BMI Calculator
Your Body Mass Index in metric or imperial units — with a straight answer about what the number does and doesn't tell you.
Runs entirely in your browser. Nothing you type here is uploaded, stored or logged.
How BMI is calculated
Metric: weight (kg) ÷ height (m)². Imperial:
703 × weight (lb) ÷ height (in)². Both give the same figure — the 703 is
just a unit-conversion constant.
The formula dates from the 1830s, devised by the Belgian statistician Adolphe Quetelet to describe populations. It was never designed to assess an individual, which is the root of most of its problems.
The standard categories
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy 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 |
Where BMI gets it wrong
Muscle. Muscle is denser than fat. A rugby player or a serious weightlifter will often score 28–30 with single-digit body fat. The number is technically correct and completely meaningless in that context.
Fat distribution. Two people with identical BMIs can carry very different health risks depending on whether that weight sits around the abdomen or on the hips and thighs. Visceral fat around the organs is the dangerous kind, and BMI cannot see it.
Ethnicity. The risk of type 2 diabetes and cardiovascular disease rises at lower BMIs in people of South Asian, Chinese, and other Asian backgrounds. Many health bodies use 23 rather than 25 as the overweight threshold for these groups.
Age and sex. Older adults lose muscle and gain fat at a stable weight, so the same BMI means something different at 70 than at 25. Women naturally carry more body fat than men at the same BMI.
Better things to track
- Waist-to-height ratio. Keep your waist under half your height. Simple, needs only a tape measure, and predicts metabolic risk better than BMI.
- Waist circumference. Risk rises above roughly 94 cm (37 in) for men and 80 cm (31.5 in) for women, with lower thresholds for some ethnic groups.
- Trend, not snapshot. The direction your weight has moved over six months tells you more than any single reading.
- How you actually function. Resting heart rate, blood pressure, sleep quality and whether you can climb two flights of stairs comfortably.
This calculator provides general information only and is not medical advice. If you are concerned about your weight or health, speak to a doctor or registered dietitian who can assess your full picture.
Frequently asked questions
What is a healthy BMI range?
The World Health Organization classifies 18.5–24.9 as the healthy range for adults, 25–29.9 as overweight and 30 or above as obese. Below 18.5 is classified as underweight. These cut-offs are population-level guidance, not a diagnosis of any individual.
Why is BMI considered unreliable?
Because it only knows your height and weight. It cannot distinguish muscle from fat, so athletes are routinely classified as overweight. It ignores where fat is stored, though abdominal fat carries most of the health risk. And the cut-offs were derived largely from European populations — the WHO recommends lower thresholds for people of South Asian, Chinese and other Asian descent.
Does BMI work for children?
Not with these thresholds. Children and teenagers are assessed against age- and sex-specific percentile charts, because body composition changes rapidly during growth. This calculator is intended for adults from about 20 upwards; for anyone younger, use a paediatric BMI-for-age chart.
What should I use instead?
Waist circumference and waist-to-height ratio are better single predictors of metabolic risk, and both are easy to measure at home. A waist measurement under half your height is a widely used rule of thumb. Body fat percentage is better still but needs equipment.
Is my data stored?
No. Your height and weight are used by JavaScript in your browser and never leave your device.