are fine for everyday use — estimating distances, cooking, or checking weather. But engineering, science, medicine, and international trade require exact conversions.
Use our converter tools for precise results. They use standard conversion factors and show results to multiple decimal places. All calculations happen instantly in your browser.
Our Converter Tools
- Length Converter — meters, feet, inches, km, miles, and more.
- Weight Converter — kg, pounds, ounces, grams, stones.
- Temperature Converter — Celsius, Fahrenheit, Kelvin.
- Volume Converter — liters, gallons (US & UK), cups, mL.
- Speed Converter — mph, km/h, m/s, knots, Mach.
- Area Converter — sq meters, acres, hectares, sq feet.
- Time Converter — seconds through years.
- Pressure Converter — PSI, bar, atm, Pascal.
- Energy Converter — joules, calories, kWh, BTU.
- Fuel Economy Converter — MPG, L/100km, km/L.
Frequently Asked Questions
- Q: Why does the UK use a mix of both systems? — The UK officially uses metric but culturally retains imperial for some uses (miles for road distances, stones for body weight, pints for beer).
- Q: Is there a difference between US and UK gallons? — Yes. A US gallon is 3.785 liters, while a UK (imperial) gallon is 4.546 liters — about 20% larger.
- Q: What about cooking measurements? — US recipes use cups and tablespoons, while most other countries use grams and milliliters. Our Volume Converter handles both systems.
Keep Reading
More converters guides and comparisons
How to Convert Celsius to Fahrenheit and Back
Convert temperatures correctly for weather, cooking, schoolwork, and travel with simple formulas and a faster browser tool workflow.
How to Convert Time Zones for Meetings and Travel
Plan meetings and trips across time zones without mistakes by checking offsets, daylight saving changes, and local date rollover.
How to Convert Cups to Grams for Cooking and Baking
Convert cups, tablespoons, ounces, grams, and milliliters more reliably when following recipes from different countries.
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