Live demo (works without JavaScript):
Sample: Body Mass Index = weight(kg) ÷ height(m)². 70 kg, 1.75 m → BMI ≈ 22.9.
enter values in the form → exact resultWritten by Sarah Mitchell · Reviewed by Alex Chen · July 26, 2026
Calculate your Body Mass Index using WHO-classified categories — metric or imperial units.
Enter values and click Calculate.
Body mass index is a screening number derived from your height and weight that gives a rough sense of whether your weight sits in a healthy range for your frame. This calculator follows the standard formula used by health organizations worldwide: it divides weight by height squared and maps the result onto the well-known WHO category bands. Because the math runs in your browser, you can experiment freely with different heights and weights without waiting for a server response or worrying about any data leaving your device.
BMI is a height-adjusted weight metric rather than a direct measurement of body fat. The idea is simple: two people of the same weight can have very different builds, so weight alone is a poor comparison tool. Dividing by height squared partially corrects for that, producing a number that lets clinicians and researchers compare people on a common scale. A person who is 1.75 m tall and weighs 70 kg, for example, has a BMI of 22.9, which sits squarely inside the normal range. The imperial version of the formula reaches the same scale using pounds and inches with the standard 703 conversion factor.
The calculator starts in imperial units, so you can type your height in feet and inches and your weight in pounds and get an instant result. Switch to metric and the form accepts centimeters and kilograms instead. The calculation itself uses the two canonical formulas: weight divided by height in meters squared for metric input, and weight multiplied by 703 then divided by height in inches squared for imperial input. Every keystroke triggers a recalculation, and the result panel shows your BMI rounded to one decimal place alongside the matching category band. Note that switching between unit systems clears the numeric fields rather than converting what you already entered, so re-enter your measurements after switching.
The result is sorted into the standard World Health Organization bands: below 18.5 is underweight; 18.5 to 24.9 is normal; 25.0 to 29.9 is overweight; 30.0 to 34.9 is class I obesity; 35.0 to 39.9 is class II obesity; and 40.0 and above is class III obesity. Each band is color-coded in the output so you can see at a glance where the number lands. These thresholds are the same ones used by the WHO and the CDC for adult screening, and they apply to adults of both sexes.
Take a person who is 5 feet 9 inches tall, which is 69 inches, and weighs 154 pounds. Square the height first: 69 times 69 is 4,761. Then multiply the weight by 703, giving 108,262. Dividing 108,262 by 4,761 yields 22.7, again inside the normal band. Working the two systems side by side makes it clear that the scale means the same thing no matter which units you type in.
BMI is a screening tool, not a diagnosis. It does not distinguish muscle from fat, so an athlete with very low body fat can register as overweight while an older adult with reduced muscle mass can appear normal despite a higher fat percentage. It also takes no account of where fat sits on the body, even though abdominal fat carries greater health risk than fat stored elsewhere. For a fuller picture, pair BMI with waist circumference, body fat percentage, or a conversation with a healthcare professional.
Anyone tracking weight trends over time, preparing for a health screening, or simply curious about where their height and weight place them on the standard scale will find this tool useful. Because it is instant and free, it is a practical way to check the direction your weight is moving between doctor visits. If you are pregnant, under 18, or an athlete with significant muscle mass, talk to a professional before interpreting the result.
Switching units clears the input fields rather than converting the numbers, so the displayed BMI reflects whatever values are currently entered. Re-enter your measurements in the new system and the result updates immediately.
The imperial version uses the standard 703 factor: weight in pounds divided by height in inches squared, then multiplied by 703. This produces the same numeric scale as the metric formula.
Yes, the adult BMI scale and its category thresholds are identical for both sexes. The same bands are used worldwide for screening purposes.
BMI is a weight-for-height index and does not measure body composition directly. Body fat percentage, taken with calipers, bioimpedance scales, or a DEXA scan, says more about the ratio of muscle to fat, but those methods need equipment and trained hands.
This calculator applies the adult scale. For children and teenagers, BMI is interpreted using age- and sex-specific percentile charts, so the adult bands do not apply.
One decimal place matches the precision of the underlying measurements. Because BMI changes slowly with small weight shifts, a single decimal is enough to track meaningful trends without implying false accuracy.
WHO obesity and overweight fact sheet — the category bands above follow this classification.