Free BMI Calculator – Check Your Body Mass Index Online

What is BMI?

Body Mass Index (BMI) is a number calculated from your weight and height. Organizations like the World Health Organization use it as a simple screening tool to categorize underweight, normal weight, overweight, and obesity in adults. It is a starting point—not a full health assessment.

BMI does not tell you your body fat percentage, muscle mass, or where fat is stored. For personal decisions, combine BMI with waist circumference, fitness, lab results, and advice from a qualified clinician.

BMI formula

With weight in kilograms and height in meters, BMI is:

BMI = weight (kg) ÷ height (m)²

If you use pounds and inches, the same value is: 703 × weight (lbs) ÷ height (in)². This calculator converts your entries automatically so you always get a consistent result.

BMI categories (adults, WHO-style)

Frequently asked questions

What is BMI and why does it matter?

BMI (Body Mass Index) is a screening number that relates weight to height. It helps estimate whether weight might increase risk for some health conditions. It does not diagnose illness or body fat percentage.

How accurate is BMI?

BMI is useful for population trends and quick screening, but it does not measure body fat or muscle. Athletes and older adults may get misleading results; use BMI alongside other measures and professional advice.

What are the standard BMI categories?

Common WHO adult categories: Underweight below 18.5, Normal 18.5 to 24.9, Overweight 25 to 29.9, and Obese 30 and above. Some regions use adjusted cutoffs.

Should I use metric or imperial units?

Use whichever you prefer. This calculator converts height (cm or feet and inches) and weight (kg or pounds) to metric internally so your BMI matches standard formulas.