What is BMI?
Body Mass Index (BMI) is a simple screening measure that uses your height and weight to estimate whether your weight falls into a healthy range. It was developed in the 19th century by Belgian mathematician Adolphe Quetelet and is still used today by health organizations worldwide as a quick, low-cost way to flag potential weight-related health risks at a population level.
How is BMI calculated?
BMI is calculated by dividing weight in kilograms by height in meters squared: BMI = weight (kg) ÷ height (m)². In imperial units, the formula is weight (lb) ÷ height (in)² × 703. The calculator above does this conversion automatically, whichever unit system you choose.
BMI categories explained
The World Health Organization defines four standard BMI bands for adults: under 18.5 is classified as underweight, 18.5 to 24.9 is considered normal weight, 25 to 29.9 is overweight, and 30 or above is classified as obese. These bands are population-level guidelines, not individual diagnoses.
Limitations of BMI
BMI does not distinguish between muscle and fat, so athletes and very muscular individuals often score as "overweight" despite low body fat. It also doesn't account for fat distribution, bone density, age, sex, or ethnicity, all of which affect health risk independently of BMI. Doctors typically use BMI as one input alongside waist circumference, blood pressure, and other measures — not as a standalone diagnosis.
Frequently asked questions
Is BMI accurate for everyone? No. It's a useful population-level screening tool but less reliable for athletes, older adults, pregnant women, and children, who need different reference standards.
What's a healthy BMI? For most adults, 18.5–24.9 is considered the normal range, though healthy weight varies by individual factors like muscle mass and frame size.
What is BMR and why does it matter? Basal Metabolic Rate is the number of calories your body needs at complete rest just to maintain basic functions like breathing and circulation. It's the foundation for estimating your total daily calorie needs.
How is daily calorie need (TDEE) different from BMR? TDEE (Total Daily Energy Expenditure) adds your activity level on top of BMR, giving a more realistic estimate of how many calories you burn — and need to eat — in an average day.