Overview
Body Mass Index is the world's most-quoted health number and also one of its most misunderstood. It is a simple ratio — your weight divided by the square of your height — invented in the 1830s as a population statistic, not a personal verdict. This calculator gives you the number, places it in its category, and shows the healthy weight range for your height, all without sending anything anywhere.
Use it as a starting point, not a conclusion. BMI cannot distinguish muscle from fat, and it says nothing about where weight sits on your body, which matters more for health. Athletes, older adults, and growing teenagers all break its assumptions in different ways. Treat the result as one data point among many — and talk to a professional before making health decisions.
How is BMI calculated?
The formula is the same everywhere: BMI = weight in kilograms divided by height in metres squared. In imperial units it becomes 703 × weight in pounds divided by height in inches squared — the 703 is just the conversion constant that makes the two formulas agree. The calculator handles both systems and converts behind the scenes, so you never have to.
The result lands in one of four adult categories set by the World Health Organization: under 18.5 is underweight, 18.5 to 24.9 is the healthy range, 25 to 29.9 is overweight, and 30 or above is obese. These cut-offs describe populations well but individuals loosely, which is why the page also shows your personal healthy weight range — arguably the more useful number.
Features
- Metric and imperial modes with a one-tap switch — centimetres/kilos or feet-inches/pounds.
- Instant results that update as you type, with no submit button.
- Category label (underweight, healthy, overweight, obese) colour-coded for a glanceable read.
- Personal healthy range showing the weight band that keeps your BMI between 18.5 and 24.9.
- Visual scale with a marker showing exactly where you sit between 15 and 40.
- Fully private: your height and weight never leave your browser.
Benefits
- Know your number in seconds. No account, no form, no waiting.
- Both unit systems. Useful whether you think in kilos or pounds.
- Context, not just a number. The healthy range turns an abstract score into something actionable.
- Honest limits. The page says plainly what BMI cannot tell you.
Who can use it
Anyone curious about where they stand: people starting a fitness routine, tracking changes over months, or filling in a health form that asks for the number. Parents checking a teenager's growth should note that adult cut-offs do not apply the same way to children — paediatricians use age-adjusted charts instead. Coaches and trainers can use it as a quick screening step before proper body-composition measurements.
How to use
- Pick Metric or Imperial at the top of the tool.
- Enter your height (and weight) in the boxes.
- Read your BMI, category and position on the scale.
- Note the healthy weight range shown underneath — that band is your practical target zone.
- Remember the caveat: one number never tells the whole story about health.
Frequently asked questions
What is a good BMI?
For most adults, between 18.5 and 24.9 is considered the healthy range. But “good” depends on the person: a rugby player at 27 may be healthier than a sedentary person at 22. Use the number as a screening flag — if it sits well outside the healthy band, that is a reasonable prompt for a conversation with a doctor, not a diagnosis.
Why does the calculator ask for height twice in imperial mode?
Because Americans usually state height as feet plus inches (5 ft 7 in), not as 67 inches. The tool accepts the familiar form and converts it internally, so you do not have to do the arithmetic yourself.
Is BMI accurate for muscular people?
No — this is its best-known blind spot. Muscle is denser than fat, so very muscular people can land in “overweight” or even “obese” while being perfectly healthy by every other measure. For them, waist circumference or body-fat percentage are far more informative.
Does BMI work for children?
Not with adult cut-offs. Children's healthy ranges shift with age and sex, so paediatricians use percentile charts instead of the fixed 18.5–25 bands. This calculator uses the adult formula, so treat results for under-18s with extra caution.
Is my data stored anywhere?
No. The calculation happens in your browser's memory and the numbers vanish when you leave or reload the page. There is no account, no database and no analytics watching what you type.
Conclusion
BMI is a useful first glance and a poor final answer. Calculate it, note where you land and what your healthy band looks like — then judge your health by the fuller picture: how you move, sleep, eat and feel, ideally with a professional's guidance.