BMI Calculator

How BMI is calculated—and what it can and cannot tell you

Body mass index (BMI) is a screening calculation based on weight and height. It does not directly measure body fat or diagnose a health condition, and it cannot account for every difference in body composition or medical history.

Formula and example

With metric measurements, BMI = weight in kilograms ÷ (height in metres)². For example, 70 kg at 1.75 m gives 70 ÷ (1.75 × 1.75) ≈ 22.9. The imperial equivalent is weight in pounds divided by height in inches squared, multiplied by 703.

Important age limitation

The category labels used by this tool are adult BMI ranges and are intended for adults age 20 and older. For children and teens ages 2–19, BMI is interpreted using age- and sex-specific percentiles rather than adult cutoffs. This tool does not calculate those percentiles; use the CDC Child and Teen BMI Calculator for that age group.

For adults too, a BMI number is only one piece of context. The CDC Adult BMI Calculator notes that other factors—including medical history, health behaviors, physical examination findings, and laboratory results—matter when assessing health. Discuss questions about your own results with a qualified healthcare professional.

Tips for accurate results

Measure height without shoes and weigh yourself at a similar time of day, because body weight can shift by a kilogram or two between morning and evening. Check that the units match the system you selected, since entering centimeters as meters or pounds as kilograms produces results that look absurd.

Common mistakes and limits

BMI is a screening ratio, not a diagnosis. It does not separate muscle from fat, does not describe where fat is stored, and is interpreted differently for children and teenagers using growth percentiles. Use it as one of several measures alongside waist size, fitness, and medical history.

Learn more: Understanding BMI: what it measures and what it misses.