What next?
BMI is a screening number, not a diagnosis. These are the steps that turn it into something you can act on.
What your result means
Your BMI number places you in a WHO weight category — underweight, healthy, overweight or obese — marked on the scale. It is a quick screen, not a diagnosis: it cannot tell muscle from fat, so very muscular or older bodies can be misjudged. Use it as a first flag, then look at waist size and overall health.
Why this one is different
The number comes with the cases it gets wrong, stated on the page rather than buried: muscle mass, age, and the lower thresholds that apply to South Asian and some other ethnic groups, plus why the adult formula should not be read across to children. Everything runs in your browser — no height, weight or result is sent anywhere.
A 200-year-old formula that was never meant for you
Here's something most people don't know: BMI was invented in the 1830s by a Belgian astronomer and statistician, Adolphe Quetelet — who was studying populations, not individuals, and explicitly warned against using it to judge a single person. Nearly two centuries later, that's exactly what we all do.
It survived because it's brilliantly cheap: no scan, no lab, just height and weight. That same simplicity is its blind spot. BMI can't tell muscle from fat, can't see where you carry weight, and quietly assumes everyone is built the same way. A prop-forward rugby player and a sedentary office worker can share an identical BMI and completely different health.
So treat your result as a conversation starter, not a scoreboard. It's a fast first flag worth knowing — then look at the fuller picture below. Read our full guide to reading your BMI →
What is BMI?
Body Mass Index (BMI) is a simple screening measure that uses your height and weight to estimate whether you are in a healthy weight range. It is widely used by clinicians and health organisations as a quick, low-cost first check — though it does not directly measure body fat or account for muscle mass.
How it works
Enter your height and weight above. The calculator divides your weight by the square of your height and instantly maps the result onto the standard World Health Organisation categories shown on the scale.
How to use this calculator
- Pick Metric (cm, kg) or Imperial (ft/in, lb) using the toggle.
- Enter your height — in centimetres, or feet and inches.
- Enter your weight — in kilograms, or pounds.
- Read your BMI number and category; the marker on the scale shows where you sit.
Formula
For imperial units the formula is BMI = 703 × weight (lb) ÷ height (in)². Both give the same result.
Example calculation
A person who is 1.70 m tall and weighs 70 kg:
BMI = 70 ÷ 2.89
BMI = 24.2 → Normal weight
Frequently asked questions
Is BMI accurate for everyone?＋
BMI is a useful population-level screen but has limits. It can overestimate body fat in muscular people (e.g. athletes) and underestimate it in older adults with less muscle. Treat it as a starting point, not a diagnosis.
What is a healthy BMI range?＋
For most adults, a BMI between 18.5 and 24.9 is considered the healthy range. Ranges differ for children, and some populations use adjusted thresholds.
Does BMI differ for men and women?＋
The BMI formula and categories are the same for adult men and women. Body composition does differ between sexes, which is one reason BMI is only a rough guide.
Should children use this calculator?＋
No. For anyone under 18, BMI is read against age- and sex-specific centile charts rather than fixed thresholds, so an adult category label would be misleading. Ask a GP or paediatrician to plot BMI on a growth chart instead.
What should I look at alongside BMI?＋
Waist circumference is the most useful companion measure, because it reflects where fat is stored rather than how much you weigh in total. Blood pressure, resting heart rate and how your clothes fit over time all add context BMI cannot see.
How much does BMI change from day to day?＋
Body weight swings by 1-2 kg across a day from food, fluid and salt intake, which moves BMI by roughly 0.3-0.7 points at average heights. Weigh at the same time of day, ideally in the morning, and compare weekly averages rather than single readings.
Related calculators
Assumptions & limitations
Every figure here comes from a simplified model. Keep these limits in mind when reading your result:
- BMI is a screening measure, not a diagnosis.
- It uses only height and weight, so it can’t tell muscle from fat — very muscular or older people can be mis-classified.
- Standard adult cut-offs don’t apply to children, pregnancy, or every ethnic group.