Stones and Pounds to Kg: The Converter and the Maths Behind It

British bathroom scales still speak in stones. Hospitals, gyms, race entry forms and every drug dosage chart speak in kilograms. That leaves a lot of people staring at a form with "11 st 4 lb" in their head and a box that wants a metric number.

Where the two numbers come from

A stone is 14 pounds. That part is fixed and has been for a long time. A pound is defined as 0.45359237 kilograms under the international agreement that standardised the imperial pound, which makes one stone 6.35029318 kg. Neither figure is a rounding somebody invented, so a converter using 6.35029 and 0.45359 is accurate to well under a gram at human body weights.

Because you are combining two units, the sum has two parts. Do the stones, do the pounds, add them. That is the whole method.

Worked example: 11 stone 4 pounds

11 x 6.35029 = 69.853 kg. Then 4 x 0.45359 = 1.814 kg. Add them and you get 71.67 kg, which most forms would take as 71.7 kg. You can check it the long way: 11 stone 4 lb is 158 pounds in total, and 158 x 0.45359 = 71.67 kg. Same answer, which is a useful habit when the number matters.

Worked example: 9 stone 12 pounds

9 x 6.35029 = 57.15 kg, and 12 x 0.45359 = 5.44 kg, giving 62.60 kg. The cross-check: 9 stone 12 lb is 138 pounds, and 138 x 0.45359 = 62.60 kg again.

Using the converter

There are two input boxes because there are two units. Put the whole stones in the first and the leftover pounds in the second. The leftover figure should always land somewhere between 0 and 13. If you find yourself typing 14 or more, you have another whole stone that belongs in the first box.

Where do those inputs come from? An old dial scale shows something like "11:4" and that reads straight across. A digital scale set to stones is the one to watch. Many of them display decimal stones, so a reading of 11.3 st is not 11 stone 3 pounds. It is 11 stone plus 0.3 of a stone, which is 4.2 pounds. If your scale gives a single number with a decimal point, enter it as decimal stones or convert the fraction yourself before typing.

The mistake that costs you nearly a kilo

Reading 11.5 stone as 11 stone 5 pounds is the classic one. Half a stone is 7 pounds, so the correct reading is 11 stone 7 lb, or 73.03 kg. The wrong reading gives 72.12 kg. That is a difference of 0.91 kg, enough to nudge a metric weight conversion into a different BMI band or throw off a weight-based medication dose.

The reverse trip catches people in the same way. If you are American and weigh 165 lb, dividing by 14 gives 11.79 stone, which reads as 11 stone 11 lb once you multiply the 0.79 back by 14. Our guide to stones, pounds and kilograms covers that direction in more detail.

Common questions

How many kg is 10 stone? Exactly 63.50 kg. Ten stone is 140 pounds, and 140 x 0.45359 = 63.503 kg. It is a handy anchor to memorise, because from there each extra stone adds 6.35 kg and each extra pound adds 0.45 kg.

Is a stone always 14 pounds? For body weight, yes, everywhere it is still used. Historically the stone varied by trade, and a wool stone was a different thing from a meat stone, but that has not applied to people for well over a century.

Why does my doctor use kilograms when my scale uses stones? Medicine runs on metric because doses are prescribed per kilogram of body weight, and BMI is defined as kilograms divided by height in metres squared. Nobody wants to do that conversion in their head during an appointment, which is why the surgery scale reads in kg regardless of what yours says.

How do I go the other way, kg back to stones and pounds? Divide the kilograms by 6.35029 to get decimal stones, keep the whole number, then multiply whatever is left over by 14 for the pounds. 80 kg gives 12.598 stone, so 12 stone and 0.598 x 14 = 8.4 pounds, which rounds to 12 stone 8 lb.