How BMI is calculated
Body Mass Index is a simple ratio of your weight to your height:
BMI = weight(kg) ÷ height(m)²
The World Health Organization categories are:
- Under 18.5 — underweight
- 18.5 – 24.9 — healthy weight
- 25.0 – 29.9 — overweight
- 30.0 and above — obese
Estimating body fat
BMI has a well-known blind spot: it can't tell muscle from fat. To add nuance, we also estimate body fat percentage using the Deurenberg formula, which factors in your age and sex:
Body fat % = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4
(sex = 1 for male, 0 for female)
(sex = 1 for male, 0 for female)
Treat body fat here as a ballpark. For precision, a DEXA scan, calipers, or a smart scale will be more accurate — this is a quick estimate to add context to your BMI.
Why BMI isn't the whole picture
BMI is a useful population-level screening tool, but it says nothing about where you carry weight, your muscle mass, or your fitness. A lifter and a sedentary person can share a BMI and be in completely different health. Use it as one signal among several — waist measurement, resting heart rate, energy, and how your clothes fit all matter too.