Metric vs. imperial units: a practical conversion guide
Most of the world measures in metric; the US mostly doesn't. If you cook from foreign recipes, travel, follow science, or buy things internationally, you'll bump into both. Here's a practical guide to moving between them.
Why two systems exist
The metric system (meters, grams, liters, Celsius) is decimal — everything scales by tens, which makes the math easy. The imperial/US customary system (feet, pounds, gallons, Fahrenheit) grew from older traditional units and sticks around mostly in the United States. Metric dominates science and most countries; imperial persists in US daily life.
The conversions worth knowing
- Length: 1 inch = 2.54 cm · 1 foot ≈ 30 cm · 1 mile ≈ 1.61 km.
- Weight: 1 pound ≈ 0.45 kg · 1 kg ≈ 2.2 pounds.
- Volume: 1 US gallon ≈ 3.79 liters · 1 liter ≈ 0.26 gallon.
- Temperature: °C = (°F − 32) × 5/9 · °F = °C × 9/5 + 32.
Quick mental tricks
- km → miles: multiply by 0.6 (100 km ≈ 60 miles).
- °C → °F fast: double it and add 30 (20 °C ≈ 68 °F; close enough for weather).
- kg → pounds: double it and add ~10% (10 kg ≈ 22 lb).
- meters → feet: multiply by 3.3.
The gotcha: US vs. UK gallons
A UK (imperial) gallon is about 20% larger than a US gallon, and pints differ too. When a recipe or fuel figure comes from Britain vs. America, don't assume they mean the same "gallon." For anything precise, converting exactly beats a rule of thumb.
Mental tricks are great for a rough sense, but for measurements that matter — engineering, dosing, precise recipes — use a converter that handles the exact factors across length, weight, volume, temperature, and more.