What Is BMI and How Is It Calculated?
Body Mass Index (BMI) is a numerical measure of body size calculated by dividing a person's weight in kilograms by the square of their height in metres. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, it was originally used to study population-level weight distributions — not individual health. The World Health Organisation (WHO) adopted it as a global screening tool in the 1990s, and it remains the most widely used measure of body weight status worldwide.
The WHO defines the standard BMI categories for adults as follows:
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Nutritional deficiency, bone loss |
| 18.5 – 24.9 | Normal weight | Lowest risk for weight-related disease |
| 25.0 – 29.9 | Overweight | Moderately increased risk |
| 30.0 – 34.9 | Obese class I | High risk |
| 35.0 – 39.9 | Obese class II | Very high risk |
| 40.0 and above | Obese class III | Extremely high risk |
Source: World Health Organisation, Global Database on Body Mass Index (2024).
Important note for South and East Asian populations: The WHO recommends lower thresholds for people of Asian descent — a BMI of 23.0 or above is considered overweight, and 27.5 or above is considered obese. This is because these populations carry higher metabolic risk at lower BMI values than European populations.
How to Calculate BMI: The Formula
Metric Formula
BMI = weight (kg) ÷ [height (m)]²
Example: A person weighing 75 kg at 1.75 m tall: BMI = 75 ÷ (1.75 × 1.75) = 75 ÷ 3.0625 = 24.5 — Normal weight.
Imperial Formula
BMI = [weight (lb) ÷ height (in)²] × 703
The factor 703 converts the result from lb/in² to the standard kg/m² scale. Example: A person weighing 165 lbs at 5'9'' (69 inches): BMI = (165 ÷ 69²) × 703 = (165 ÷ 4,761) × 703 = 24.4 — Normal weight.
Why BMI Is Not a Perfect Measure of Health
BMI is a population-level screening tool, not a diagnostic instrument. The CDC and WHO both acknowledge several well-documented limitations:
- Muscle vs fat: BMI cannot distinguish between lean muscle mass and body fat. Elite athletes — particularly rugby players, bodybuilders, and sprinters — routinely measure as "overweight" or "obese" despite very low body fat percentages.
- Age and sex: Women naturally carry more body fat than men at the same BMI. Older adults tend to lose muscle mass (sarcopenia), so a "normal" BMI can mask a high body fat percentage in people over 60.
- Ethnicity: The standard thresholds were established using predominantly European population data. As noted above, South and East Asian populations face higher health risks at lower BMI values.
- Fat distribution: Waist circumference is often a better predictor of metabolic risk than BMI. The WHO considers a waist over 94 cm (37 in) for men or 80 cm (31.5 in) for women as a marker of elevated cardiovascular risk, regardless of BMI.
What Should You Do With Your BMI Result?
A BMI result is a starting point for a conversation with your doctor — not a diagnosis. If your BMI falls outside the normal range, a GP or physician will consider the full picture: blood pressure, cholesterol, blood glucose, body composition, fitness level, family history, and lifestyle factors.
If you want a more precise measure of body composition, your doctor can arrange a DEXA scan, hydrostatic weighing, or bioelectrical impedance analysis — all of which measure fat, muscle, and bone density directly rather than inferring body composition from weight and height alone.
For related calculators: use the Body Fat Calculator to estimate body fat percentage, the Waist-to-Hip Ratio Calculator for metabolic risk, and the Ideal Weight Calculator for your healthy weight range.
Frequently Asked Questions About BMI
Is BMI an accurate measure of health?
BMI is a useful population-level screening tool but not a diagnostic instrument. It correlates well with body fat in most people, but cannot distinguish between muscle and fat — a highly muscular athlete may be classified as "overweight" despite low body fat. The CDC recommends BMI as a screening tool only, followed by further assessment to determine health status.
Why does the BMI calculator ask for age and sex?
While the core BMI formula only requires height and weight, age and sex are important for interpreting results. Body composition changes with age, and men and women naturally carry different proportions of fat and muscle at the same BMI. These inputs provide context that helps a healthcare professional give a more accurate assessment.
Should I be worried if my BMI is high?
A high BMI is a signal, not a sentence. Use it to prompt a conversation with your doctor rather than drawing conclusions on your own. A clinician can assess your full health picture — including blood pressure, cholesterol, and fitness level — and determine whether your BMI reflects genuine health risk or simply a muscular build.
Want to Go Deeper on BMI?
Our full guide covers what the BMI number can and can't tell you — including the muscle mass problem, ethnic group thresholds, waist circumference as a better metabolic risk predictor, and which additional metrics give a more complete picture. Read: What Is a Healthy BMI? How to Calculate It and What It Really Means →
