How to Calculate BMI (and What Your Number Means)

Your Body Mass Index, or BMI, is your weight divided by your height squared. It is a quick screening number: it puts you in a weight category — underweight, healthy, overweight or obese — but it is not a diagnosis and it says nothing about how much of you is muscle and how much is fat. The formula is simple, the arithmetic takes ten seconds, and the hard part is knowing what the result does and does not mean.

By Calcly Team · Last updated 8 November 2026 · Free guide

The BMI formula

In metric units, the formula is: BMI = weight in kilograms ÷ (height in metres × height in metres). If you weigh 70 kg and you are 1.75 m tall, you square the height first — 1.75 × 1.75 = 3.0625 — then divide: 70 ÷ 3.0625 = 22.9. In imperial units the formula looks stranger because of the unit conversion: BMI = 703 × weight in pounds ÷ (height in inches × height in inches). The 703 is just the number that makes pounds and inches line up with the metric formula. A person who weighs 154 lb and is 5 ft 9 in tall (69 inches) works out at 703 × 154 ÷ (69 × 69) = 108,262 ÷ 4,761 = 22.7 — the same person, near enough, as the metric example above. Whichever system you use, measure your height without shoes and weigh yourself in the morning, in light clothing, for the most consistent result.

Worked example

Take someone who weighs 70 kg and stands 1.75 m tall. Square the height: 1.75 × 1.75 = 3.0625. Divide the weight by that number: 70 ÷ 3.0625 = 22.857, which rounds to 22.9. That sits inside the healthy range of 18.5 to 24.9. Now work out the healthy weight range for that same height. The bottom of the range is 18.5 × 3.0625 = 56.7 kg, and the top is 24.9 × 3.0625 = 76.3 kg. So at 1.75 m, anything from about 57 kg to about 76 kg counts as a healthy weight. If that same person weighed 90 kg instead, their BMI would be 90 ÷ 3.0625 = 29.4 — overweight, but close to the top of that band rather than far into it. Small changes in weight move BMI slowly: losing 5 kg from 90 kg takes this person from 29.4 to 27.8.

BMI categories

Doctors and health organisations around the world use the same four adult categories, set by the World Health Organization. They are screening bands, not verdicts — two people with the same BMI can be in very different health — but they are the starting point for almost every conversation about weight.

Obesity is split further when it matters clinically: class I is 30 to 34.9, class II is 35 to 39.9, and class III is 40 and above. The higher the class, the higher the average risk of type 2 diabetes, heart disease and joint problems — which is why a doctor who sees a BMI of 32 will usually also check your blood pressure, blood sugar, cholesterol and waist measurement before saying anything about your health.

What BMI does not tell you

BMI has one blind spot that matters: it cannot see what your weight is made of. Muscle weighs more than fat, so a person who lifts weights can be classed as overweight or even obese while carrying very little body fat. The reverse is also true — an older person can have a perfectly healthy BMI and still carry too little muscle, which brings its own problems. BMI also ignores where fat sits. Fat around the waist is linked to more health risk than fat on the hips and thighs, so doctors often measure waist circumference alongside BMI: roughly speaking, a waist above 94 cm (37 in) for men or 80 cm (31.5 in) for women adds risk whatever the BMI says. Age matters too. The adult categories do not apply to children, whose BMI is read against percentile charts, and they fit less well for very tall, very short or older people. None of this makes BMI useless — it is still the fastest screening tool there is — but it does mean a single number should never be the whole story.

What to do with your number

If your BMI is in the healthy range, the practical goal is to stay there: regular movement, sensible portions and a weight check every month or two will do it. If it is above 25, the number that matters most is the direction it moves, not how fast it falls. Losing 5 to 10 percent of your body weight — 5 to 9 kg for a 90 kg person — is enough to improve blood pressure and blood sugar noticeably, and it is far easier to keep off than a crash diet. If it is below 18.5, the same logic applies in reverse: small, steady gains built on regular meals and strength work beat trying to eat everything at once. Whatever your number, treat it as a prompt to look at the wider picture — how you move, sleep and eat — rather than as a grade. And if you have a medical condition, are pregnant, or are under 18, talk to a doctor instead of reading your health off a chart. BMI is general information, not medical advice.

Want your number without the arithmetic? Use our BMI calculator — enter your weight and height and it shows your BMI and your healthy weight range instantly. To see the other side of the picture, try our calorie calculator for your daily needs, or our sleep calculator to sort out your bedtime.