What is BMI?
Body Mass Index (BMI) is a simple number calculated from your height and weight that screens for weight categories associated with health risks. It's used by the WHO, doctors and researchers worldwide because it's fast, free and works reasonably well at the population level.
BMI formula
- Metric: BMI = weight (kg) ÷ height (m)²
- Imperial: BMI = 703 × weight (lb) ÷ height (in)²
What is BMI Prime?
BMI Prime is your BMI divided by 25 (the upper healthy limit). A value under 1.0 means you're within or below the healthy range; 1.1 means you're 10% above it. It makes it easy to see at a glance how far from the healthy boundary you are.
What is the Ponderal Index?
The Ponderal Index (kg/m³) divides weight by height cubed instead of squared. It corrects a known BMI weakness: BMI tends to overestimate for tall people and underestimate for short people. A typical healthy PI is roughly 11–15 kg/m³.
Limitations of BMI
- Doesn't separate muscle from fat — athletes often read high while lean
- Doesn't show where fat is stored (belly fat is riskier)
- Adult thresholds don't apply to children, and may shift slightly for some ethnic groups
For a fuller picture, pair BMI with the body fat calculator and a waist measurement.
Is BMI accurate?
As a quick screen, yes — but it's one data point, not a diagnosis. Trends over time and body composition matter more than a single reading.
What BMI should I aim for?
Anywhere in the 18.5–24.9 healthy band. Within that band, the weight where you feel strong and energetic is the right one.
Does age or gender change BMI?
The formula is identical for adult men and women. Past ~65, slightly higher BMIs (up to ~27) are often considered acceptable.