How BMI is calculated
Body Mass Index = weight (kg) ÷ height (m)². It's a quick screening tool that estimates whether your weight is proportionate to your height, using standard World Health Organization categories: underweight, healthy range, overweight, and obese.
Limitations of BMI
BMI doesn't distinguish between muscle and fat mass, so athletes or very muscular individuals can show a "high" BMI despite having low body fat. It also doesn't account for fat distribution, age, or sex differences in body composition. For a more complete picture, pair it with our Body Fat Calculator.
BMI categories
Under 18.5 is classified as underweight, 18.5–24.9 as healthy range, 25–29.9 as overweight, and 30+ as obese. These thresholds are general population guidelines and may not apply equally to all ethnicities, ages, or body types.
Frequently asked questions
Is BMI accurate for children? No — children and teens need age- and sex-specific BMI percentile charts, not the adult categories used here.
Should I worry about a single BMI reading? A single number is just a starting point — trends over time, along with other health markers, give a much more useful picture than one measurement alone.