BMI explained: how to calculate it and what it means

Body mass index, or BMI, is a simple number that compares your weight with your height. Doctors, nurses and public health teams use it as a quick screening tool, and you will often be asked for it on forms. It is easy to calculate by hand, but it is also easy to misread, because it does not measure body fat directly and it works differently for some groups of people. This guide shows the formula, works through an example in stones and pounds, sets out the ranges the NHS uses, explains where BMI falls short, and suggests other measures that can add useful context. It is general information, and anyone worried about their weight should speak to a GP.

By Valtrinity. Updated 18 October 2026.

The formula

BMI is your weight in kilograms divided by your height in metres, squared. In symbols: BMI = weight ÷ (height × height). Squaring the height means multiplying the number by itself.

For a person who weighs 70 kg and is 1.75 m tall, the height squared is 1.75 × 1.75, which is 3.0625. Divide 70 by 3.0625 and you get 22.86. The BMI is 22.9 when rounded to one decimal place.

The unit of BMI is kilograms per square metre, but it is usually written without a unit. The result does not depend on sex or age for adults, though how it is interpreted can.

A worked example in stones and pounds

Many people in the UK know their weight in stones and pounds and their height in feet and inches. Convert first. Take someone who is 12 stone 6 pounds and 5 ft 10.

Weight: 12 stone is 168 pounds, plus 6 is 174 pounds. Multiply by 0.45359 to get kilograms: 174 × 0.45359 is 78.93 kg. Height: 5 ft 10 is 70 inches. Multiply by 0.0254 to get metres: 70 × 0.0254 is 1.778 m.

Square the height: 1.778 × 1.778 is 3.161. Divide the weight: 78.93 ÷ 3.161 is 24.97. The BMI is about 25.0, right at the boundary between the healthy range and the overweight range. This shows how close the lines are: gaining or losing 1 kg moves the BMI by about 0.3.

The ranges the NHS uses

For most adults, the NHS gives these ranges: below 18.5 is underweight, 18.5 to 24.9 is a healthy weight, 25 to 29.9 is overweight, 30 to 39.9 is obese and 40 or above is severely obese. Further obesity classes are used in clinical settings.

For adults from Black, Asian and other minority ethnic backgrounds, lower thresholds are used, because the risk of conditions such as type 2 diabetes starts at a lower BMI. NICE guidance sets the overweight range at 23 to 27.4 and obesity at 27.5 and above for some of these groups. If this applies to you, mention it when you talk to a health professional.

Children and young people are assessed differently. Their BMI is compared with charts for their age and sex, and a single adult range does not apply. The NHS has a separate tool for parents.

What BMI cannot tell you

BMI uses only weight and height, so it cannot tell muscle from fat. A fit person with a lot of muscle can have a BMI in the overweight range without carrying excess fat. A person who is older and has lost muscle can have a normal BMI but a high proportion of body fat.

It also does not say where fat is stored. Fat around the middle is linked with higher risk of heart disease and type 2 diabetes than fat on the hips or thighs, but BMI treats them the same.

Pregnancy, certain health conditions and some medicines affect weight in ways that BMI does not allow for. For those reasons, BMI should be treated as a first look, not a diagnosis. A health professional will take other factors into account.

Waist measurements as an extra check

A simple extra check is the waist-to-height ratio. NICE recommends keeping your waist measurement to less than half your height. Measure around the middle, halfway between the bottom of your ribs and the top of your hips, after breathing out.

For a person 175 cm tall, half their height is 87.5 cm. A waist of 80 cm gives a ratio of 0.46, which is below 0.5. A waist of 95 cm gives 0.54, which suggests extra risk even if the BMI looks normal.

The NHS also gives waist guidance in absolute terms, with higher risk for men above 94 cm (37 inches) and for women above 80 cm (31.5 inches), and a very high risk above 102 cm (40 inches) and 88 cm (34.5 inches) respectively. Using both BMI and a waist measure gives a fuller picture than either alone.

Working out a healthy weight range for your height

You can flip the formula to find the weights that correspond to the healthy range. Multiply the height squared by 18.5 for the lower end, and by 24.9 for the upper end.

For a height of 1.65 m, the square is 2.7225. The lower end is 2.7225 × 18.5, which is 50.4 kg. The upper end is 2.7225 × 24.9, which is 67.8 kg. In stones and pounds, that is about 7 stone 13 pounds to 10 stone 9 pounds.

A range is more useful than a single target. Where you sit within it depends on your build, your age and your health, and many people are well at several different weights inside the band.

When to talk to someone

If your BMI is outside the healthy range, it is not an emergency, but it is worth discussing, particularly if you also have symptoms, a family history of heart disease or diabetes, or you are planning a pregnancy. Your GP or practice nurse can check other measures such as blood pressure and blood sugar.

If you are underweight and losing weight without trying, or you have a difficult relationship with food, seek support. Services such as your GP, and the NHS website, set out where to go.

Use BMI for what it is: a quick screen that works best for populations, and a rough guide for individuals. Track trends over time rather than fixating on a single reading.

Tools mentioned in this guide

Frequently asked questions