How BMI is calculated
Body mass index divides your weight in kilograms by the square of your height in metres:
BMI = weight (kg) ÷ height (m)²
Example: 75 kg ÷ 1.70² = 75 ÷ 2.89 = 25.95
The result is a single number that places you in one of four NHS categories. It was designed in the 1830s by Belgian mathematician Adolphe Quetelet as a population-level statistical tool — not as an individual diagnostic.
NHS BMI categories
The limits of BMI
BMI has three well-documented blind spots:
- It does not distinguish muscle from fat. A rugby player and a sedentary person can share the same BMI with completely different body compositions and health profiles.
- Ethnic variations. Research shows that South Asian and Black populations may experience metabolic risk at lower BMI thresholds. The NHS acknowledges this but still uses universal cut-offs in its standard guidance.
- Age limitations. Older adults tend to lose muscle and gain fat without their BMI changing much. A "healthy" BMI in a 75-year-old may mask significant sarcopenia (muscle loss).
Waist circumference is a better predictor of visceral fat and metabolic risk. For men, above 94 cm increases risk; above 102 cm substantially increases it. For women, the thresholds are 80 cm and 88 cm.
When BMI matters and when it does not
BMI is useful as a quick population-level screening tool and for tracking your own trend over time. If your BMI has risen steadily over several years and you are not strength training, that trend is worth paying attention to regardless of the absolute number.
BMI is not useful for assessing fitness, athletic performance, or body composition. If you train regularly, use body-fat percentage, waist measurements, or progress photos alongside (or instead of) BMI.