Metric vs Imperial Units: Exact Conversions You Can Rely On

By the SAA Tool editor, an IT support engineer · Last reviewed 1 October 2026 · 6 min read

Imperial and US units sound old-fashioned, but they're defined in metric terms, and exactly. An inch is 2.54 centimetres by definition, and a pound is 0.45359237 kilograms by definition. That means every conversion has one right answer, and any error comes from rounding. This guide gives the exact figures, the places where US and UK units differ, and worked examples you can check on a calculator.

Three systems, not two

• Metric units, such as the metre, the kilogram and the litre, scale by powers of ten: a kilometre is 1,000 metres and a millilitre is a thousandth of a litre. Most of the world uses it for trade and everyday life.
• US customary units are used day to day in the United States: inches, feet, miles, ounces, pounds, cups and US gallons.
• Imperial units are the British system, defined in UK law by the Weights and Measures Act 1985. Metric is the norm for trade in the UK, but you'll still see miles on road signs, pints in pubs and body weight in stones.

Other countries mix systems too. In Canada, for example, metric is official, but many people give their height in feet and inches. For length and weight, US and imperial units are identical: an inch, a foot, a mile, an ounce and a pound are the same in both. The trouble is volume, where the US and UK gallon, pint and fluid ounce are different sizes.

The exact definitions

The inch, pound and US gallon values come from the US National Institute of Standards and Technology (NIST Handbook 44), and the yard, pound, pint and gallon from the UK's 1985 Act. Every other row follows by exact multiplication. The UK gallon and pint are about 20% larger than their US namesakes, but a UK fluid ounce is slightly smaller than a US one, because the UK pint holds 20 fluid ounces and the US pint only 16.

Going the other way, the factors never end neatly, so here they are rounded to six significant figures: 1 cm = 0.393701 in, 1 m = 3.28084 ft, 1 km = 0.621371 mi, 1 kg = 2.20462 lb, and 1 L = 0.264172 US gallons or 0.219969 UK gallons.

Worked examples

• Height: 175 cm ÷ 2.54 = 68.90 in, which is 5 ft 8.9 in. Going the other way, 6 ft is 72 in × 2.54 = 182.88 cm exactly.
• Body weight: 70 kg ÷ 0.45359237 = 154.32 lb, or 11 st 0.32 lb. And 11 st 4 lb is 158 lb × 0.45359237 = 71.67 kg.
• Luggage: 23 kg is 50.71 lb, while a 50 lb limit is 22.68 kg, a little under 23 kg.
• Running: 5 km is 3.107 mi, and the 42.195 km marathon is 26.219 mi.
• Speed: 100 km/h is 62.14 mph.
• Drinks: a 2 L bottle holds 0.528 US gallons or 0.440 UK gallons, and 500 mL is 2.11 US cups.
• Fuel economy: 40 miles per gallon means different things in each country. With UK gallons it's 7.06 litres per 100 km; with US gallons it's 5.88. To convert, divide 282.481 by UK mpg, or 235.215 by US mpg.

Cooking is its own trap. A US cup is 236.6 mL, but some countries' recipes use a 250 mL metric cup, so check which cup a recipe means. For baking, weighing in grams avoids the question altogether.

Temperature: not a simple multiplication

Celsius and Fahrenheit have different zero points as well as different step sizes, so you multiply and add. These are the formulas NIST gives:

• °F = °C × 1.8 + 32
• °C = (°F − 32) ÷ 1.8
• K = °C + 273.15

Two things to watch. First, a temperature difference converts without the 32: a rise of 10 °C is a rise of 18 °F. Second, the popular shortcut of doubling and adding 30 is fine for weather (20 °C gives 70 °F against an exact 68 °F) but drifts at the ends (35 °C gives 100 °F against an exact 95 °F). Don't use it for cooking or medicine.

Rounding without surprises

Rounded factors are fine for a rough idea, but the errors add up. Using 2.2 lb per kg is about 0.21% low, so 100 kg comes out as 220 lb instead of 220.46 lb. Using 1.6 km per mile is 0.58% low. Using 2.5 cm per inch is 1.57% low, turning 30 cm into 12 in when the true figure is 11.81 in. Keep the exact factor through every step and round only the final answer, to a precision that suits the job.

Also watch for units that share a name:

• Tonne, ton and long ton. A tonne is 1,000 kg. A US (short) ton is 2,000 lb, or 907.18474 kg. A UK (long) ton is 2,240 lb, or 1,016.0469088 kg.
• Troy ounces. Gold and silver are priced per troy ounce, 31.1034768 g, which is heavier than the ordinary 28.35 g ounce.
• The US survey foot. Old US land records sometimes use a survey foot of 1200/3937 m, about 2 parts per million longer than the standard foot. NIST says it should be avoided after 31 December 2022 except for historic and legacy data.

Converting with our tools

1. Open the converter for what you're measuring: the Length Converter, Weight Converter, Volume Converter or Temperature Converter. For the two most common jobs there are quick pages too: CM to Inches and KG to LBS.
2. Type a number in Value and choose the From and To units. The answer updates as you type, with the one-unit factor underneath, such as 1 kg = 2.204622622 lb.
3. Click ⇄ Swap to convert the other way, or Copy result to copy the answer.
4. Scroll to In every unit to see the same value in every other unit of that kind at once.
5. For lengths, the tools also show feet and inches (175 cm shows as 5 ft 8.9 in). CM to Inches and KG to LBS add a table of common values, and the Temperature Converter shows the formula and warns you if a value is below absolute zero.

The converters run in your browser and round results to 10 significant figures. The Volume Converter lists US and UK gallons and fluid ounces separately, and its cups and spoons are US measures. The Weight Converter shows stones as a decimal (70 kg is 11.02 st) rather than stones and pounds, and also covers tonnes, US and UK tons, carats, tolas and troy ounces. If you're converting for a health measure, our BMI guide shows the metric formula.

Quick answers

• Is an inch exactly 2.54 cm? Yes, by definition, so 12 inches is exactly 30.48 cm.
• How many pounds are in a kilogram? 2.20462262, which is 1 ÷ 0.45359237.
• Are US and UK pints the same? No. A UK pint is 568 mL and a US pint 473 mL.
• How tall is 1 metre in feet? 3 ft 3.37 in, or 39.37 in.
• Is a ton the same as a tonne? No. A tonne is 1,000 kg, and the two tons are 907 kg (US) and 1,016 kg (UK).
• Why does a converter show so many decimals? Because the definitions are exact but the reverse factors aren't. Round the final answer to what you need.

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