BMI Calculator: What Body Mass Index Tells You (And What It Misses)
BMI is weight in kilograms divided by height in metres squared. It is a fast population-level screen for whether body weight sits in a healthy range for height — useful as a starting point, but not a diagnosis and not a measure of body fat.
Standard adult BMI categories
- Under 18.5 — underweight
- 18.5 to 24.9 — healthy weight
- 25.0 to 29.9 — overweight
- 30.0 and above — obese (classes I, II and III at 30, 35 and 40)
Where BMI is misleading
BMI cannot tell muscle from fat. A trained athlete can score in the overweight range with very low body fat, while someone with little muscle can sit inside the healthy band while carrying excess visceral fat. It also under-estimates risk in some South Asian populations, where thresholds are commonly lowered by about 2 points, and it is not valid for children, pregnancy or the very elderly without age-specific charts.
Better measures to pair with it
Waist circumference and waist-to-height ratio track abdominal fat, which is the fat most closely linked to cardiovascular and metabolic risk. A simple rule is to keep your waist under half your height. Combining BMI with waist measurement, blood pressure and blood glucose gives a far more useful picture than BMI alone.
Key takeaways — BMI Calculator
- BMI = kg ÷ m². Use pounds × 703 ÷ inches² for imperial units.
- It screens populations well but describes individuals poorly.
- Pair it with waist-to-height ratio for a realistic risk picture.