The 2.2 shortcut is wrong, and sometimes it matters

How far off is it?

The pound is defined as exactly 0.45359237 kilograms, making one kilogram 2.2046226 pounds. Rounding to 2.2 understates the result by about 0.21 per cent.

Where it does not matter

Everyday conversation, rough cooking quantities, telling a friend what you lifted. A fifth of a per cent is well inside the noise of a bathroom scale.

Where it does

• Baggage allowances. A 23 kg limit is 50.71 lb. Pack to exactly 50.6 lb believing you are at the limit and you are 0.1 lb over the real figure — and airport scales round up.
• Weight-class sport. Boxing and weightlifting classes are defined precisely. Two hundredths of a kilogram has ended weigh-ins.
• Medication dosing. Drugs dosed per kilogram of body weight compound the error across the calculation, which is why clinical software never uses the shortcut.

A better mental shortcut

Double the kilograms and add ten per cent of the doubled figure, then subtract a whisker. For 70 kg: double is 140, ten per cent is 14, total 154 — against the exact 154.32. Close enough to say out loud, and it fails in the safe direction.

Where the definition comes from

Until 1959 the pound meant slightly different things in different countries. The international yard and pound agreement, signed by the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa, fixed the pound at exactly 0.45359237 kilograms and the yard at exactly 0.9144 metres.

That defined the pound in terms of the kilogram rather than the other way round, which is why the conversion is exact in one direction and a repeating decimal in the other. One pound is a clean 0.45359237 kg; one kilogram is 2.204622621848... pounds.

Stone, and why it survives

British and Irish speakers rarely give body weight in pounds alone. Fourteen pounds make a stone, so 11 stone 4 pounds is 158 pounds or 71.67 kg. The unit persists because it produces convenient two-digit numbers across the human range — most adults land between 8 and 20 stone.

Converting from stone needs the intermediate step:

1. Multiply the stone by 14 and add the loose pounds.
2. Multiply that total by 0.45359237 for kilograms.

Going the other way, multiply kilograms by 2.2046226, divide by 14 for whole stone, and multiply the remainder by 14 for pounds.

Reference table

Mass and weight are not the same thing

Strictly, kilograms and pounds measure mass — the amount of matter — while weight is the force gravity exerts on it, measured in newtons. A 70 kg person has the same mass on the Moon and weighs about a sixth as much.

The distinction almost never matters on Earth, which is why gym plates, bathroom scales and airline limits all use mass units and call them weight. It matters in physics problems, in aerospace, and in the occasional pub argument.

The takeaway

Use 2.2 for conversation and 2.2046226 for anything with a threshold attached. The extra keystrokes cost nothing; a failed weigh-in costs more.