What is BMI?
Body Mass Index — BMI — is a simple number worked out from an adult's height and weight. It was devised in the 19th century and adopted by health organizations worldwide because it is cheap, quick and good enough to flag possible weight-related risk across large populations. Doctors use it as a first screening step: a BMI outside the normal band is a prompt to look more closely, not a verdict in itself. This calculator gives you the same number, in metric or imperial units, along with the standard category it falls into and a scale showing where it sits.
The BMI formula
- Metric: BMI = weight (kg) ÷ height² (metres)
- Imperial: BMI = 703 × weight (lb) ÷ height² (inches)
The 703 in the imperial version is simply a conversion factor that makes the two formulas agree. Worked example, metric: a person weighing 70 kg and standing 175 cm tall has a height of 1.75 m, so BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9. The same person in imperial units — 154 lb, 5 ft 9 in (69 inches) — gives 703 × 154 ÷ (69 × 69) = 22.7; the tiny difference is rounding in the conversions, and both results land in the same category.
The BMI categories
| BMI | Weight category (adults) |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25 – 29.9 | Overweight |
| 30 and above | Obese |
These are the standard adult ranges used by the World Health Organization and the US Centers for Disease Control and Prevention. They apply to adults aged 20 and over. Children and teenagers are assessed differently, using BMI-for-age percentiles, so do not use this chart — or this calculator — to judge a child's weight; a pediatrician has the right growth charts.
What BMI can and cannot tell you
BMI's strength is simplicity, and that is also its weakness. It cannot distinguish fat from muscle, bone or water, and it says nothing about where fat is carried, which matters for health. As a result it misjudges some people in both directions: a rugby player or weightlifter may land in the overweight band with very little body fat, while an older adult who has lost muscle may read as normal despite carrying more fat than is healthy. Pregnancy, some medical conditions and simply being very tall or very short can also shift the number. Treat your BMI as one data point — a reason, if it sits outside the normal band, to have a fuller conversation with a doctor who can add waist measurement, blood pressure, blood tests and your history into the picture.
Using your result sensibly
If your BMI is in the normal band, that is reassuring but not a certificate of health — fitness, diet, sleep and not smoking matter at every weight. If it falls outside the band, do not panic and do not start a crash diet on the strength of one number. Recheck your height and weight entries (a common slip is entering height in inches into the cm box, or the other way round), then, if the result still concerns you, book a routine check-up and discuss it there. Small, sustained changes — a daily walk, one fewer sugary drink — move real health markers far more reliably than dramatic short-lived efforts, and a professional can tailor the plan to you.
Frequently asked questions
How is BMI calculated?
Metric: weight in kilograms divided by height in metres squared. Imperial: 703 × weight in pounds divided by height in inches squared. 70 kg at 175 cm gives a BMI of 22.9.
What is a normal (healthy) BMI?
For adults, 18.5 to 24.9 is the normal weight band; below 18.5 is underweight, 25 to 29.9 overweight, and 30 or above obese. These are screening guidelines, not a diagnosis.
Is BMI accurate for athletes and muscular people?
Often not — BMI cannot tell muscle from fat, so muscular people can score high with low body fat. Read it alongside other measures and professional advice.
Is this BMI calculator suitable for children?
No. Children are assessed with BMI-for-age percentiles by age and sex. This calculator uses the adult categories only.
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