Overview
BMI is a two-century-old ratio that refuses to die: weight divided by height squared, yielding a single number the whole world recognises. This calculator produces that number, labels its category, and — more usefully — shows the weight band that would keep your BMI in the healthy zone.
One honest framing before you use it: BMI is a screening number, not a medical verdict. It cannot see muscle, bone or fat distribution, and it was designed for populations, not people. Use it as a prompt for questions, never as an answer — and talk to a qualified professional about health decisions.
What does the BMI formula do?
It divides your weight in kilograms by the square of your height in metres. The squaring is the clever bit: it normalises for height, so a tall and a short person with proportional builds get similar numbers. Imperial users get the identical result through the equivalent formula with its 703 conversion constant.
The output falls into WHO bands: under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight, 30+ obese. The bands describe populations well — across millions of people they track health risk — while saying much less about any one individual, which is the number's permanent tension.
Features
- Metric and imperial inputs with a single tap to switch systems.
- Instant calculation — the number updates as you type.
- Category label with colour coding for a glanceable read.
- Personal healthy band showing the weight range for your height.
- Visual scale with a marker from 15 to 40 so you can see position, not just digits.
- Sensible input limits that reject impossible heights and weights.
- Clear health framing stating what the number cannot tell you.
- Fully private: your measurements never leave the browser.
Benefits
- One number, instantly. No forms, no accounts, no waiting rooms.
- Both unit systems. Centimetres or feet-and-inches, kilos or pounds.
- Context included. The healthy band is more actionable than the bare score.
- Honest limits. The page tells you what BMI misses, upfront.
Who can use it
Anyone curious where they stand, people tracking changes over months of training, and anyone filling in a form that asks for the number. Athletes should read their result with extra scepticism — dense muscle reads as “overweight” on this scale. Parents: adult bands don't apply to children, whose healthy ranges shift with age; paediatric charts are the right tool there.
How to use
- Choose Metric or Imperial at the top of the tool.
- Enter your height — and weight — in the boxes provided.
- Read your BMI, its category and your position on the scale.
- Note the healthy weight band shown beneath: that range is the practical takeaway.
- Remember the caveat, and discuss any health decision with a professional.
Frequently asked questions
What counts as a healthy BMI?
For adults, 18.5 to 24.9 is the band the WHO calls healthy. But bands are population statistics: a fit rugby player can sit at 27 and be healthier than a sedentary person at 22. Treat an out-of-band result as a reason to ask questions — of a doctor, not of the number.
Why is BMI criticised?
Because it mistakes density for excess. Muscle weighs more than fat per litre, so muscular people score high; it also ignores where fat sits — abdominal fat carries more risk than the same weight elsewhere. It remains useful as a cheap first screen precisely because it is cheap, not because it is complete.
Does it work for children or older adults?
Not directly. Children's healthy ranges move with age and sex, so doctors use percentile charts instead of fixed bands. In older adults, some extra weight correlates with better outcomes, and muscle loss can hide inside a “healthy” score. This calculator uses the adult formula throughout.
How is the healthy weight band computed?
By running the formula backwards: the weights that would put your height at BMIs of 18.5 and 24.9. It is the same arithmetic, just solved for weight instead of the index — a range most people find more meaningful than the single number.
Is my data stored anywhere?
No. Height and weight live in the page's memory while you use it and vanish when you leave. No account, no database, no analytics on your inputs.
Conclusion
Take the number, take the band, and take the caveat with equal seriousness. BMI is a decent first question about weight and a poor final answer — the rest of the answer involves how you move, eat and sleep, ideally discussed with someone qualified.