How to Calculate BMI & What Your Number Means

This guide shows the formula, works through an example, explains the categories, and — just as importantly — explains where BMI gets things wrong, so you can read your number with the right amount of scepticism.

The BMI formula

BMI = weight in kilograms ÷ (height in metres)². Height is squared, which means small differences in height matter more than you might expect. If you measure in feet and pounds, the formula is BMI = 703 × weight in pounds ÷ (height in inches)². Either way, you only need two numbers, and the calculation is the same for adults of any age — though the interpretation differs for children, whose BMI is compared against age-based charts instead.

Measure your height without shoes, standing straight, and weigh yourself in the morning, before eating, for the most consistent reading. A height error of just 2 cm can shift the result by about half a point, so it is worth measuring properly rather than guessing.

Worked example: 70 kg at 1.75 m

A person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9. That falls in the healthy range, which the World Health Organization defines as 18.5 to 24.9. Below 18.5 is underweight, 25.0 to 29.9 is overweight, and 30.0 and above is obese, with further bands at 35 and 40.

To see how sensitive the number is, keep the same height and raise the weight to 85 kg: BMI becomes 27.8, which is overweight. Drop the weight to 55 kg and it becomes 18.0, just underweight. A 15 kg change moves the category — which is a reminder that BMI bands are broad, and a single point either side of a boundary does not mean much on its own.

Step by step

1. Measure your height in metres (or centimetres) without shoes.
2. Weigh yourself in kilograms, ideally in the morning.
3. Enter both numbers into the BMI Calculator.
4. Read your BMI and the category it falls in.
5. Compare with waist circumference and how you feel day to day — BMI alone is not the whole picture.

Common mistakes

• Guessing your height. Most people overestimate their height, which makes BMI look lower than it is. Measure it.
• Treating the category as a diagnosis. BMI is a screening tool. It flags who should look closer, it does not diagnose disease.
• Ignoring muscle. Muscle is denser than fat, so a muscular person can have a high BMI while carrying little body fat.
• Using adult bands for children or teenagers. Young people's BMI must be read against age and sex charts, not adult cut-offs.
• Weighing at different times of day. Weight shifts by a kilogram or more during the day. Weigh at the same time, in similar clothing, to compare fairly.

What BMI does not tell you

BMI says nothing about where fat is carried, and that matters: fat around the waist carries more health risk than fat on the hips or thighs, at the same BMI. That is why many doctors also measure waist circumference — roughly, a waist over 94 cm for men or 80 cm for women is a flag worth discussing with a clinician, whatever the BMI says. BMI also does not account for age, sex, ethnicity or fitness: older adults naturally carry more fat at the same BMI, and some populations show higher health risks at lower BMIs. Use the number as a prompt to pay attention, not as a verdict — and if it worries you, the next step is a conversation with a health professional, not a crash diet.

Frequently asked questions

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