Pounds and Kilograms: Converting Weight

The exact relationship

One avoirdupois pound is exactly 0.45359237 kilograms. Like the inch and the mile, this is a definition rather than a measurement: it was fixed by the 1959 international agreement between the United States, the United Kingdom and four other countries, which unified the national definitions of the yard and the pound. One kilogram is therefore 1 ÷ 0.45359237 pounds, which is about 2.2046226.

Everyday weighing rarely needs that many digits. The values 0.4536 kg per pound and 2.2046 lb per kilogram are accurate to better than one part in ten thousand, and 2.2 is a reasonable figure for quick estimates.

Converting in both directions

Multiply pounds by 0.45359237. For 150 lb, 150 × 0.45359237 = 68.0388555 kg, which you would normally report as 68.04 kg or simply 68 kg. Treat the trailing digits with care: a bathroom scale that reads to the nearest pound does not justify seven decimal places in kilograms. For 10 lb the answer is 4.5359237 kg, and for 1 lb it is 0.45359237 kg.

Because a kilogram is the bigger unit, the number of kilograms is always smaller than the number of pounds. A result of 68 kg for 150 lb passes that test: it is a little under half. If your answer came out larger than the starting number, you divided when you should have multiplied.

For kilograms to pounds, multiply kilograms by 2.2046226, or divide by 0.45359237. A 70 kg person is 70 × 2.2046226 = 154.32 lb. A 20 kg bag is 44.09 lb. A tonne, which is 1,000 kg, is about 2,204.62 lb. This conversion is the one travelers meet most often, for example when a luggage allowance is quoted in kilograms and a scale shows pounds, or the reverse. Check the exact limit with the airline or carrier, because allowances differ and are stated in their own terms.

Pounds and ounces

A pound is made of 16 ounces. One ounce is therefore 0.45359237 ÷ 16 = 0.028349523125 kg, or 28.349523125 grams. Half a pound, 8 ounces, is therefore 226.796 grams, so 226.8 g or 227 g are sensible roundings for a kitchen scale. The ounce in this sense is the avoirdupois ounce; it is not the same as a fluid ounce, which measures volume, and not the same as the troy ounce used for precious metals.

Quantities written as pounds and ounces must be combined before converting. A baby weighing 7 lb 8 oz weighs 7.5 lb, because 8 ÷ 16 = 0.5. Then 7.5 × 0.45359237 = 3.4019 kg, or 3.40 kg to two decimals. In reverse, 3.4 kg is about 7.50 lb, which is 7 lb and 7.9 oz (0.4957 × 16 = 7.93).

Mental shortcuts

To turn kilograms into pounds, double the number and add 10 percent of the result. For 70 kg, double is 140, a tenth of that is 14, and the estimate is 154 lb, against an exact 154.32 lb. To turn pounds into kilograms, halve the number and subtract a tenth of the half: 150 lb halves to 75, minus 7.5 gives 67.5 kg, against an exact 68.04 kg.

Halving and subtracting a tenth is the same as multiplying by 0.45, which runs slightly low. A closer estimate is to multiply by 0.45 and then add 1 percent. For 150 lb that gives 67.5 plus 0.675, about 68.2 kg, which is closer to the exact 68.04 kg.

Mass, weight and other pounds

In everyday language people say weight when they mean mass, and the pound and kilogram are both units of mass in this conversion. A pound-force, a unit of force, is a different quantity again, and it cannot be converted into kilograms with this factor. If a specification mentions lbf rather than lb, it needs a different calculation.

Other pounds exist too. The troy pound, used for precious metals, contains 12 troy ounces instead of 16 ounces, and is not the same weight as the avoirdupois pound. Unless the context is bullion, assume the avoirdupois pound, which is also the pound used for body weight, groceries and parcels. Heavier loads may be quoted in US tons, which equal 2,000 pounds, or in metric tons, which equal 1,000 kilograms.

Using a converter, and when to check the source

A unit converter such as this one has pound, ounce, kilogram, gram and several other units in its Mass category. You can enter a value with a dot or a comma, swap the two units, and see the same mass in every unit at once. Use the decimal-places setting to match the precision you need.

As a worked check, enter 154.32 lb and the result should be close to 70 kg, and entering 70 kg should bring you back to about 154.32 lb. Round trips like this reveal swapped units quickly, and they show how much precision is lost when you round in the middle of a chain of calculations.

For medicine, dosing, laboratory work, shipping documents or legal trade, do not rely on a converted number alone. Check the original source and the unit it uses.