BMI Formula Explained: How to Calculate Body Mass Index
BMI is one of the most widely used health screening tools in the world. Here's what it measures, how the formula works, what your result means, and its important limitations.
What Is BMI?
Body Mass Index (BMI) is a simple numerical value derived from your height and weight. It was developed by Belgian mathematician Adolphe Quetelet in the 19th century and is now used by the World Health Organization (WHO) and most health systems worldwide as a quick screening tool to categorise weight status.
BMI doesn't directly measure body fat — it estimates whether your weight is proportionate to your height, which correlates with health risk at the population level.
The BMI Formula
Metric (kg/m²)
BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625
BMI = 22.9
Imperial (lb/in²)
BMI = 703 × 154 ÷ (69²) = 108,262 ÷ 4,761
BMI ≈ 22.7
BMI Categories (Adults)
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Malnutrition, osteoporosis, weakened immune system |
| 18.5 – 24.9 | Normal weight | Lowest health risk |
| 25.0 – 29.9 | Overweight | Moderately increased risk of metabolic disease |
| 30.0 – 34.9 | Obese (Class I) | High risk of diabetes, heart disease, hypertension |
| 35.0 – 39.9 | Obese (Class II) | Very high risk |
| 40.0 and above | Obese (Class III) | Extremely high risk; may qualify for bariatric surgery |
WHO classifications. Some countries (e.g., parts of Asia) use lower thresholds due to population-specific risk profiles.
BMI for Children and Teens
For people under 18, standard adult BMI categories do not apply. Instead, BMI-for-age percentiles are used — comparing a child's BMI to others of the same age and sex.
Limitations of BMI
BMI is a useful screening tool, but it has well-documented limitations:
Does not distinguish muscle from fat
Athletes and bodybuilders often have a high BMI despite low body fat. A 90 kg sprinter and a 90 kg sedentary person have the same BMI but very different health profiles.
Doesn't account for fat distribution
Abdominal (visceral) fat is more dangerous than fat stored elsewhere. Two people with the same BMI can have very different fat distributions.
Age and sex differences
Women generally have more body fat than men at the same BMI. Older adults tend to have more body fat at the same BMI than younger people.
Ethnic variations
South Asian, East Asian, and some other populations face higher health risks at lower BMI thresholds than the WHO standards suggest.
Better Metrics to Use Alongside BMI
Frequently Asked Questions
What is a healthy BMI range for adults?
For adults, a BMI between 18.5 and 24.9 is considered normal weight with the lowest health risk. Below 18.5 is underweight, 25–29.9 is overweight, and 30 and above is classified as obese.
How do I calculate my BMI?
In metric units: BMI = weight (kg) ÷ height (m)². In imperial units: BMI = 703 × weight (lb) ÷ height (inches)². For example, 70 kg at 1.75 m gives BMI = 70 ÷ 3.0625 = 22.9 (normal weight).
Is BMI an accurate health indicator?
BMI is a useful screening tool but not a perfect measure. It does not distinguish between muscle and fat mass, does not account for fat distribution, and uses the same thresholds for different ages, sexes, and ethnicities. Always use it alongside other health markers.
Can a muscular person have a high BMI?
Yes. Athletes and bodybuilders often have a high BMI despite low body fat because muscle weighs more than fat per volume. A professional rugby player might have BMI of 28 (overweight) while being in excellent health. For such individuals, body fat percentage is a better metric.
What BMI is considered obese?
A BMI of 30.0 or above is classified as obese. Class I obesity is 30–34.9, Class II is 35–39.9, and Class III (severe obesity) is 40 and above. Each class carries progressively higher risk of diabetes, heart disease, and other conditions.
Does BMI apply to children and teenagers?
Standard adult BMI categories do not apply to children under 18. For children, BMI-for-age percentiles are used, comparing a child's BMI to others of the same age and sex. Below 5th percentile is underweight; 85th–95th is overweight; 95th and above is obese.
Related Calculators
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