BMI Calculator
Body Mass Index and weight category — metric or imperial.
About BMI Calculator
A BMI calculator in metric or imperial units estimates your Body Mass Index from your height and weight and places you in a category — underweight, normal, overweight, or obese. Enter your measurements in kg and cm or in pounds, feet, and inches, and get your Body Mass Index and weight category instantly, right in your browser.
The formula and the categories
BMI is weight in kilograms divided by height in metres squared. Someone 1.75 m and 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 22.9. In imperial units the same calculation needs a conversion factor of 703: weight in pounds divided by height in inches squared, times 703. The units are kg/m², which is why the number has no intuitive meaning on its own.
The widely used adult thresholds are: under 18.5 underweight, 18.5 to 24.9 normal, 25 to 29.9 overweight, and 30 or above obese. These are not natural boundaries. They are cut-points chosen because population studies showed risk rising around them, and someone at 24.9 is not meaningfully different from someone at 25.1.
It was never designed for individuals
The formula comes from Adolphe Quetelet, a Belgian statistician working in the 1830s who was describing the distribution of a population — not assessing patients. It was adopted as a health screening tool much later precisely because it is cheap and needs only two easy measurements. That history explains its main weakness: it is a good instrument for comparing groups and a blunt one for judging a person.
It cannot distinguish muscle from fat, or where fat is carried. A muscular athlete routinely lands in the overweight or obese range while carrying very little body fat. Someone with low muscle mass can sit in the normal range while carrying enough visceral fat to matter. The number is a starting point for a conversation, not a conclusion.
Where the standard thresholds do not apply
The cut-points above are for adults aged roughly 20 to 65. Children and teenagers are assessed against age- and sex-specific percentile charts instead, because normal body composition changes throughout growth — an adult BMI category applied to a twelve-year-old is meaningless.
Population matters too. The World Health Organization has noted that health risk rises at lower BMI values in many Asian populations, and several countries use lower thresholds accordingly — often around 23 for overweight rather than 25. Pregnancy, older age, and limb loss all distort the calculation as well. If any of these apply, the standard bands are not the right yardstick.
Better measurements to pair it with
Waist circumference is the usual companion because it captures abdominal fat, which is the distribution most strongly associated with metabolic risk — something BMI is blind to. Waist-to-height ratio is simpler still and needs no chart: keeping your waist under half your height is a reasonable rule of thumb across most adults.
Tracking direction over time is generally more informative than any single reading. A BMI that has moved several points in one direction over a year says more than the absolute value on any given day. And none of this substitutes for a clinician who can see the whole picture — this calculator gives you a number, not a diagnosis.
Frequently asked questions
- How is BMI calculated from kg and cm?
- BMI is weight in kilograms divided by height in metres squared (kg/m²) — so a height in centimetres is divided by 100 first. In imperial units the tool uses weight in pounds times 703, divided by height in inches squared.
- What are the BMI categories?
- Under 18.5 is underweight, 18.5–24.9 is normal weight, 25–29.9 is overweight, and 30 or above is obese, per WHO ranges for adults.
- Is BMI accurate for everyone?
- BMI is a general screen for adults and doesn't distinguish muscle from fat, so it can misclassify athletes or older people. Treat it as a rough guide, not a diagnosis.
- Does this work in both metric and imperial units?
- Yes. Switch between metric (kilograms and centimetres) and imperial (pounds, feet, and inches) and the calculator converts and updates your BMI instantly.