BMI Calculator

How to use the BMI calculator

1. Pick your units. Metric takes kilograms and centimetres; imperial takes pounds plus feet and inches. Switching units converts whatever you have already typed, so you never have to do the arithmetic yourself.
2. Enter your weight and height. Everything recalculates as you type — there is no button to press.
3. Optionally add your waist and age. The waist adds a waist-to-height ratio; the age only changes which caveat is shown, because the adult bands do not apply under 20.
4. Read the scale bar. It shows where your number falls across the four WHO bands from 15 to 40, with a marker at your BMI.
5. Check the healthy weight range. The tool converts the 18.5–24.9 band back into kilograms or pounds for your exact height, which is usually more useful than the index itself.

The formula

Body mass index is deliberately simple. In metric units:

BMI = weight in kilograms ÷ (height in metres)²

In imperial units the same quantity is written with a conversion factor baked in:

BMI = 703 × weight in pounds ÷ (height in inches)²

The categories below come from the World Health Organization’s adult classification and are used by most national health services.

Because BMI scales with the square of height, tall people get slightly higher numbers than a purely volumetric measure would give, and short people slightly lower. That is a known limitation of the index rather than a bug in the maths.

Worked examples

Metric. Someone weighing 70 kg at 175 cm: 1.75 m squared is 3.0625, and 70 ÷ 3.0625 is 22.9. That is inside the normal band, with a BMI prime of 0.91. At that height the 18.5–24.9 band runs from 56.7 kg to 76.3 kg.

Imperial. Someone weighing 154 lb at 5 ft 9 in: 69 inches squared is 4,761, and 703 × 154 ÷ 4,761 is 22.7. Converting to metric first — 69.9 kg at 175.3 cm — gives the same 22.7, which is a useful check that the two formulas really are the same calculation.

A number above the band. 90 kg at 175 cm gives a BMI of 29.4, inside the overweight band and 13.7 kg above the top of the healthy range for that height. The tool shows that distance directly, because “13.7 kg” is easier to reason about than “29.4”.

With a waist measurement. A waist of 85 cm at 175 cm height gives a waist-to-height ratio of 0.49 — just under the half-your-height line that is commonly quoted. The same person at 95 cm would be at 0.54.

What BMI cannot tell you

• Body composition. Muscle is denser than fat, so two people with identical BMIs can have very different bodies. Athletes and heavily muscled people are the classic false positive.
• Fat distribution. Weight carried around the middle behaves differently from weight carried on the hips and legs, and BMI is blind to the difference. That is why the optional waist measurement is there.
• Age. Muscle mass falls and body composition shifts with age, so the adult bands become less predictive in older people.
• Pregnancy and breastfeeding. BMI is not meaningful here at all; weight is tracked separately by a midwife or doctor.
• Children and teenagers. Growth makes a fixed cut-off meaningless. Percentile charts specific to age and sex are used instead.
• Ancestry. Several health bodies use lower action points for people of South Asian, Chinese and some other backgrounds, because cardiometabolic risk rises at a lower BMI. Local guidance varies, so check what your own health service uses.

Tips for a measurement you can trust

• Weigh yourself consistently — same scale, same time of day, ideally in the morning. Day-to-day swings of a kilogram or two are mostly water.
• Measure your height properly. Guessing by two centimetres moves a BMI of 22.9 by about 0.3. Stand against a wall without shoes and mark the top of your head.
• Measure your waist at the right place: midway between the lowest rib and the top of the hip bone, at the end of a normal breath out, with the tape snug but not compressing the skin.
• Watch the trend, not the reading. A single BMI is a snapshot. Several months of readings taken the same way say far more.

Glossary

• BMI – body mass index, mass divided by height squared.
• BMI prime – BMI expressed as a fraction of 25.
• WHtR – waist-to-height ratio, waist circumference divided by height.
• Obesity class – the WHO subdivision of BMI 30 and above into class I (30–34.9), II (35–39.9) and III (40+).

A note on what this page is

This is a screening tool, not a diagnosis; talk to a healthcare professional before drawing any conclusion about your health. This page deliberately gives no diet, exercise or medical advice — it does one calculation and explains what the number does and does not mean. Everything runs in your browser, so the height, weight and waist you type are never uploaded, logged or stored anywhere.

Last reviewed: September 21, 2026