BMI Calculator
Body mass index, metric or imperial.
BMI Calculator at a glance
BMI Calculator is a free online calculator you can use right now to body mass index, metric or imperial โ no account, no install and no usage limit. It runs entirely inside your browser, so whatever you enter stays on your own device.
- Price
- Free โ no trial, no paid tier, no watermark
- Sign-up
- Not required
- Where it runs
- In your browser โ nothing is uploaded to a server
- Works on
- Chrome, Safari, Firefox and Edge โ desktop, tablet and phone
- Category
- Calculators
About BMI Calculator
The BMI Calculator works out your Body Mass Index from your height and weight, in either metric or imperial units, and shows where it falls on the standard World Health Organization scale.
The formula is weight in kilograms divided by height in metres squared. The WHO bands are: under 18.5 underweight, 18.5-24.9 healthy weight, 25-29.9 overweight, and 30 or above obese. Those thresholds are population-level guidance drawn from large studies, not a diagnosis of any individual.
It's worth understanding the limits, because BMI is widely misread. It takes no account of body composition, so muscular people are routinely classed as overweight despite low body fat โ this is a well-known issue with athletes. It's also less reliable for older adults, pregnant people, and children, who need age- and sex-specific percentile charts rather than the adult formula.
How to use BMI Calculator
- Type your numbers into the fields.
- The result updates the moment you change a value.
- Adjust any input to compare different scenarios.
Frequently asked questions
What is a healthy BMI range?
The WHO defines 18.5 to 24.9 as the healthy range for adults. Below 18.5 is classed as underweight, 25 to 29.9 as overweight, and 30 or above as obese.
Why does BMI say I'm overweight when I'm fit?
BMI can't distinguish muscle from fat, and muscle is denser. Athletes and regular strength trainers often land in the overweight band with low body fat. Waist circumference or a body composition measurement gives a far better picture.
Is BMI accurate for children?
No โ children need age- and sex-specific percentile charts, because healthy body composition changes throughout growth. Ask a paediatrician rather than applying the adult formula.
How is BMI calculated?
Weight in kilograms divided by height in metres squared. In imperial units it's weight in pounds divided by height in inches squared, multiplied by 703.