Body Mass Index (BMI)
A quick measure of body weight relative to height.
What is BMI?
Body Mass Index (BMI) is a simple screening number that relates your weight to your height. It is calculated as your weight in kilograms divided by the square of your height in metres (kg/m²). Because it needs only two measurements, it is the most widely used first-pass indicator of whether someone is under- or over-weight.
BMI categories
- Under 18.5 — underweight
- 18.5–24.9 — normal weight
- 25–29.9 — overweight
- 30 and above — obese
BMI is a population-level estimate, not a diagnosis. It does not distinguish muscle from fat, so very muscular people are often flagged as "overweight" despite being lean. If that's you, try the muscle-adjusted Smart BMI calculator or measure your body fat percentage instead.
Frequently asked questions
How is BMI calculated?
BMI equals your weight in kilograms divided by your height in metres squared (kg/m²). If you use imperial units, the calculator converts pounds and feet/inches to metric first, so you get the same result either way.
What is a healthy BMI range?
A BMI between 18.5 and 24.9 is generally considered the healthy range for adults. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is classified as obese.
Is BMI accurate for athletes and muscular people?
No. BMI cannot tell muscle from fat, so muscular athletes are frequently misclassified as overweight. For a more accurate picture, use the Smart BMI calculator (which adjusts for muscle mass) or measure body fat percentage.