Unit conversion guide — metric vs imperial and the mistakes everyone makes

📅 May 2026⏱ 5 min read🏷 Reference

Unit conversion sounds like a school maths problem — but it comes up constantly in real life. You're following a recipe from a US cookbook in grams. You're checking a car's fuel efficiency in km per litre when the listing says miles per gallon. You're converting Fahrenheit to Celsius because a weather app changed. Getting these wrong causes genuine problems. This guide gives you the most useful conversions and the shortcuts that actually stick.

Metric vs imperial — the basics

Most of the world uses the metric system (SI units): kilometres, kilograms, litres, Celsius. The United States uses the imperial system for most everyday measurements: miles, pounds, fluid ounces, Fahrenheit. The UK uses a mixture — metric for most things, but miles for road distances and pints for beer.

Metric units scale in powers of 10, which makes maths easy: 1 kilometre = 1,000 metres = 100,000 centimetres. Imperial units have irregular relationships (12 inches in a foot, 3 feet in a yard, 1,760 yards in a mile), which is why conversion errors are common.

Length conversions

From	To	Multiply by
Inches	Centimetres	2.54
Centimetres	Inches	0.394
Feet	Metres	0.305
Metres	Feet	3.281
Miles	Kilometres	1.609
Kilometres	Miles	0.621

Quick trick for km to miles: multiply by 6, then divide by 10. So 50 km × 6 = 300 ÷ 10 = 30 miles. Close enough for most purposes (the exact answer is 31.1).

Weight conversions

From	To	Multiply by
Ounces (oz)	Grams	28.35
Grams	Ounces	0.035
Pounds (lb)	Kilograms	0.454
Kilograms	Pounds	2.205
Stone	Kilograms	6.35

Quick trick for kg to lbs: double it and add 10%. So 70 kg × 2 = 140, plus 14 = 154 lbs (exact: 154.3).

Temperature conversions

This one catches people most often. The formulas:

Celsius to Fahrenheit: (°C × 9/5) + 32
Fahrenheit to Celsius: (°F − 32) × 5/9

Mental shortcuts: 0°C = 32°F (freezing), 100°C = 212°F (boiling), 37°C = 98.6°F (body temp). For everyday outdoor temps, a rough guide: 20°C ≈ 68°F (warm), 30°C ≈ 86°F (hot), -10°C ≈ 14°F (very cold).

Volume conversions

From	To	Multiply by
US fluid ounces	Millilitres	29.57
US cups	Millilitres	236.6
US gallons	Litres	3.785
UK pints	Millilitres	568
Litres	US gallons	0.264

Note: US and UK fluid measurements differ. A US pint is 473ml; a UK (imperial) pint is 568ml. If a recipe says "1 pint" without specifying, context (source country) matters.

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The most common conversion mistakes

Confusing US and UK measurements — cups, pints and fluid ounces differ. Always check which system a recipe uses.
Forgetting to square or cube for area and volume — 1 foot = 0.305 metres, but 1 square foot = 0.0929 square metres (0.305²). People often just multiply the linear factor.
Using "billion" without specifying country — a US billion is 1,000,000,000. A traditional UK billion used to mean 1,000,000,000,000. In modern usage both mean 10⁹, but older British texts may differ.
Degrees: °C vs °F vs Kelvin — scientific contexts use Kelvin (K = °C + 273.15). Never add a degree symbol to Kelvin.
Speed: km/h vs mph vs m/s — 100 km/h ≈ 62 mph ≈ 27.8 m/s. All three appear in different contexts.

Useful reference anchors to memorise

1 inch = 2.54 cm exactly (defined, not approximate)
1 mile = 1.609 km
1 kg = 2.205 lbs
1 litre of water = 1 kg
0°C = 32°F, 100°C = 212°F