BMI Calculator

Body Mass Index (BMI) is a simple number calculated from your height and weight that the World Health Organization (WHO) uses as a screening tool to identify whether an adult is underweight, at a healthy weight, overweight, or obese. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, BMI was initially called the "Quetelet Index" and was designed for population-level health statistics — not individual clinical diagnosis.

BMI works by dividing a person's weight in kilograms by the square of their height in metres. The resulting dimensionless number correlates reasonably well with body fatness in most adults, which is why it has been adopted globally as an inexpensive, non-invasive screening measure. Research consistently shows that BMI above 25 is associated with increased risk of type 2 diabetes, hypertension, cardiovascular disease, and certain cancers. Below 18.5, risks shift toward malnutrition, bone density loss, and compromised immune function.

Despite its widespread use, BMI has significant known limitations. Most critically, it cannot distinguish between fat mass and muscle mass. A 100 kg competitive powerlifter and a sedentary 100 kg person of the same height will have identical BMI scores, but completely different health profiles. BMI also ignores fat distribution — visceral fat (around the organs) carries far higher metabolic risk than subcutaneous fat — and does not account for differences due to age, sex, or ethnicity. Several health authorities now recommend using BMI alongside waist circumference and body fat percentage for a more complete assessment.

Used appropriately, BMI remains a useful first-pass screening tool. If your BMI falls outside the healthy range, it's a signal worth investigating further — not a diagnosis. Always consult a healthcare professional for clinical assessment. Our BMI calculator uses the identical formula as the WHO standard, giving you the same result your doctor would calculate.

The BMI formula itself is one of the simplest in medicine. However, the units matter precisely — using centimetres instead of metres is the most common calculation error. Below are both the metric and imperial versions with fully worked examples.

Step-by-Step Summary

1. Get your height in metres — divide centimetres by 100 (e.g. 175 cm → 1.75 m)
2. Square your height — multiply it by itself (e.g. 1.75 × 1.75 = 3.0625)
3. Divide your weight (kg) by that number — this gives your BMI
4. Round to one decimal place and compare against the WHO categories below

Note that the multiplication constant of 703 in the imperial formula is simply the conversion factor for (kg/m²) → (lb/in²). Both formulas produce identical results when the same person's measurements are used — it's purely a unit conversion.

The WHO BMI classification system divides body weight into six categories using the same cut-off points for all adult populations. Each category is associated with different levels of health risk for conditions like type 2 diabetes, cardiovascular disease, and all-cause mortality.

BMI Ranges for Children and Teenagers

The adult BMI categories above do not apply to people under 18. Children and adolescents use a different system based on age- and sex-specific BMI percentile charts developed by the CDC and WHO. A child's BMI is compared to others of the same age and sex, then assigned to a percentile range (underweight <5th, healthy weight 5th–85th, overweight 85th–95th, obese >95th). Use a dedicated pediatric BMI calculator for anyone under 18 years.

Ethnic-Specific BMI Considerations

Research shows that people of South Asian (Indian, Pakistani, Bangladeshi) and East Asian (Chinese, Japanese, Korean) descent develop metabolic complications — including type 2 diabetes and cardiovascular risk — at lower BMI values than people of European descent. The WHO Expert Consultation on BMI in Asian Populations proposed lower action points for these groups:

• BMI ≥ 23 may indicate increased risk (compared to ≥ 25 in European populations)
• BMI ≥ 27.5 may indicate high risk (compared to ≥ 30 in European populations)

If you are of South or East Asian descent, consider these lower thresholds when interpreting your result, and discuss your individual risk with a healthcare professional.

BMI for Older Adults

For adults over 65, a slightly higher BMI of 25–27 is associated with lower mortality than the standard "healthy" range. This is because low BMI in older populations often indicates loss of muscle mass (sarcopenia) or malnutrition, which carries significant health risks. Some geriatric guidelines use 22 as the lower cut-off for underweight in elderly populations.