BMI Calculator
🔒 Runs in your browser
Add age for the body-fat estimate; ages 2–19 use children percentiles.
For information only — not a substitute for medical advice.
The BMI formula
Body Mass Index is your weight in kilograms divided by your height in metres, squared:
BMI = weight (kg) / height (m)²
For example, someone who weighs 70 kg and is 1.75 m tall has a BMI of 70 / (1.75 × 1.75) = 70 / 3.0625 = 22.9, which is a normal weight. In imperial units, pounds are first converted to kilograms (1 lb = 0.45359237 kg) and feet/inches to centimetres (1 in = 2.54 cm) before the same formula is applied — the result is identical either way.
BMI for women and men
The BMI number and the World Health Organization weight categories are the same for women and men — there is no separate BMI formula by sex. What does change with sex is the estimated body-fat percentage and the ideal-weight figures: at the same BMI, women naturally carry a higher body-fat percentage than men, which the Deurenberg body-fat estimate reflects. That is why the calculator asks for your sex even though the BMI value itself is unisex.
BMI and age
For adults the BMI thresholds do not depend on age: a BMI of 27 is overweight at 25 as at 65. Age still matters for the body-fat estimate, which rises with age at the same BMI, and it matters decisively for children and teenagers, whose healthy BMI changes month by month as they grow.
BMI for children (WHO percentiles)
For anyone aged 2–19, a fixed cut-off like 25 is meaningless — a healthy 6-year-old and a healthy 16-year-old have very different BMIs. Instead, the child's BMI is compared with the WHO BMI-for-age reference for that exact age and sex, giving a percentile and a z-score. Enter an age in the 2–19 range and the calculator switches to this mode automatically.
| Percentile / z-score | Category |
|---|---|
| z < −3 (below ~0.1st) | Severe thinness |
| −3 to −2 (~0.1st–2nd) | Thinness |
| −2 to +1 (~2nd–85th) | Normal weight |
| +1 to +2 (~85th–97th) | Overweight |
| above +2 (above ~97th) | Obesity |
Cut-offs shown are the WHO 5–19 reference; for ages 2–5 the WHO Child Growth Standards apply a slightly different overweight/obesity band, which the calculator uses automatically.
BMI table (WHO adult categories)
| BMI | Category |
|---|---|
| Below 16.0 | Severe thinness |
| 16.0 – 16.9 | Moderate thinness |
| 17.0 – 18.4 | Mild thinness |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
How much should I weigh?
The most defensible answer to your ideal weight is a range, not a single number: the weights that keep your BMI between 18.5 and 25 for your height. For 1.75 m that is roughly 56.7–76.6 kg. The calculator shows this healthy range for your height, alongside the classic Lorentz and Devine ideal-weight estimates, which give a single target figure for those who prefer one.
BMI is a quick screening indicator, not a diagnosis — it does not distinguish muscle from fat and ignores build and fat distribution, so very muscular people can read as overweight. This result is for information only and does not replace medical advice. Every calculation runs entirely in your browser; nothing is sent to a server.
Frequently asked questions
What is the BMI formula?
BMI = weight in kilograms divided by height in metres squared (kg/m²). For example, 70 kg at 1.75 m gives 70 / (1.75 × 1.75) = 22.9. Imperial pounds and feet/inches are converted to metric first (1 lb = 0.45359237 kg, 1 in = 2.54 cm).
Is BMI different for women and men?
The BMI number and the WHO categories are the same for both sexes. Sex does change the body-fat estimate and the ideal-weight figures this tool also shows, which is why you can pick female or male.
How does BMI change with age?
For adults the BMI thresholds do not change with age, but the body-fat estimate (Deurenberg) rises with age at the same BMI. For children and teens aged 2–19, BMI is read against age- and sex-specific WHO percentiles instead of the adult cutoffs.
How is BMI calculated for children?
Enter the child age (2–19). The tool computes a WHO BMI-for-age z-score and percentile for that exact age and sex — for example, above the 85th percentile is overweight — because a fixed number like 25 does not fit a growing child.
How much should I weigh for my height?
A healthy weight is roughly the range that keeps BMI between 18.5 and 25 for your height. This tool shows that range in kg (or lb), plus the Lorentz and Devine ideal-weight estimates.
Is BMI a perfect measure of health?
No — BMI does not distinguish muscle from fat and ignores build and fat distribution. It is only a quick screening indicator; this result is informative and does not replace medical advice.
Is my data sent to a server?
No — every calculation, including the child percentile tables, runs entirely in your browser. Nothing is transmitted or stored.