Metric vs Imperial Units: A Practical Guide

Two systems, one planet

The metric system (SI) is decimal: a kilometre is 1000 metres, a kilogram is 1000 grams and a litre is 1000 millilitres. Nearly every country uses it, and science and medicine work in metric units everywhere.

Imperial and US customary units grew out of old English measures: the foot, inch, pound and gallon. Their ratios are not decimal: a foot has 12 inches, a yard 3 feet, a mile 1760 yards and a pound 16 ounces. That is why mental arithmetic with them is harder.

Where imperial and US units are still used

In everyday life, miles, pounds, feet and degrees Fahrenheit are standard in the United States. The United Kingdom is a mixed case: road signs and speed limits use miles and mph, beer and milk are often sold in pints, and people often give their body weight in stones, but most shop goods are also labelled in metric.

In aviation, altitude is normally given in feet and speed in knots worldwide. Screen sizes of TVs and monitors and tyre diameters are also commonly quoted in inches, even in metric countries.

A short history

The metric system was created in France after the Revolution, in the late 18th century, to provide uniform and rational measures. The British Empire kept its own measures for a long time, and the United States inherited the English system and stayed with it.

Interestingly, US units have been officially defined through metric ones since the late 19th century, and in 1959 several countries agreed on the international yard and pound: 1 pound is exactly 0.45359237 kg and 1 inch is exactly 2.54 cm. So conversions are exact, only inconvenient.

The most common conversions

This table collects the factors you will need most often. Some numbers are rounded, which is enough for everyday use.

A gallon is not always a gallon

The US gallon (about 3.785 litres) and the imperial or British gallon (about 4.546 litres) differ by roughly 20 percent. The same goes for pints: a US pint is about 473 ml, a British pint about 568 ml. So fuel economy quoted as 30 miles per gallon can mean very different things: with a US gallon it is about 7.8 L/100 km, with an imperial gallon about 9.4 L/100 km.

If you are unsure which unit is meant, check the country of origin first. For the US it is almost always the US gallon.

Mental-math shortcuts

To turn kilometres into miles, multiply by 0.6, or multiply by 5 and divide by 8. For 100 km: 100 × 5 ÷ 8 = 62.5, while the exact value is 62.14 miles. Going the other way, multiply miles by 1.6: 50 miles is about 80 km (exactly 80.47).

For kilograms to pounds, double the number and add 10 percent: 70 kg gives 140 + 14 = 154 lb (exactly 154.3). One inch is about 2.5 cm, so a 55-inch TV has a diagonal of about 140 cm (exactly 139.7). One foot is about 30 cm, so a height of 6 feet is about 183 cm (exactly 182.88).

• km to miles: x0.6 (or x5 / 8)
• miles to km: x1.6
• kg to pounds: x2.2
• cm to inches: / 2.5
• metres to feet: x3.3

Common mistakes when shopping, travelling and cooking

When shopping online, a frequent mistake is mixing up fluid ounces (a volume) with ounces (a weight): 8 fl oz is about 237 ml, while 8 oz by weight is about 227 g. Another is assuming clothing and shoe sizes convert mathematically: manufacturers use their own tables, so always check the maker's size chart.

When travelling in the US, remember that 65 mph is about 105 km/h and that fuel is priced per gallon: 4 USD per gallon is about 1.06 USD per litre. When cooking from American recipes, cups and spoons are not perfectly precise, so look for a weight in grams or use kitchen scales where you can.

Worked examples

A height of 5 feet 9 inches: 5 × 12 + 9 = 69 inches, and 69 × 2.54 = 175.26 cm. A weight of 180 pounds: 180 × 0.45359237 ≈ 81.65 kg. A trip of 250 miles: 250 × 1.609344 ≈ 402.3 km. An American recipe asking for 16 ounces of flour means about 453.6 g.

For runners: 10 km is about 6.21 miles, a half marathon (21.0975 km) is about 13.1 miles and a marathon (42.195 km) about 26.2 miles. Fuel economy: 35 miles per US gallon is about 6.7 L/100 km (235.215 / 35), while 35 miles per imperial gallon is about 8.1 L/100 km (282.481 / 35). The same number gives noticeably different results.

Why confusion can be costly

Unit mix-ups are not only a household problem. In 1999 NASA's Mars Climate Orbiter was lost because one team used pound-force seconds and another used newton seconds. Your mistakes will probably be smaller, but the principle is the same: always check which unit a number is given in, and write the unit next to every result.

A short checklist

Before buying a foreign product or following a foreign recipe, run through these points.

• Is the measurement in inches or centimetres? Check the product photos and size tables.
• Are the ounces a weight (oz) or a volume (fl oz)?
• Is the gallon a US or an imperial one?
• Are height and weight given in feet and pounds, or metric units?
• Did you round only in the final step, not after each intermediate one?