How to work out your BMI by hand

The formula

BMI is your weight divided by your height squared. That's genuinely all there is to it. A Belgian mathematician called Adolphe Quetelet came up with it in the 1830s while studying averages across whole populations, and it was only much later that doctors started using it on individuals.

BMI = weight (kg) ÷ height (m)²

Squaring your height is the clever bit. A woman who is 5ft 10in will naturally weigh more than a woman who is 5ft 2in, even if they're built the same way, and the squared height evens that out so the two numbers can be compared fairly.

In kilograms and metres

This is the easiest version because the units already work together.

1. Write down your height in metres. If you know it in centimetres, divide by 100, so 165 cm becomes 1.65 m.
2. Multiply your height by itself.
3. Divide your weight in kilograms by the number you got in step two.

That one lands exactly on the line between healthy and overweight. It's a useful reminder that a single decimal point isn't a verdict on your health. Someone at 24.9 and someone at 25.0 are, for any practical purpose, in the same place.

In pounds and inches

The imperial version needs one extra number, 703, because pounds and inches don't line up the way kilograms and metres do. Multiplying by 703 converts the answer into the same scale the metric formula gives you.

BMI = weight (lb) × 703 ÷ height (in)²

In stone and pounds

Most of us in the UK still think in stone, which makes things slightly awkward because there isn't a direct formula. The fix is simple: turn it into pounds first. Multiply the stone by 14, then add the leftover pounds.

If you'd rather not do any sums at all, our BMI chart in stone and pounds has the healthy range already worked out for every height from 4ft 10in to 6ft.

What your number means

These are the categories the NHS uses for adults. The WHO chart on our homepage uses the same cut-off points but splits the obese range into three classes. To see every height and weight at once, use our colour-coded BMI chart.

One thing a lot of calculators leave out: if your family background is South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean, NICE advises lower thresholds. For you, overweight starts at a BMI of 23 and obesity at 27.5, because conditions like type 2 diabetes tend to appear at lower weights in these groups.

Mistakes that throw the number off

• Guessing your height. It matters more than you might think, since an inch changes your healthy weight range by several pounds. Plenty of us haven't been properly measured since school, and height can drop slightly as we get older. Stand against a wall in bare feet, rest a hardback book flat on your head, mark the wall underneath it and measure from the floor.
• Weighing at random times. Your weight can shift by a kilo or more over a single day, and fluid retention before a period can add a pound or two on top. For numbers you can compare, weigh first thing in the morning, after the loo and before breakfast.
• Rounding too early. Keep every decimal until the last step. In the metric example above, rounding 2.7225 down to 2.7 nudges the answer from 25.0 to 25.2.
• Forgetting to square. Divide by your height only once and you'll get a number somewhere in the forties. If your answer looks alarming, check this first.