Body Mass Index is the most widely used screening measure of whether a person’s weight is in a healthy range for their height. It is quick, needs only two numbers, and gives a single value that maps onto the World Health Organization’s weight categories.
How it works
BMI divides body mass by the square of height, which removes most of the effect of being tall or short and leaves a number that tracks body fatness across the population.
metric: BMI = weight(kg) / height(m)^2
imperial: BMI = 703 x weight(lb) / height(in)^2The calculator also works backwards from your height to show the weight range that keeps BMI between 18.5 and 24.9, the WHO normal band.
Categories, example, and notes
A worked example: a person 1.75 m tall weighing 72 kg has a BMI of 72 / (1.75 x 1.75) which is about 23.5, placing them in the normal band. The full set of WHO cut-offs runs from underweight below 18.5 up to obese class III at 40 and above. Remember that BMI cannot tell muscle from fat and ignores where fat is stored, so a muscular athlete may register as overweight while carrying little fat. For a fuller picture, combine BMI with waist circumference and, where relevant, a lean body weight estimate.
WHO BMI categories in full
| BMI range | Category | Notes |
|---|---|---|
| Below 18.5 | Underweight | Associated with malnutrition, frailty risk |
| 18.5 – 24.9 | Normal weight | The reference range for cardiovascular and metabolic risk |
| 25.0 – 29.9 | Overweight | Increased risk — waist circumference adds context |
| 30.0 – 34.9 | Obese class I | Moderate risk; lifestyle intervention typically recommended |
| 35.0 – 39.9 | Obese class II | High risk; pharmacological or surgical options considered |
| 40.0 and above | Obese class III (severe) | Very high risk; bariatric surgery may be appropriate |
Some clinicians further divide class III into class III (40–49.9) and class IV or “super obesity” (50 and above), though this is not part of the standard WHO nomenclature.
When BMI is and isn’t the right measure
Where BMI works well BMI is a good population-level screening tool and is consistent across large studies, making it the backbone of epidemiological research. For a typical sedentary adult, BMI correlates reasonably well with body fat percentage and cardiometabolic risk.
Where BMI gives misleading results
- Muscular athletes — resistance-trained athletes commonly have BMIs in the overweight range with low body fat. A rugby player at 1.80 m and 95 kg has a BMI of 29.3 (overweight) but may have 12% body fat.
- Older adults — BMI tends to underestimate adiposity in older people because muscle mass naturally declines with age (sarcopenia) while fat may remain constant or increase. A BMI of 22 in a 75-year-old may reflect more fat and less muscle than the same BMI at age 25.
- South Asian and East Asian populations — the standard WHO cut-offs may underestimate risk. Some regional health bodies use lower thresholds: overweight starting at BMI 23 and obesity at BMI 27.5 for South Asian populations.
- Pregnancy — BMI changes rapidly and reference ranges are replaced by gestational weight gain recommendations based on pre-pregnancy BMI.
- Children — child BMI is age- and sex-specific (percentile-based) and uses different cut-offs from the adult tables.
Complementary measures
For a fuller picture beyond BMI:
- Waist circumference — fat around the abdomen carries higher cardiovascular risk than the same fat on hips and thighs. Common risk thresholds are above 94 cm (men) or 80 cm (women) for increased risk, and above 102 cm (men) or 88 cm (women) for substantially increased risk.
- Waist-to-height ratio — a ratio below 0.5 is a simple, age-independent proxy for cardiovascular risk that some researchers argue outperforms BMI.
- Body fat percentage — measured by DEXA scan, hydrostatic weighing, or bioelectrical impedance, this directly distinguishes fat from lean mass and is more accurate than BMI for individuals.