BMI Calculator
Calculate body mass index from height and weight, then see where it falls against both the WHO international cutoffs and the lower Asian-Pacific thresholds used across South and East Asia.
Calculate your BMI
Enter height and weight. The result updates as you type.
How BMI is calculated
Body mass index divides your weight by the square of your height. Squaring the height is what makes the index roughly comparable between a tall person and a short one. Body mass tends to scale with height squared rather than height itself.
Worked example (70 kg, 175 cm):
70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9
If you work in pounds and inches, the same relationship needs a conversion factor of 703: BMI = 703 × weight (lb) ÷ height (in)². This calculator converts your imperial entries to metric first, so both paths give the same answer.
Source: World Health Organization, Obesity: Preventing and Managing the Global Epidemic, WHO Technical Report Series 894 (2000); WHO Expert Consultation, The Lancet 363:157–163 (2004).
What the categories mean
The WHO thresholds below are the international standard. They come from large cohort studies linking BMI to mortality and to conditions like type 2 diabetes and cardiovascular disease. The relationship is a shallow U-curve: risk is lowest across a broad middle band and rises at both ends.
| BMI | WHO category | What it broadly indicates |
|---|---|---|
| Under 18.5 | Underweight | Possible undernutrition, low bone density or an underlying condition |
| 18.5 – 24.9 | Healthy weight | The band with the lowest all-cause mortality in most populations |
| 25.0 – 29.9 | Overweight | Modestly raised risk, though heavily dependent on fat distribution |
| 30.0 – 34.9 | Obesity class I | Clearly raised risk of metabolic and cardiovascular disease |
| 35.0 – 39.9 | Obesity class II | Substantially raised risk |
| 40.0 and above | Obesity class III | Highest risk band; clinical management usually indicated |
Why this calculator also shows Asian-Pacific thresholds
At any given BMI, people of South and East Asian descent tend to carry more visceral fat and show higher rates of type 2 diabetes and cardiovascular disease than white European populations. Reviewing this evidence, the WHO Expert Consultation identified additional public-health action points at 23.0 and 27.5 rather than the usual 25 and 30.
That is why a BMI of 24 can read as "healthy weight" on the international scale and "increased risk" on the Asian-Pacific one. Neither number is wrong. They are calibrated to different populations. If your ancestry is South or East Asian, the lower thresholds are the more informative of the two.
Where BMI misleads
BMI knows two things about you: your height and your weight. Everything it appears to say about body composition is inference, and there are well-documented cases where that inference fails.
- Muscular bodies. Muscle is denser than fat. A trained rugby player or powerlifter can land in the "obese" band with single-digit body fat. This is the most common and most legitimate objection to BMI.
- Older adults. Muscle mass falls with age while fat rises. Someone can hold a stable BMI for thirty years while their body composition changes substantially underneath it.
- Fat distribution. BMI cannot tell where fat sits. Visceral fat around the organs is far more metabolically harmful than subcutaneous fat on hips and thighs, and two people at BMI 28 can have very different risk profiles.
- Height extremes. The height-squared relationship holds well through the middle of the range and drifts at the tails, mildly overstating BMI for very tall people and understating it for very short ones.
- Pregnancy and childhood. BMI is not valid during pregnancy. For children and teenagers, age-and-sex percentile charts are used instead of fixed adult cutoffs.
The practical fix is to pair BMI with a waist measurement. Waist-to-height ratio in particular captures the visceral fat that BMI misses, and the two together are far more informative than either alone. Our body fat calculator takes that second step.