How to Calculate BMI (and What It Actually Tells You)
The body mass index formula in metric and imperial units, the standard categories, and where the measure falls short.
4 min read
Step by step
- Metric: divide weight in kilograms by height in meters squared.
- Imperial: divide weight in pounds by height in inches squared, then multiply by 703.
- Example: 70 kg at 1.75 m is 70 ÷ 3.0625 = 22.9.
- Compare against the standard bands: 18.5–24.9 is the healthy range.
Key takeaway: BMI = weight ÷ height² — a screening tool, not a diagnosis.
What BMI measures
Body mass index is weight divided by height squared: kg ÷ m², or (lb ÷ in²) × 703 in imperial units. It is a screening measure that places adults into standard categories: under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight, 30 and above obese.
Example: 70 kg at 1.75 m gives 70 ÷ 3.0625 = 22.9, comfortably inside the healthy band.
Where BMI falls short
BMI cannot distinguish muscle from fat, so muscular athletes often read as 'overweight', and it says nothing about where fat is carried, which matters for health risk. It is also calibrated for adults, not children or teenagers.
Treat BMI as a first-pass screen. Waist circumference, body composition and blood markers add the context BMI lacks — and a clinician adds more.
Using the number sensibly
Track the trend, not the single reading. A BMI moving from 31 toward 27 over six months is meaningful progress even though both readings sit in the same broad conversation about weight.
Set goals in small steps: for most adults, a 5–10% change in body weight is a realistic and health-relevant first target.
Do it automatically
Body mass index and weight category from height and weight.
Open the BMI Calculator