BMI Calculator
Body Mass Index in either unit system, plus the weight range considered healthy for your height.
What BMI is — and isn't
BMI is weight divided by height squared (kg/m²), a quick population-level screening number developed in the 1830s. It correlates with body fat across large groups, which is why clinics and insurers use it — but for an individual it knows nothing about muscle, bone density, age, sex or fat distribution. A muscular athlete can read "overweight"; an older adult with low muscle can read "normal" while carrying excess fat.
The standard categories (WHO, adults)
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy range |
| 25.0 – 29.9 | Overweight |
| 30.0+ | Obesity |
Some populations use adjusted cutoffs — for example, health agencies recommend a lower overweight threshold (23) for many Asian populations due to different cardiometabolic risk profiles at the same BMI.
Better numbers to pair it with
Waist circumference (risk rises above ~102 cm / 40 in for men, 88 cm / 35 in for women), waist-to-height ratio (keep waist under half your height), blood pressure and bloodwork tell far more than BMI alone. Treat BMI as a rough gauge and trends over time as the signal — and talk to a healthcare professional before acting on any of it.
Measurements that tell you more than BMI
BMI can't distinguish muscle from fat, or tell where fat is stored — and location matters more for health risk than total weight. These are worth measuring alongside it:
| Measure | How | Guideline |
|---|---|---|
| Waist circumference | At the navel, relaxed, after breathing out | Risk rises above ~102 cm (40 in) for men, ~88 cm (35 in) for women |
| Waist-to-height ratio | Waist ÷ height | Keep waist under half your height |
| Waist-to-hip ratio | Waist ÷ hip | Below ~0.90 (men) / ~0.85 (women) |
Waist-to-height is the easiest to remember and among the most useful: if your waist is less than half your height, that's a reassuring sign regardless of what BMI says. Fat stored around the abdomen is more strongly associated with cardiometabolic risk than fat stored on hips and thighs, which is exactly the distinction BMI cannot make.
Who BMI misclassifies
- Muscular people — athletes and regular lifters frequently read "overweight" with low body fat.
- Older adults — muscle is lost with age, so a "normal" BMI can conceal a high fat percentage.
- Very tall or very short people — the height-squared formula distorts at the extremes.
- Different ethnic groups — health agencies recommend a lower overweight threshold (around 23) for many South and East Asian populations, who show elevated risk at lower BMI values.
- Pregnant or breastfeeding women — BMI categories don't apply.
- Children and teens — must be assessed against age- and sex-specific percentile charts, not adult cutoffs.
If you want the number to change
Health authorities generally recommend 0.25–1 kg (0.5–2 lb) per week through changes you can maintain indefinitely. Faster loss tends to come from water and muscle and reliably rebounds. Note too that fitness improvements often arrive well before weight changes — blood pressure, resting heart rate, blood sugar and energy all respond to activity even when the scale doesn't move. Trends over months matter more than any single reading, and a healthcare professional can interpret yours in context.