Work out your body mass index from your height and weight, see where it falls on the adult BMI scale, and the healthy weight range for your height.
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BMI is your weight in kilograms divided by the square of your height in metres:
At 170 cm and 70 kg that's 70 ÷ (1.70 × 1.70) = 24.2. With pounds and inches the formula becomes 703 × weight ÷ height². You can type either, and the calculator converts to metric before it starts.
It rounds the result to one decimal, then reads it against the World Health Organization's adult scale1:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 to 34.9 | Obesity, class 1 |
| 35 to 39.9 | Obesity, class 2 |
| 40 or more | Obesity, class 3 |
Because of the rounding, a BMI shown as 25.0 always counts as overweight. Your healthy weight is anything from 18.5 × height² to 24.9 × height². The Figure option only changes the drawing. Women and men share one formula and one scale.
In 1832 the Belgian statistician Adolphe Quetelet noticed that adult weight tends to rise with the square of height, and the ratio carried his name for 140 years2. In 1972 Ancel Keys tested it against other weight-for-height measures in 7,424 men from five countries. It tracked body fat slightly better than the rest and was the easiest to work out, so he renamed it the body mass index3.
BMI can't tell muscle from fat. A muscular person can score high with little body fat, and NICE advises caution in that case4. It also ignores where fat sits. A waist measurement covers that, and the Body Fat Calculator uses one.
For many people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background, health risk starts at a lower BMI. NICE counts overweight from 23 and obesity from 27.5 for them4, close to the action points a WHO expert group proposed for Asian populations5.
A 2025 Lancet commission went further: it treats BMI as a screening number, and says excess fat should be confirmed with a waist measurement or a direct body fat test6.
Warning
The calculator is for adults aged 20 and over. Children and teens need BMI centiles for their age and sex7. BMI isn't a good measure during pregnancy or for athletes8.
World Health Organization (2000). Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894. ↩
Eknoyan G (2008). Adolphe Quetelet (1796-1874): the average man and indices of obesity. Nephrology Dialysis Transplantation 23(1):47-51. ↩
Keys A, Fidanza F, Karvonen MJ, Kimura N, Taylor HL (1972). Indices of relative weight and obesity. Journal of Chronic Diseases 25(6):329-343. ↩
National Institute for Health and Care Excellence (2025). Overweight and obesity management. NICE guideline NG246. ↩ ↩2
WHO Expert Consultation (2004). Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. The Lancet 363(9403):157-163. ↩
Rubino F, Cummings DE, Eckel RH, et al. (2025). Definition and diagnostic criteria of clinical obesity. The Lancet Diabetes & Endocrinology 13(3):221-262. ↩
Centers for Disease Control and Prevention. Adult BMI categories. ↩
NHS inform. Body mass index (BMI). ↩
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