Health & Medicine · Biometrics
BMI Calculator
Calculate your Body Mass Index (BMI) using the standard WHO formula. Returns BMI value and weight classification.
Calculator
Formula
BMI is calculated by dividing body weight in kilograms by the square of height in metres. The result is expressed in kg/m². This formula was developed by Belgian mathematician Adolphe Quetelet in the 19th century and adopted by the WHO as a population-level screening tool.
Source: World Health Organization (WHO). Physical status: the use and interpretation of anthropometry. Geneva, 1995.
How it works
BMI is computed by dividing body mass in kilograms by the square of the height in metres. The resulting number is dimensionally equivalent to kg/m² and falls into one of four WHO-defined categories.
The WHO classification system is: Underweight (BMI below 18.5), Normal weight (18.5–24.9), Overweight (25.0–29.9), and Obese (30.0 and above). Obesity is further subdivided into Class I (30–34.9), Class II (35–39.9), and Class III (40+).
BMI is a screening tool, not a diagnostic instrument. It does not directly measure body fat percentage, and two individuals with identical BMIs may have very different body compositions.
Worked example
A person weighing 80 kg and standing 1.78 m tall has a BMI of:
BMI = 80 ÷ (1.78)² = 80 ÷ 3.1684 = 25.2 kg/m²
This places them in the Overweight category (25.0–29.9) according to WHO classification.
Limitations & notes
BMI does not account for muscle mass — a muscular athlete may have a high BMI without excess body fat. It also does not distinguish between fat distribution patterns (visceral vs subcutaneous), which carry different health risks.
BMI thresholds may not apply equally across all ethnic groups. Some Asian populations have elevated cardiovascular risk at BMI values below the standard overweight threshold. For children and adolescents, age- and sex-specific BMI percentiles should be used rather than adult cutoffs.
Frequently asked questions
What is a healthy BMI range for adults?
According to the World Health Organization, a BMI between 18.5 and 24.9 kg/m² is considered normal weight for adults. Values below 18.5 are classified as underweight, 25–29.9 as overweight, and 30 or above as obese.
Is BMI an accurate measure of body fat?
BMI is a proxy measure, not a direct measurement of body fat. It correlates reasonably well with body fat at the population level but can misclassify individuals with high muscle mass as overweight or obese. More precise measures include DEXA scans, hydrostatic weighing, or skinfold measurements.
Does BMI apply to children?
BMI can be calculated for children and adolescents using the same formula, but interpretation uses age- and sex-specific BMI-for-age percentiles rather than fixed adult cutoffs. A BMI at or above the 95th percentile for age and sex is classified as obesity in children.
How do I convert my weight from pounds and height from inches?
To convert pounds to kilograms, divide by 2.205. To convert inches to centimetres, multiply by 2.54. Alternatively, the imperial BMI formula is: BMI = (weight in lbs × 703) ÷ (height in inches)².
What are the BMI cutoffs for Asian populations?
The WHO notes that for many Asian populations, health risks increase at lower BMI values. Some health authorities use action points of 23 kg/m² for overweight and 27.5 kg/m² for obesity in Asian populations, though global standards remain at 25 and 30 respectively.
Last updated: 2025-01-15 · Formula verified against primary sources.