BMI calculator
Body mass index compares your weight to your height. It is a screening measure, not a diagnosis — useful as a rough signal at population level, and easy to misread for any one individual. This calculator gives you the number, where it sits on the scale, and an honest account of what it cannot tell you.
Your measurements
Your BMI
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Healthy range for your height
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Difference from that range
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Show the working
BMI is weight in kilograms divided by height in metres squared:
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This is a screening tool, not a medical diagnosis. BMI cannot tell you whether you are healthy. It does not measure body fat, muscle, bone density or where fat is stored, and it does not account for age, sex, ethnicity or medical history. Talk to a doctor or a registered dietitian before making decisions about your weight, and treat any single number here as one small input among many.
How to use this calculator
- Choose your units — metric takes centimetres and kilograms, imperial takes feet, inches and pounds. Your choice stays in the shared link.
- Enter your height — measure without shoes, standing straight against a wall. A two-centimetre error moves an adult BMI by roughly half a point.
- Enter your weight — ideally in the morning before eating, since normal daily fluctuation of one to two kilograms is common and will shift the result.
What the number means
The standard adult categories are: below 18.5 underweight, 18.5 to 24.9 healthy, 25 to 29.9 overweight, and 30 or above obese. These cut-offs are conventions drawn from population studies, not biological boundaries. Nothing meaningful changes between a BMI of 24.9 and 25.1.
They also are not universal. Health bodies in several countries apply lower thresholds for people of South Asian, Chinese and some South East Asian ancestry, because cardiometabolic risk in these populations tends to rise at lower BMI values. If that applies to you, the conventional healthy ceiling of 25 may be too generous.
Where BMI misleads
BMI has one input beyond height: total mass. It cannot see what that mass is made of, which produces two predictable failures.
It penalises muscle. A trained athlete carrying substantial lean tissue will often land in the overweight or obese band while having low body fat and excellent metabolic health. The number is arithmetically correct and clinically meaningless.
It misses hidden fat. The opposite case is more common and more consequential. Someone with a BMI of 23 can carry significant visceral fat around the organs, which is the deposit most strongly linked to cardiovascular and metabolic risk. BMI reports them as healthy.
It is also blind to fat distribution generally. Two people at identical BMI, one carrying weight around the abdomen and one around the hips, face measurably different risk profiles. This is why waist circumference and the waist-to-height ratio have gained ground as complements — a waist measurement under half your height is a simple and surprisingly robust screen.
What to do with the result
If your BMI sits inside the healthy band, it is weak evidence that nothing is wrong and no evidence that everything is right. Blood pressure, resting heart rate, lipid panel and fasting glucose all tell you more.
If it sits outside, treat it as a prompt to look further rather than a verdict. Take a waist measurement. Consider what has changed recently in sleep, activity and stress. And if the number concerns you, bring it to a clinician who can interpret it alongside everything BMI cannot see.
Frequently asked questions
What is a healthy BMI?
Conventionally 18.5 to 24.9 for adults. That range comes from population-level associations between BMI and mortality, so it describes where risk is lowest on average — not where any individual should be.
Is BMI accurate for everyone?
No. It performs reasonably across large populations and poorly for individuals, particularly for athletes, older adults losing muscle, pregnant people, and anyone whose body composition differs from the average the thresholds were built on.
Does BMI work for children?
Not with these thresholds. Children and adolescents are assessed on age-and-sex percentile charts, because normal body composition shifts throughout growth. Use a paediatric chart or ask a paediatrician.
BMI or body fat percentage?
Body fat percentage is more informative, since it separates fat from lean tissue. It is also harder to measure well — home scales using bioelectrical impedance are unreliable, and accurate methods like DEXA need a clinic. Our body fat calculator uses the US Navy tape method, which is a reasonable middle ground.
How quickly can BMI change?
Slowly, if the change is real. Weight fluctuates one to two kilograms daily from food, fluid and salt, so day-to-day movement in BMI is mostly noise. Weekly measurements under the same conditions give a far more honest trend.
Why does my BMI differ between calculators?
Almost always rounding, or a unit conversion done at a different point in the arithmetic. Differences under 0.2 are not meaningful. Larger gaps usually mean one calculator is using a different formula variant, such as the Trefethen index.