Free BMI Calculator — body mass index, metric & imperial
Calculate your body mass index from height and weight in metric or imperial units, and see which WHO category the result falls into. Free and instant.
About the BMI calculator tool
Body mass index is weight in kilograms divided by height in metres squared. It was devised in the 1830s by a Belgian statistician studying the characteristics of populations, and that origin explains both its usefulness and its limits: it was built to describe groups, not to assess the individual standing in front of you. It remains widely used because it needs only two measurements anyone can take, which makes it practical at a scale no better method matches.
The World Health Organization categories are the ones shown here — under 18.5 underweight, 18.5 to 24.9 normal, 25 to 29.9 overweight, and 30 or above obese. These are population thresholds where health risks begin to shift statistically, not diagnostic boundaries. Crossing one does not mean something is wrong, and sitting inside the normal band does not mean everything is right; both are probabilities applied to a large group rather than statements about a person.
The formula's best-known flaw is that it cannot distinguish muscle from fat. Muscle is considerably denser, so athletes and physically active people routinely register as overweight or obese while carrying very little body fat. The same insensitivity works the other way: someone with low muscle mass and higher fat can score in the normal range while carrying genuine metabolic risk. BMI measures a ratio of mass to height and nothing about what the mass consists of.
It also does not know where fat is stored, which is what much of the health risk actually depends on. Fat around the abdomen and internal organs is associated with considerably worse outcomes than the same amount carried on hips and thighs, and BMI cannot tell the difference. Waist circumference and waist-to-height ratio capture that and are useful alongside it. Ethnicity matters too — risk thresholds occur at lower BMI values in South and East Asian populations, and several countries use adjusted cut-offs.
How it works
Choose your units
Metric for centimetres and kilograms, imperial for inches and pounds. Both give the same result.
Enter height and weight
Two measurements are all the formula needs. Nothing is sent anywhere or recorded.
Read it in context
The number and its WHO category appear — a rough population indicator, not a personal diagnosis.
Frequently asked questions
- What are the BMI categories?
- The World Health Organization defines under 18.5 as underweight, 18.5 to 24.9 as normal, 25 to 29.9 as overweight and 30 or above as obese. These mark where risk shifts across a population rather than diagnosing an individual, and several countries apply lower thresholds for people of South and East Asian descent, where risk appears at lower values.
- Is BMI accurate for athletes?
- Often not. Muscle is much denser than fat, so a lean, muscular person can register as overweight or even obese while carrying very little body fat. The index measures the ratio of total mass to height and has no way to distinguish what that mass is made of. For anyone with substantial muscle, it is the wrong tool.
- What is a healthy BMI?
- The normal band runs from 18.5 to 24.9, but treat it as a rough guide rather than a target. Health is determined by fitness, diet, blood pressure, blood markers, sleep and much else that BMI cannot see. Someone at 26 who exercises regularly may be considerably healthier than someone at 23 who does not.
- Does BMI work for children?
- Not with these categories. Children and adolescents are assessed against age and sex-specific percentile charts, because body composition changes substantially through growth and a fixed threshold means nothing at eight years old. Use the growth charts your national health service publishes, and interpret them with a paediatrician rather than alone.
- What should I use alongside it?
- Waist circumference is the most useful addition, because abdominal fat carries considerably more risk than fat elsewhere and BMI cannot detect the difference. Waist-to-height ratio is a simple and well-supported measure — keeping your waist under half your height is a reasonable rule. Body fat percentage is more informative still where it can be measured properly.
- Does it apply during pregnancy?
- No. Weight gain in pregnancy is expected and necessary, and the standard categories are not meaningful during it or in the period shortly after. Guidance on healthy gain during pregnancy is specific to your circumstances and starting point, and should come from the midwife or doctor managing your care.
- Is my height and weight stored anywhere?
- No. The calculation happens in your browser tab and nothing is transmitted, saved or logged. Body measurements are health data, and a calculator has no legitimate reason to collect them. Closing the page discards everything you typed.