About BMI Calculator
BMI Calculator computes your body mass index from your height and weight, in either metric (cm/kg) or imperial (ft-in/lb) units. Alongside the number it shows which WHO category you fall into — underweight, healthy, overweight, obese — and the healthy weight range for your height. It's a quick, browser-only screening tool with no sign-up, useful for tracking trends over time or filling out a health form.
- No uploads
- Browser-only
- Works offline
- 100% free
How it works
- 1
Pick your units
Toggle between metric (centimetres, kilograms) and imperial (feet/inches, pounds).
- 2
Enter height and weight
Type the values or use the steppers. The BMI updates live with every change.
- 3
Read your result
See your BMI, the WHO category, and the weight range you'd need to be in to land in the 'healthy' band for your height.
What BMI actually measures — and what it doesn't
BMI is weight divided by height squared. That's the entire formula. It was devised in the 1830s by Adolphe Quetelet, a Belgian astronomer and statistician, to describe populations — not to assess individuals — and it became a clinical shorthand in the 1970s because it was cheap and needed no equipment.
It doesn't know the difference between muscle and fat, where the weight sits, your age, your sex or your frame. A rugby forward and a sedentary person of the same height and weight get the same number and are not in the same health. The number is a rough screen, useful for spotting a trend across a lot of people, and it says less about you individually than almost anyone assumes.
The standard categories, and who they were drawn for
The WHO cut-offs are the ones almost every calculator uses. They were derived largely from European-descended populations, which matters: cardiometabolic risk rises at lower BMIs in South and East Asian populations, and several health bodies use lower thresholds accordingly — often 23 for overweight and 27.5 for obesity rather than 25 and 30.
| BMI | WHO category |
|---|---|
| Under 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 over | Obesity class III |
Where BMI is known to mislead
- Muscle. Dense muscle weighs more than fat by volume, so athletes routinely score 'overweight' while carrying very little fat.
- Height. The height-squared term under-reads tall people and over-reads short ones; the exponent that actually fits human bodies is nearer 2.5 than 2.
- Children and teenagers. Adult categories don't apply at all — growth charts and age-and-sex percentiles are what paediatricians use.
- Older adults. Muscle mass falls with age, so the same BMI can hide a very different body composition.
- Pregnancy. The number is meaningless during it; pre-pregnancy BMI is what's used clinically.
- Where the weight sits. Waist circumference predicts cardiometabolic risk better than BMI does, because abdominal fat behaves differently from fat elsewhere. It's a tape measure and thirty seconds.
How to use the number sensibly
Treat it as one cheap data point, not a verdict. If it lands somewhere that concerns you, the useful next steps are a tape measure round the waist and a conversation with a doctor who can see the rest of you — not a diet chosen on the strength of a single ratio.
This calculator runs entirely in your browser: the height and weight you type never leave the tab and aren't stored anywhere. That's deliberate. Body measurements are health data, and health data shouldn't go to a server because you wanted a number divided by another number.
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Frequently asked questions about BMI Calculator
Is this medical advice?
No. BMI is a rough screening metric, not a diagnosis. It doesn't distinguish muscle from fat, doesn't account for age, sex, ethnicity, frame size, or body composition, and can mis-categorise athletes and the elderly. This tool is for educational purposes only — for medical guidance speak to a qualified healthcare professional. See our /disclaimer page for full terms.
What BMI range is considered 'healthy'?
The WHO standard for adults is: under 18.5 underweight, 18.5–24.9 healthy, 25.0–29.9 overweight, 30+ obese. Some regional bodies (e.g. for South-Asian populations) use slightly lower cut-offs for the overweight and obese bands due to differing health-risk profiles at lower BMI values.
Does this work for children?
Adult BMI categories don't apply to children and teenagers — paediatric BMI is interpreted on age-and-sex percentile charts (BMI-for-age), not against fixed bands. Use a paediatric BMI tool or speak to your child's doctor.
Privacy, offline use, browser support, and pricing questions are answered on the site-wide FAQ.