How to Calculate BMI (Body Mass Index)
BMI is weight divided by height squared. It is a **screening** tool for weight categories, not a direct measure of body fat or health. Pair it with waist circumference, body composition, or clinical context when decisions matter.
Core formula
BMI = kg / m² · US: BMI = 703 × lb / in²Step by step
1. Use consistent units
Metric: weight in kg, height in meters. US: weight in pounds, height in inches, then multiply by 703.
2. Compute and read category
Under 18.5 underweight; 18.5–24.9 normal (adult screening bands); 25–29.9 overweight; 30+ obesity categories.
3. Interpret limits
Muscular builds may read “overweight” with low fat; older adults may have normal BMI with low muscle mass.
BMI vs other body metrics
Use BMI for quick screening; use other tools when precision affects training or medical plans.
- BMI: Fast, population screening; misses muscle vs fat split.
- Body fat %: Better for composition; methods vary (Navy, bioimpedance).
- Waist circumference: Central adiposity signal; useful with BMI.
- Ideal weight formulas: Heuristic targets—not diagnoses.
Use our calculators
Common mistakes
- Mixing cm with inches
- Using shoes-on height
- Treating BMI as a diagnosis
FAQ
Is BMI accurate for athletes?
Often no—muscle increases weight without the same health risks as high fat.
Children?
Use age/sex percentiles—not adult cutoffs.