How this is calculated
BMI = weight in kilograms ÷ (height in metres)². For imperial units the formula is weight in pounds ÷ (height in inches)² × 703, which is the same relationship with a unit conversion folded in.
Worked example at 170 cm and 70 kg: height in metres is 1.70, squared is 2.89. BMI = 70 ÷ 2.89 = 24.2, which falls in the normal-weight range.
The WHO categories are: under 18.5 underweight, 18.5 to 24.9 normal, 25.0 to 29.9 overweight, and 30.0 and above obese, subdivided into three classes. The boundaries are round numbers chosen for convenience, not thresholds where anything biological changes — 24.9 and 25.1 are not meaningfully different.
Where BMI gets it wrong
Muscle is denser than fat, so muscular people are routinely classified as overweight or obese while carrying low body fat. Many professional athletes score above 30. In the other direction, someone who has lost muscle with age can sit in the normal range while carrying a high fat percentage — sometimes called normal-weight obesity.
BMI also says nothing about fat distribution, which is what most strongly predicts metabolic risk. Visceral fat around the abdomen is considerably more dangerous than the same mass carried on hips and thighs, and BMI cannot see the difference.
Population differences matter too. Several health authorities apply lower thresholds for people of South and East Asian descent, who show elevated metabolic risk at BMI values within the standard normal range. If that applies to you, treat the standard categories as optimistic.
Better measures to use alongside it
Waist circumference is the simplest and most informative addition. Above roughly 94 cm for men and 80 cm for women indicates raised risk, with higher thresholds marking substantially raised risk — and these numbers capture the abdominal fat that BMI misses entirely.
Waist-to-height ratio is simpler still and arguably better: keep your waist under half your height. It needs no tables, works across most adult populations, and correlates with metabolic risk more closely than BMI does. Body fat percentage, measured by the Navy method or by a scan, gives the most direct answer if you can obtain it.
How to use the bmi calculator
- Enter your height in centimetres. Measure without shoes. A few centimetres of error changes BMI noticeably, because height is squared.
- Enter your weight in kilograms. Weigh yourself at a consistent time of day — morning, before eating, is the usual convention.
- Read the category as a screen, not a verdict. If you are muscular, or of South or East Asian descent, check your waist circumference before drawing any conclusion from the category.