BMI Calculator
Calculate your Body Mass Index using metric or imperial units. Get WHO classification and health insights.
Enter Your Measurements
Your Result
—
Enter your measurements to see your BMI
How BMI is calculated
Body Mass Index divides body mass by the square of height: BMI = kg / m². For imperial inputs the same ratio is applied with a 703 conversion factor, so BMI = (lb / in²) × 703. This calculator converts your entered height and weight to SI units first, then rounds the result to one decimal place, which is the precision used in clinical charts. Because the formula depends on the square of height, small measurement errors in height affect the result roughly twice as much as the same relative error in weight — measure height without shoes and against a flat wall for the most reliable figure.
Reading the WHO categories
The World Health Organization classifies adult BMI as underweight below 18.5, healthy weight from 18.5 to 24.9, overweight from 25.0 to 29.9, and obese at 30.0 and above, with obesity further split into class I (30.0–34.9), class II (35.0–39.9) and class III (40.0 and over). These cut-off points describe population-level risk, not an individual diagnosis. Some national health bodies apply lower thresholds for people of South Asian, Chinese and Japanese descent, where cardiometabolic risk rises at a lower BMI, commonly using 23.0 as the overweight boundary.
Limits of the measure
BMI does not distinguish muscle from fat, nor does it describe where fat is stored. Trained athletes frequently score in the overweight range while carrying low body fat, and older adults can lose muscle while their BMI stays flat. Waist circumference, waist-to-height ratio and body-fat measurement add information BMI cannot provide. BMI is also not valid for children and teenagers, who are assessed against age- and sex-specific percentile charts instead, nor for pregnant people. Treat the number here as one screening signal and discuss any concern with a clinician.
Cite this page
UnitConvertLab. (2026). BMI Calculator. UnitConvertLab. Retrieved from https://unitconvertlab.com/ar/tools/bmi-calculator