Metric vs Imperial: What Actually Differs

Almost every metric–imperial conversion is exact by definition, not an approximation. Since the international yard and pound agreement of 1959, the imperial units have been defined in terms of metric ones.

That means every factor below is a definition, and results are exact to as many decimal places as you want.

Length

Unit	Exactly
1 inch	25.4 mm
1 foot	0.3048 m
1 yard	0.9144 m
1 mile	1,609.344 m
1 nautical mile	1,852 m

Converters: cm to inches · inches to cm · mm to inches · meters to feet · feet to meters · km to miles · miles to km · cm to feet

Weight

Unit	Exactly
1 pound	0.45359237 kg
1 ounce	28.349523125 g
1 stone	6.35029318 kg (14 lb)
1 metric tonne	1,000 kg (2,204.62 lb)

Converters: kg to lbs · lbs to kg · kg to stone · stone to lbs · tonnes to lbs

The three people get wrong

1. Area factors are squared. A metre is 3.28 feet, so a square metre is 10.76 square feet — not 3.28. Dividing a floor area by 3.28 instead of 10.76 gives an answer three times too large, and it looks plausible enough to act on.

See square feet to square metres and square metres to square feet.

2. Decimal feet are not feet and inches. "5.9 feet" is not 5 feet 9 inches. The decimal is tenths of a foot, and a foot has twelve inches: 5.9 ft is 5 ft 10.8 in. This costs an inch or two every time, and it is the single most common error in height conversion.

3. Temperature needs two steps, in order. °C = (°F − 32) × 5/9. Subtract the 32 first. Multiplying first gives an answer wrong by about 18 degrees — enough to turn a fever into a normal temperature.

Note that a temperature difference is different again: for a change rather than a reading, multiply by 5/9 and skip the 32 entirely.

Temperature anchors

°C	°F
−40	−40	the only point they agree
0	32	water freezes
20	68	room temperature
37	98.6	body temperature
100	212	water boils

Volume, where the systems genuinely disagree

Length and weight are shared. Volume is not:

	US	Imperial
Gallon	3.785 L	4.546 L
Pint	473 ml	568 ml
Fluid ounce	29.6 ml	28.4 ml

A US gallon is 20% smaller than an imperial one. The US kept an old English wine gallon of 231 cubic inches; Britain redefined its own in 1824 as the volume of ten pounds of water. They have been apart ever since.

Converters: liters to gallons · gallons to liters · ml to oz · liters to ml

Mental shortcuts worth knowing

• km to miles: multiply by 0.6. Or use consecutive Fibonacci numbers — 5 km ≈ 3 mi, 8 ≈ 5, 13 ≈ 8, 21 ≈ 13. The coincidence is real: the golden ratio is 1.618 and the factor is 1.609.
• kg to lbs: double it and add 10%. 70 kg → 140 → 154, against a true 154.3.
• °C to °F: double and add 30. 20°C → 70°F against a true 68. Fine for weather, not for cooking or medicine.
• cm to inches: divide by 2.5 and subtract 2%.

Where imperial still wins

Not on the arithmetic, but on the domain:

• Aviation uses feet for altitude worldwide, including in fully metric countries, because international air navigation standardised on it and changing would be more dangerous than the inconsistency.
• Marine navigation uses nautical miles because one equals one minute of latitude — the only distance unit that maps directly onto a chart.
• Screen and tyre sizes are inches everywhere, including in metric countries.

The US, Liberia and Myanmar are the only countries that have not formally adopted metric — though in practice the US uses metric throughout science, medicine and the military.