BMI calculator
Enter your height and weight to see your body mass index, the range it falls into and the healthy weight range for your height. If you know your waist circumference, the reading gets considerably more useful.
Fill in the fields
Your result appears here as soon as the numbers are complete.
Your BMI
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WHO ranges for adults
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Waist-to-height ratio
The maths happens inside your browser. No measurement is sent to our servers or stored anywhere.
BMI cannot tell muscle from fat, nor where the fat sits. It does not apply to people under 20, during pregnancy or breastfeeding, or to athletes carrying a lot of muscle. This result is an educational estimate and does not replace assessment by a physician or dietitian, who weighs history, lab work and context no formula can see.
What BMI actually is
Body mass index is weight divided by height in metres squared. Someone who is 70 kg and 1.75 m has a BMI of 22.9 — because 70 divided by 1.75 × 1.75 works out at 22.86.
The formula dates from the nineteenth century and the Belgian statistician Adolphe Quetelet, and it was never meant to diagnose anyone: it described populations. What made it universal was convenience. Two measurements any clinic already has, a calculation you can do in your head, and a genuine population-level correlation between the higher ranges and the risk of type 2 diabetes, hypertension and cardiovascular disease.
That explains both why BMI is still in use and why, on its own, it never closes an assessment.
Reading the result
The classification used here is the World Health Organization one for adults. The ranges are the same for men and women — the index does not distinguish sex.
It is worth noticing that the boundaries are useful conventions, not walls. A BMI of 24.9 and one of 25.1 describe practically the same body; what changes between them is the word printed on the chart, not the risk. The honest reading follows the trend over time rather than today’s digit.
The WHO also recognises lower cut-off points for Asian populations — overweight from 23 and obesity from 27.5 — because metabolic risk appears at lower body weight in those groups.
What BMI cannot see
The index knows two numbers: weight and height. It has no idea where the weight comes from. A rugby player at 12% body fat and a sedentary person of the same weight and height get exactly the same classification, and the conclusion is useful for one and absurd for the other.
It also has no idea where the fat sits — and that is the limitation that costs most. Fat stored in the abdomen, around the organs, tracks far more directly with insulin resistance and cardiovascular disease than fat carried on hips and thighs. Two people at BMI 27 can have quite different risk profiles.
Finally, it does not apply to people under 20, who are read against age- and sex-specific percentile curves, nor during pregnancy or breastfeeding, nor to anyone who has lost muscle mass to age or illness.
Waist completes the picture
The waist-to-height ratio covers much of what BMI leaves out, and with the same economy of means: a tape measure. Divide waist circumference by height, in the same units.
The reference is 0.5 — in other words, keep your waist under half your height. Above 0.6, cardiometabolic risk is considered high. That is the guidance adopted by NICE, the UK clinical guidelines institute, and it works without a separate table per sex or ethnicity, which makes it easy to remember and hard to apply wrongly.
To measure: tape at navel height, standing, at the end of a normal exhale, without pulling it tight and without holding your stomach in.
WHO ranges for adults
| BMI | Classification |
|---|---|
| below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 to 34.9 | Obesity class I |
| 35.0 to 39.9 | Obesity class II |
| 40.0 or above | Obesity class III |
Frequently asked questions
How do I work out BMI by hand?
Divide weight in kilos by height in metres squared. For 68 kg and 1.70 m: 1.70 × 1.70 = 2.89, then 68 ÷ 2.89 = 23.5. The result has no unit — it is just an index.
What is my ideal weight for my height?
There is no ideal weight, there is a range. It corresponds to a BMI between 18.5 and 24.9, and the calculator shows that interval in kilos or pounds as soon as you enter your height. For 1.70 m it runs from roughly 53.5 kg to 72 kg — a spread of nearly 19 kg, all of it inside what counts as healthy.
Does BMI work for people who train?
Poorly. Muscle is denser than fat, so someone carrying plenty of it weighs more at the same height and moves up a category with nothing wrong happening. In those cases body fat percentage and waist circumference say far more than the index does.
Is a BMI of 27 something to worry about?
It is a prompt to look at everything else, not a diagnosis. A BMI of 27 with a waist under half your height, normal blood pressure, clean bloodwork and good fitness is a different picture from a BMI of 27 with a high waist measurement and borderline fasting glucose. The index suggests where to look; it concludes nothing on its own.
Does this work for children and teenagers?
No. Up to about age 20, BMI is interpreted against percentile curves that vary with age and sex, because body composition and height are still changing. A fixed threshold applied to a teenager misclassifies easily. That reading is done with the WHO growth curves, in consultation.
Is my data sent to a server?
No. The maths happens entirely inside your browser, on your own device. No measurement leaves the page, nothing is stored and nothing is tied to you. You can check by going offline: the calculator keeps working.