BMI Calculator
Find your BMI, your healthy weight range, and exactly how many pounds separate you from it.
About this tool
What this BMI calculator does
Body Mass Index (BMI) is a quick screening number that compares your weight to your height. It won't tell you your body fat percentage or how much of your weight is muscle, but it's still the number doctors, insurers, and fitness apps reach for first because it takes ten seconds to calculate and correlates reasonably well with health risk across large populations. This calculator works in both metric (kilograms and centimeters) and imperial (pounds, feet and inches) units, and goes a step further than a bare number: it shows your BMI category, your BMI Prime, your ponderal index, and the actual weight range that counts as 'healthy' for your height.
How to use the BMI calculator
- Pick metric or imperial units from the dropdown.
- Enter your height and current weight.
- Optionally add your age and sex — we don't change the BMI formula for these, but we'll flag when standard adult ranges don't really apply (mainly for under-18s).
- Your BMI, category, and healthy weight range update instantly as you type.
Reading your result
The standard adult categories (WHO) are: under 18.5 is underweight, 18.5–24.9 is normal weight, 25–29.9 is overweight, and 30+ is obese, split into Class I, II and III. We plot your exact position on a gradient bar so you can see how close you are to the next band, not just which one you landed in.
What is BMI Prime?
BMI Prime is your BMI divided by 25 (the upper edge of the healthy range). A BMI Prime of 1.0 means you're sitting exactly at that upper healthy cutoff — anything below 1.0 is inside the healthy band, and it's an easy way to see by what fraction you're over or under.
What is Ponderal Index?
Ponderal Index divides weight by height cubed instead of squared. It's less commonly used than BMI but scales a little more sensibly for people who are very tall or very short, where BMI can skew high or low even at a perfectly healthy build.
Common questions
Is BMI accurate for everyone?
No. BMI doesn't distinguish muscle from fat, so it tends to overestimate body fat in muscular athletes and can underestimate it in older adults who've lost muscle mass. It's a population-level screening tool, not a diagnosis — for a more direct estimate of body fat, use our Body Fat Percentage Calculator alongside this one.
What's a healthy BMI range?
For most adults, 18.5 to 24.9 is considered the healthy range. This calculator converts that range into an actual weight for your specific height, so instead of just a number you get 'you'd need to gain/lose X kg (or lb) to reach it.'
Does BMI work the same for men and women?
The formula itself doesn't change by sex, though body composition norms differ — women naturally carry a higher percentage of body fat than men at the same BMI. We collect sex as optional context, not as an input to the formula.
Why do I need my age?
Standard BMI cutoffs are calibrated for adults (18+). For children and teens, doctors use age- and sex-specific growth percentiles instead of fixed numbers, so we show a note rather than a misleading category if you enter an age under 18.
How is BMI calculated?
Metric: BMI = weight (kg) ÷ height (m)². Imperial: BMI = weight (lb) ÷ height (in)² × 703. Both give the same result — we just handle the unit conversion for you.