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.

Why two systems still exist

Metric won almost everywhere. The United States is the only large economy still using imperial units for everyday life, with the UK in a permanent halfway state — road signs in miles, petrol in litres, body weight in stones, milk in pints, and school science entirely metric.

The reason is inertia rather than argument. Changing units means changing every road sign, machine tool, textbook, recipe and spare part in a country. Britain began metrication in 1965 and has never finished it; the US passed the Metric Conversion Act in 1975 but made it voluntary, which meant it largely did not happen.

Metric's practical advantage is that it is decimal throughout. Converting metres to kilometres is a decimal shift, while converting feet to miles requires remembering 5,280. Imperial units evolved separately from body parts, farming practice and trade, which is why the relationships between them are so irregular.

When getting it wrong is expensive

Unit confusion is not a trivial problem. NASA's Mars Climate Orbiter was lost in 1999 when navigation software supplied thruster impulse figures in pound-force seconds while the receiving system expected newton-seconds; the spacecraft approached Mars at the wrong altitude and was destroyed. A 1983 Air Canada flight ran out of fuel mid-flight after a metric-imperial mix-up during refuelling and had to glide to a landing.

Those are famous because they were dramatic. The everyday versions are quieter: parts ordered in the wrong size, medication dosed in the wrong unit, timber cut short. The lesson is consistent — write the unit next to the number, every time, and never assume a bare number carries its unit by context.

Full reference table

Every figure in that table is exact by international agreement rather than measured, which is the point worth taking away: since 1959 the imperial units have been defined in terms of metric ones. An inch is not approximately 25.4 mm; it is 25.4 mm, by definition. Both systems have been metric underneath for over sixty years.

The NIST unit conversion reference and the BIPM definition of the SI are the authoritative sources for these values.

The gallon trap

The single most common expensive mistake in this area. A US gallon is 3.785 litres and a UK gallon is 4.546 — a 20% difference. US and UK pints and fluid ounces differ too, in different directions.

This matters for recipes, fuel economy figures and anything bought by volume. A car rated at "40 miles per gallon" is meaningfully more efficient in the UK than the same number in the US, because the gallon is bigger.

For the specific factors: the length and weight conversion guides list them exactly, and the complete unit conversion guide covers every category in one place.

Frequently asked questions

Why does the US still use imperial units?

Mainly inertia. The Metric Conversion Act of 1975 made metrication voluntary rather than mandatory, so it never happened at scale. Changing units means replacing signage, tooling, textbooks and parts inventories across an entire economy.

Are imperial units defined using metric ones?

Yes. Since the international yard and pound agreement of 1959, an inch has been exactly 25.4 mm and a pound exactly 0.45359237 kg. Imperial units are formally defined in terms of metric units, which is why the conversion factors are exact rather than measured.

Is a US gallon the same as a UK gallon?

No, and the difference is large. A US gallon is 3.785 litres and a UK gallon is 4.546 — about 20% bigger. US and UK pints and fluid ounces also differ, so recipes and fuel figures do not transfer between them.

What is the easiest way to convert metric to imperial in my head?

Learn three anchors: a metre is about 3.3 feet, a kilogram is about 2.2 pounds, and a litre is about 1.75 UK pints. For temperature, double the Celsius figure and add 30 for a rough Fahrenheit estimate.

Does the UK use metric or imperial?

Both. Road distances are in miles, draught beer in pints and body weight often in stones, while fuel is sold in litres, food is labelled in grams and all science and engineering is metric.