Body mass index compares your weight to your height with a single formula. It is a screening measure used at population scale, not a diagnosis — useful as one rough signal among several, and genuinely misleading for some people. This calculator shows the number, its category and the healthy weight range for your height.
How BMI is calculated
Metric: weight in kilograms divided by height in metres squared. Someone 1.80 m and 75 kg has a BMI of 75 ÷ 1.8², which is 23.1.
Imperial: weight in pounds divided by height in inches squared, multiplied by 703. The 703 exists purely to make imperial units land on the same scale.
The categories
The World Health Organization thresholds for adults:
- Under 18.5 — underweight
- 18.5 to 24.9 — healthy weight
- 25.0 to 29.9 — overweight
- 30.0 and above — obese
Where BMI misleads
The formula knows only height and weight. It cannot tell muscle from fat, so a trained athlete is routinely classed as overweight or obese while carrying very little fat.
It runs the other way with age. Muscle is lost and fat gained over decades at a constant weight, so an older person can sit in the healthy band with a body composition that is anything but.
It also ignores where fat is carried, which is what actually matters for metabolic risk. Fat around the abdomen carries far more risk than the same weight on hips and thighs — a waist measurement or waist-to-height ratio captures that, and BMI does not.
The thresholds themselves come from studies of European-descent populations, and health risk begins at lower BMI values for people of South and East Asian descent. Several countries use lower thresholds for that reason.
What to do with the number
Treat it as one input, not a verdict. A BMI outside the healthy band is a reason to look at other measures — waist circumference, blood pressure, blood glucose, activity, how you actually feel — not a diagnosis on its own.
And if the number surprises you, that is worth a conversation with a doctor rather than a change of diet based on a web page.
FAQ
Is BMI accurate?
It is accurate as arithmetic and limited as a health measure. It reliably describes weight relative to height across a population, which is what it was designed for. For an individual it can be badly wrong in either direction, because it cannot distinguish muscle from fat or account for where fat is carried.
Does BMI work for children?
Not with these categories. Children and teenagers are assessed with age-and-sex-specific percentile charts, because body composition changes substantially during growth. Use a paediatric BMI chart instead.
What about during pregnancy?
BMI is not meaningful during pregnancy, and the categories here do not apply. Weight gain in pregnancy is monitored differently and should be discussed with a midwife or doctor.
What is a better measure?
No single number replaces it, but waist-to-height ratio is a useful companion and is easy to measure: keep your waist under half your height. Combined with blood pressure and blood glucose it gives a far better picture of metabolic risk than BMI alone.
Is my data stored?
No. The calculation runs entirely in your browser. Your height and weight are never transmitted, logged or stored anywhere.