Converting between metric and imperial units, in the kitchen and on the road

When converting between units, you multiply or divide by a fixed factor: 1 kilometre is 1,000 metres, 1 inch is exactly 2.54 centimetres and 1 mile is about 1.6 kilometres.

In the UK the two systems live side by side. Shops sell in grams and litres, older recipes ask for ounces and pints, and people still give their height in feet and their weight in stones. This guide covers the metric units first, then a single rule that works for any conversion, the imperial equivalents worth knowing, the kitchen, distances, height and weight, and what the law and the school curriculum say.

Metric units: moving the decimal point

The metric system is built on powers of 10, so converting inside it never needs a calculator. The prefixes tell you the factor: kilo means 1,000, centi means a hundredth and milli means a thousandth.

• Length: 1 km = 1,000 m, 1 m = 100 cm, 1 cm = 10 mm. So 3.5 km is 3,500 m, and 45 mm is 4.5 cm.
• Mass: 1 kg = 1,000 g. So 750 g is 0.75 kg.
• Capacity: 1 litre = 1,000 ml. So 1.5 litres is 1,500 ml, and a 330 ml can is 0.33 litres.

Multiplying by 1,000 moves the decimal point three places to the right; dividing by 1,000 moves it three places to the left. The one trap is area and volume. A square metre is 100 cm by 100 cm, so 1 m² is 10,000 cm², not 100. The same goes for prices: to compare two packs fairly, put them in the same unit first, as in our guide to working out the unit price.

A simple rule for converting any units

Every conversion comes down to one question: is the new unit bigger or smaller than the old one?

• Going to a smaller unit, the number gets bigger, so you multiply. 4 feet in inches: 4 × 12 = 48.
• Going to a bigger unit, the number gets smaller, so you divide. 48 inches in feet: 48 ÷ 12 = 4.

If you are unsure, write the units into the calculation and let them cancel. 5 miles × 1.609 km per mile = 8.05 km: the miles cancel and kilometres are left. If the units do not cancel, the calculation is upside down.

Then check that the answer makes sense. A kilometre is shorter than a mile, so a distance in kilometres is always the larger number; a kilogram is heavier than a pound, so a weight in kilograms is always the smaller number. The national curriculum's guidance for Year 6 puts it the same way: pupils know approximate conversions and are able to tell if an answer is sensible.

The metric and imperial equivalents worth knowing

The imperial units are defined in metric terms by law. Schedule 1 to the Weights and Measures Act 1985 sets the yard at 0.9144 metre and the pound at 0.45359237 kilogram, and defines the other units of length and weight from those two; the pint is defined directly as 0.56826125 litre. That is why some conversions are exact and others are rounded.

For everyday use, rounded versions are enough: an inch is about 2.5 cm, a kilogram is about 2.2 pounds, a pint is a little over half a litre. In the shops, the law sets the order of the two systems. GOV.UK's guide to weights and measures law says goods in England, Scotland and Wales must be sold in metric units. The only exceptions are draught beer or cider and milk in returnable containers, sold by the pint, and precious metals, sold by the troy ounce. An imperial measurement can be shown next to the metric one, but it cannot stand out more.

Cooking conversions for ounces, pounds and pints

Older British recipes are often written in ounces and pints, while kitchen scales and measuring jugs show grams and millilitres. Three numbers cover most of it:

• 1 ounce ≈ 28 g. So 4 oz of sugar is about 113 g and 8 oz of flour about 227 g.
• 1 pound ≈ 454 g. A pound of butter is just under half a kilo.
• 1 pint ≈ 568 ml. Half a pint of milk is about 284 ml. A fluid ounce is 1/20 of a pint, about 28 ml.

You can round an ounce to 25 g or 30 g to keep the numbers simple. That works as long as you round every ingredient the same way, so that the proportions between them stay the same. Mixing rounded and exact conversions in the same recipe is what goes wrong.

The simplest option is to avoid converting at all: if your scales can switch between grams and ounces, weigh in the unit the recipe uses.

Km to miles and back, for travel and running

A mile is 1.609344 km, so to turn miles into kilometres you multiply by 1.609, and to turn kilometres into miles you divide by 1.609, or multiply by about 0.62. In your head, the ratio 8 to 5 is close enough: 8 km is about 5 miles, 40 km about 25 miles.

Speeds work the same way, because the hours do not change. 30 mph is about 48 km/h, 50 mph about 80 km/h and 70 mph about 113 km/h. Going the other way, 100 km/h is about 62 mph. Multiplying by 1.6 is quick if you break it up: 1.6 is 1 plus 0.6, so 45 × 1.6 is 45 + 27 = 72. Our mental maths shortcuts work through more splits like this one.

Height and weight in feet, inches and stones

Height and body weight are often still given in imperial units. Both need two steps, because they mix two units.

• Height: turn feet and inches into inches, then multiply by 2.54. 5 feet 10 inches is 5 × 12 + 10 = 70 inches, and 70 × 2.54 = 177.8 cm.
• Weight: turn stones and pounds into pounds, then multiply by 0.4536. 11 stone is 11 × 14 = 154 pounds, about 69.9 kg; 12 stone 6 pounds is 174 pounds, about 78.9 kg.

Going back from metric, divide first and keep the remainder. 180 cm ÷ 2.54 is about 70.9 inches, which is 5 feet and about 11 inches. 80 kg is about 176 pounds, and 176 ÷ 14 is 12 remainder 8, so 12 stone 8 pounds.

What pupils learn about units in Years 5 and 6

In England, the national curriculum for mathematics spreads conversions over the last two years of primary school. In Year 5, pupils convert between metric units, such as kilometres and metres or grams and kilograms, and learn approximate equivalences between metric units and common imperial units such as inches, pounds and pints.

In Year 6, they convert measurements of length, mass, volume and time between smaller and larger units, using decimals with up to three decimal places, and they convert between miles and kilometres. The non-statutory guidance adds that they connect the conversion from kilometres to miles to a graph, as preparation for proportional graphs. For the tests at the end of Year 6, see our guide to the Year 6 maths SATs.

Frequently asked questions

How do you convert kilometres to miles?

Divide by 1.609, or multiply by about 0.62: 10 km is about 6.2 miles. In your head, 8 km is roughly 5 miles.

How many grams are in an ounce?

About 28.35 g. An ounce is 1/16 of a pound, and a pound is 453.59237 g.

How many millilitres are in a pint?

A UK pint is 0.56826125 litre, about 568 ml. Half a pint is about 284 ml.

How do you convert stones to kilograms?

Multiply the stones by 14 to get pounds, then by 0.4536: 11 stone is 154 pounds, about 69.9 kg.

When do children learn to convert units in England?

In Year 5 they convert metric units and learn approximate imperial equivalents; in Year 6 they convert between miles and kilometres.