Health
BMI calculator
Body mass index compares your weight to your height. It's a screening figure, not a diagnosis — the notes below explain what it can and can't tell you.
The formula
Imperial: BMI = 703 × weight (lb) ÷ height (in)²
The 703 in the imperial version is just the unit conversion folded into a single constant, so both versions give the same number for the same person. Height is squared, which is why small differences in height move the result more than you'd expect.
The standard categories
These ranges come from the World Health Organization and apply to adults aged 20 and over.
| Category | BMI range |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obese | 30.0 and above |
Some health authorities in Asia use lower thresholds — often 23 for overweight and 27.5 for obese — because cardiovascular and diabetes risk tends to appear at a lower BMI in South and East Asian populations.
Where BMI stops being useful
BMI knows your weight and your height. It doesn't know what that weight is made of, or where it sits. That produces predictable blind spots:
- Muscle reads as excess weight. Muscle is denser than fat, so athletes and people who lift regularly often land in the overweight range while carrying very little body fat.
- Fat distribution is invisible to it. Two people with identical BMI can carry very different amounts of abdominal fat, which is the pattern most strongly linked to metabolic risk. Waist circumference captures this better.
- Older adults lose muscle. Body composition shifts with age, so an unchanged BMI can hide a real change underneath.
- It doesn't apply to children. Under 20, BMI has to be read against age-and-sex percentile charts, not these fixed ranges.
- Pregnancy makes it meaningless. Standard BMI ranges don't apply during pregnancy.
BMI is used widely because it's cheap and quick across large populations, not because it's precise for individuals. Treat a result near a boundary as a prompt to ask a clinician, not as a verdict.
Common questions
What's a healthy weight for my height?
The tape shows the weight range that corresponds to a BMI of 18.5 to 24.9 at the height you entered. It's a range rather than a target, and where you sit within it matters less than the other markers a doctor would look at.
Does BMI differ for men and women?
The formula and the thresholds are the same. Women typically carry a higher percentage of body fat at the same BMI, which is one more reason the number is a rough screen rather than a measure of body composition.
What should I measure instead?
Waist circumference, or waist-to-height ratio, tracks abdominal fat more directly and takes seconds with a tape measure. Body fat percentage via DEXA or bioimpedance is more precise again. A clinician will usually look at blood pressure, blood glucose and lipids alongside any of these.