Metric vs Imperial: A No-Nonsense Guide to Unit Conversion

The world officially runs on the metric system — only the United States, Liberia and Myanmar still default to imperial units. Yet in practice everyone bumps into both: American recipes measure in cups, British roads in miles, aircraft altitude in feet, and scientific data in metric everywhere. Being able to convert quickly, at least approximately, is a small superpower.

Length: the 5-to-8 rule

The magic ratio for miles and kilometers is roughly 5 miles ≈ 8 km. So to convert miles to kilometers, multiply by 1.6; kilometers to miles, multiply by 0.6.

• 1 inch = 2.54 cm (exactly — it's defined that way)
• 1 foot = 30.48 cm, so "about 30 cm" is fine for daily life
• 1 meter ≈ 3.28 feet — a bit more than a yard
• 1 mile = 1.609 km

Fun fact for runners: consecutive Fibonacci numbers approximate the miles-to-km ratio surprisingly well — 5 miles ≈ 8 km, 8 miles ≈ 13 km, 13 miles ≈ 21 km.

Weight: 2.2 is your friend

One kilogram is 2.205 pounds. For everyday purposes, double the kilograms and add 10%: 70 kg → 140 + 14 = 154 lb (exact answer: 154.3). Going the other way, halve the pounds and knock a little off.

• 1 ounce = 28.35 g ("about 30 grams")
• 1 pound = 453.6 g ("about half a kilo")
• 1 stone (UK) = 14 lb = 6.35 kg

Temperature: the only formula with an offset

Temperature is the one conversion you can't do with a single multiplication, because the scales start at different zeros:

°F = °C × 9/5 + 32
°C = (°F − 32) × 5/9

Quick approximation: double the Celsius and add 30. For 20°C that gives 70°F (exact: 68). It's accurate to within a couple of degrees across the range of normal weather.

Anchor points worth memorizing: 0°C = 32°F (freezing), 37°C = 98.6°F (body temperature), 100°C = 212°F (boiling).

Volume: where cooking gets messy

Volume is the most chaotic category because the US and UK define some units differently. A US gallon is 3.785 liters, but an imperial (UK) gallon is 4.546 liters — a 20% difference. For cooking:

• 1 US cup = 240 ml
• 1 tablespoon = 15 ml, 1 teaspoon = 5 ml
• 1 US fluid ounce = 29.6 ml ("about 30")

When a recipe's origin is unclear, assume US measures — the internet's recipes overwhelmingly are.

When precision matters, don't do mental math

Approximations are great for intuition, terrible for engineering, medicine or baking bread. For exact figures across length, weight, temperature, volume, area, speed and data storage, use our free unit converter — it converts as you type, with no rounding surprises.