BMI Calculator

What is BMI?

Body Mass Index (BMI) is a simple measure that uses your height and weight to determine if you are in a healthy weight range. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and remains one of the most widely used health screening tools worldwide.

How is BMI Calculated?

The formula is straightforward: BMI = weight (kg) / height² (m²). For imperial units: BMI = 703 × weight (lb) / height² (in²). Our calculator handles both systems automatically — just enter your measurements and get instant results.

BMI Categories (WHO Standards)

Underweight: BMI below 18.5. Normal weight: BMI 18.5–24.9. Overweight: BMI 25–29.9. Obesity Class I: BMI 30–34.9. Obesity Class II: BMI 35–39.9. Obesity Class III: BMI 40 and above. These categories help identify potential health risks associated with weight status.

BMI for Children and Teens

For children aged 2-20, BMI is calculated the same way but interpreted differently using age- and sex-specific percentiles. A child in the 85th-95th percentile is considered overweight, above 95th is obese. Use our Growth Chart tool for pediatric assessment.

Limitations of BMI

BMI does not distinguish between muscle and fat. Athletes with high muscle mass may have elevated BMI without excess body fat. BMI also does not account for body fat distribution (where fat is stored). For a more complete picture, combine BMI with our Body Fat and Waist-to-Hip Ratio calculators.

Why Use Our BMI Calculator?

Instant results in both metric and imperial units. No sign-up, no data collection, 100% private — all calculations happen in your browser. Available in 14 languages. Mobile-friendly design. Based on WHO and CDC standards. Use it anywhere, anytime, completely free.

BMI Calculator for Women

BMI ranges are the same for men and women, but women naturally carry more body fat. A healthy BMI of 18.5-24.9 applies to both genders. However, women should also track body fat percentage and waist-to-hip ratio for a complete health picture. Pregnancy, menopause, and hormonal changes all affect body composition in ways BMI alone cannot capture.

BMI Calculator by Age — Does Age Matter?

BMI does not directly account for age, but healthy BMI ranges shift slightly with age. Older adults may benefit from a slightly higher BMI (23-27) as some studies suggest this range offers protective benefits. For children and teens, BMI is interpreted using age- and sex-specific percentiles. Use our Growth Chart tool for pediatric BMI assessment.

Is BMI Accurate for Athletes and Muscular People?

BMI often overestimates body fat in muscular individuals because muscle weighs more than fat by volume. A bodybuilder with 8% body fat can have a BMI of 30+, technically "obese" by BMI standards. For athletes, body fat percentage (use our Body Fat Calculator) and waist-to-hip ratio are more accurate health indicators than BMI alone.