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Body weight in kilograms, pounds and stone

Three systems are still in daily use for body weight: kilograms almost everywhere, pounds in the United States, and stone plus pounds in Britain and Ireland. Each converts exactly, but the shortcuts people rely on introduce errors that matter for medication, weight classes and baggage allowances. Here is the accurate method.

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The three systems and how they relate

The kilogram is the base unit of mass in the metric system. The pound is defined against it as exactly 0.45359237 kilograms, a figure agreed internationally in 1959. One kilogram is therefore 2.2046226 pounds.

The stone adds a third layer in the British Isles. One stone is fourteen pounds, so a weight of 11 stone 4 pounds is 158 pounds, or 71.67 kilograms. British medical records use kilograms, but patients typically report their weight in stone, and both figures often appear on the same form.

The familiar shortcut of multiplying by 2.2 is convenient and wrong by about a fifth of a pound at 100 kilograms. For everyday conversation that is irrelevant; for a 23-kilogram baggage limit, a boxing weight class or a dose calculated per kilogram of body weight, use the full factor.

Key facts

Exact definition

One pound is exactly 0.45359237 kg, which makes one kilogram 2.2046226 pounds.

Stone arithmetic

One stone is fourteen pounds, so 11 st 4 lb is 158 lb or 71.67 kg.

The 2.2 shortcut drifts

At 100 kg it is off by roughly 0.2 lb, and the gap grows with weight.

Mass, not weight

Kilograms and pounds both measure mass; the newton measures force, which is why gym plates are labelled in kilograms.

Baggage limits are strict

A 23 kg allowance is 50.7 lb — a suitcase weighed at 51 lb will usually be flagged.

Medical dosing

Many drugs are dosed per kilogram, so an imprecise conversion becomes a clinical issue rather than a rounding one.

How to get it right

1

Pick a direction

Kilograms to pounds multiplies by 2.2046226; pounds to kilograms multiplies by 0.45359237.

2

Handle stone first

Multiply stone by fourteen and add the loose pounds before converting to kilograms.

3

Use the full factor

Reserve the 2.2 shortcut for rough mental maths only.

4

Round at the end

Convert with full precision and round once, at the final step, to avoid stacking errors.

Questions people ask

154.32 pounds. The calculation is 70 × 2.2046226. Using the 2.2 shortcut gives 154, which is close but not exact.

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Convert your exact weight

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