⚖️ BMI Calculator
Body mass index with WHO adult categories. Runs privately in your browser.
What BMI actually measures
BMI divides your weight in kilograms by the square of your height in metres. The imperial path converts to the same computation using the familiar 703 multiplier. The result is compared against WHO bands for adults: below 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, 30 and above obese.
BMI was designed for populations, not individuals, and it knows nothing about body composition. A rugby player and a sedentary person of identical height and weight get identical BMIs. Treat it as a coarse screening signal that costs nothing, not a diagnosis — and read the limitations guide on this site for where it breaks down.
The formula, metric and imperial
Metric: BMI = weight (kg) ÷ height (m)². Imperial: BMI = 703 × weight (lb) ÷ height (in)². Example anchor: 1.75 m and 70 kg gives 70 ÷ (1.75 × 1.75) = 22.9 — normal weight.
Same weight, different heights
A 1.70 m person weighing 95 kg: BMI = 95 ÷ 2.89 = 32.9, in the obese band. The same weight at 1.85 m gives 27.8 — overweight. Height matters more than intuition suggests because it is squared: an extra 10 cm of height lowers BMI by roughly 10% at constant weight.
Where BMI breaks down
- Using BMI for children — paediatric charts use age- and sex-specific percentiles, not the adult bands.
- Trusting it for very muscular people, whose lean mass inflates the score.
- Ignoring fat distribution: waist circumference and waist-to-height ratio add real risk information BMI misses.
- Chasing a single decimal. The bands are wide; the difference between 24.8 and 25.1 is not clinically meaningful.
Common BMI questions
What is a healthy BMI range?
For adults, 18.5 to 24.9 on the WHO scale. It is a screening band for populations, not a diagnosis for any individual.
Is BMI accurate for athletes?
Often not — muscle is denser than fat, so muscular people routinely score 'overweight' while carrying little excess fat. Waist measures or body-composition tests are better signals for them.
Does the calculator handle imperial units?
Yes. Switch to imperial and enter feet-and-inches style height in inches plus pounds; the 703 conversion is applied automatically.
Why does the same weight give different BMIs for different people?
Because BMI divides by height squared. Taller people carry the same weight across a bigger frame, so their BMI is lower.
What should I do with my result?
Use trends, not single readings. If your BMI sits outside 18.5–24.9, a waist measurement and a conversation with a healthcare professional tell you far more than the number itself.
Other health tools
Tip: double-check inputs before relying on results for important decisions.