BMI calculator
Calculate body mass index from height and weight in metric or imperial units, see which WHO category it falls in, and get the healthy weight range for your height.
Body mass index
22.9
Healthy weight
A BMI in the healthy range at this height corresponds to roughly 56.7–76.3 kg.
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How BMI is calculated
Body mass index is a single ratio of weight to height squared:
BMI = weight (kg) ÷ height (m)²For imperial units the same relationship carries a conversion constant:
BMI = 703 × weight (lb) ÷ height (in)²Someone 175 cm tall weighing 70 kg has a BMI of 70 ÷ 1.75² = 22.9. The same person measured as 5 ft 9 in and 154 lb gives 703 × 154 ÷ 69² = 22.7, the small difference coming from rounding the conversions.
The categories
| BMI | WHO category |
|---|---|
| Under 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese, class I |
| 35.0 – 39.9 | Obese, class II |
| 40.0 and above | Obese, class III |
At 175 cm, the healthy band of 18.5–24.9 corresponds to roughly 56.7 kg to 76.3 kg - a 20 kg span. That width is worth noticing: BMI describes a broad range, not a target.
What BMI is good for, and what it is not
BMI was devised in the 1830s by Adolphe Quetelet as a way to describe populations, not to assess individuals. It survives because it needs only a tape measure and a scale, which makes it usable at national scale.
It is a reasonable screening signal for groups. It is a poor instrument for any individual, because it knows nothing about:
- Muscle mass. Dense tissue raises the number without raising risk.
- Fat distribution. Visceral fat around the organs carries far more metabolic risk than the same mass on hips and thighs, and BMI cannot see the difference.
- Age. Older adults lose lean mass, so an identical BMI represents more fat at 70 than at 30.
- Frame and limb proportion. Because height is squared rather than cubed, the formula systematically overstates BMI for tall people and understates it for short people.
Better measurements to pair it with
Waist-to-height ratio. Keep your waist under half your height. It is a single measurement, needs no chart, and predicts cardiometabolic risk better than BMI in most studies.
Waist circumference. Risk rises above 94 cm for men and 80 cm for women, with lower thresholds recommended for South Asian populations.
If your BMI sits near a category boundary, the boundary is not a cliff. A BMI of 25.1 and one of 24.9 describe effectively identical bodies. Treat the number as one input among several, and take any decision about your health to a clinician who can see the rest of the picture.
Common questions
Is BMI accurate for athletes?
No. BMI cannot distinguish muscle from fat, and muscle is denser than fat. A well-trained rugby player or sprinter routinely records a BMI in the overweight or obese range while carrying very little body fat. For anyone with substantial muscle mass, waist circumference or a body composition measurement is far more informative.
Where do the BMI categories come from?
The World Health Organization set the adult cut-offs of 18.5, 25 and 30 based on population studies of mortality risk. They are statistical thresholds for groups, not diagnoses for individuals, and the WHO itself notes that the risk associated with a given BMI differs across populations.
Does BMI work the same for every ethnicity?
No. Research on East and South Asian populations shows elevated metabolic risk at lower BMI values, which is why the WHO proposed additional trigger points at 23 and 27.5 for Asian populations. Several countries, including Singapore and Japan, use lower national thresholds as a result.
Can I use this for a child?
Not directly. Children's BMI is interpreted against age-and-sex percentile charts, because healthy body composition changes as a child grows. The raw number this calculator gives is correct arithmetic, but the adult categories do not apply to anyone under 20.
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