BMI Calculator
Type your height and weight and the calculator returns your BMI (kilograms per square metre) along with the World Health Organization category: underweight, normal, overweight, or one of three obesity classes. It also shows the weight range that would land you in the normal band for your height, so you can see the gap in concrete kilos or pounds — useful for a baseline, not a diagnosis.
How BMI is calculated
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1
Enter height and weight
Switch between metric (cm / kg) and imperial (ft-in / lb). Input precision: 0.5 cm and 0.1 kg.
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2
Formula is applied
BMI = weight (kg) / height (m)^2. Imperial is converted internally; the output BMI is always in kg/m².
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3
Category is looked up
The WHO cut-offs (18.5, 25, 30, 35, 40) classify the result into one of six bands.
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4
Healthy-weight range appears
For your specific height, the calculator shows what you would weigh at BMI 18.5 and BMI 25 — your "normal" window.
WHO adult BMI categories
| BMI (kg/m²) | Category |
|---|---|
| < 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| ≥ 40.0 | Obesity class III |
Asian-population guidelines (IOTF, WHO Asia Pacific) use lower thresholds — 23 for overweight, 27.5 for obesity — because body fat percentages tend to run higher at any given BMI.
Where BMI falls short
- Muscle mass: athletes and lifters often test “overweight” by BMI despite low body fat. Rugby forwards and bodybuilders routinely land in the 28-32 range.
- Age and sarcopenia: older adults lose muscle, so a “normal” BMI can mask under-muscled frames.
- Body composition: BMI says nothing about where fat is distributed. Abdominal (visceral) fat is a bigger cardiometabolic risk than hip/thigh fat at the same BMI.
- Pregnancy: BMI is not meaningful during pregnancy or the first few months postpartum.
Better companion measures
- Waist-to-height ratio: keep waist < half your height.
- Body fat percentage: via DEXA, hydrostatic weighing or a good bioimpedance scale.
- Waist circumference: > 102 cm (40 in) for men and > 88 cm (35 in) for women flags elevated risk regardless of BMI.
Frequently Asked Questions
It’s a population-level screening tool, not a diagnosis. Muscular, pregnant, very tall/short, elderly and some ethnic groups routinely get misclassified. Use it as one data point among several.
Asian populations show elevated metabolic risk at lower BMI values, so WHO Asia Pacific sets overweight at 23 and obesity at 27.5. The calculator shows both scales when that region is selected.
Only with age- and sex-specific percentile charts — adult categories do not apply. The calculator is for adults 18 and older; use a paediatric BMI-for-age tool for children.
The span from BMI 18.5 to 24.9 is what WHO calls normal. For your height, the calculator prints that as a specific kg or lb window, and the lower half often maps to better long-term cardiovascular markers.
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