Calculate your Body Mass Index instantly. See your BMI category, healthy weight range, and where you fall on the BMI scale — for men and women, all ages.
| Category | BMI Range | Your Height |
|---|---|---|
| Underweight | < 18.5 | — |
| Normal | 18.5 – 24.9 | — |
| Overweight | 25 – 29.9 | — |
| Obese I | 30 – 34.9 | — |
| Obese II | 35 – 39.9 | — |
| Obese III | ≥ 40 | — |
Body Mass Index (BMI) is a numerical value calculated from your weight and height. It's used worldwide as a quick screening tool to categorize adults into weight categories: underweight, normal weight, overweight, and obese. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, BMI remains the most widely used population-level measure of weight status despite its known limitations.
BMI does not measure body fat directly. It uses a simple formula that correlates reasonably well with more expensive and invasive body fat measurements for most people. Health organizations including the CDC, WHO, and NHS use BMI as a first-pass screening tool.
The BMI formula divides weight by height squared. There are two versions depending on your unit system:
For example, a person who is 5 feet 10 inches (70 inches) and weighs 170 pounds: BMI = (170 × 703) ÷ (70²) = 119,510 ÷ 4,900 = 24.4 — which falls in the "normal weight" category.
The World Health Organization defines these adult BMI categories, which apply equally to men and women:
| Category | BMI Range |
|---|---|
| Severe Thinness | < 16.0 |
| Underweight | 16.0 – 18.4 |
| Normal weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obese Class I | 30.0 – 34.9 |
| Obese Class II | 35.0 – 39.9 |
| Obese Class III | ≥ 40.0 |
The standard BMI formula and categories are the same for both sexes. However, women naturally carry a higher percentage of body fat than men at the same BMI. A woman with a BMI of 24 typically has more body fat than a man with the same BMI. This is one reason many health professionals recommend using BMI alongside other measurements like waist circumference or body fat percentage for a more complete picture — especially for women.
For adults 20 and older, BMI is interpreted using the standard categories regardless of age or sex. For children and teens (ages 2–19), BMI is age- and sex-specific and expressed as a percentile relative to other children of the same age.
Research suggests that for older adults (65+), a slightly higher BMI (25–27) may be associated with lower mortality risk compared to the standard "normal" range. This is sometimes called the "obesity paradox." However, the official BMI categories have not been adjusted for age.
BMI is a useful starting point but has well-documented blind spots. It doesn't distinguish between muscle mass and fat mass — a bodybuilder and a sedentary person of the same height and weight get the same BMI. It doesn't account for where body fat is stored (visceral fat around organs is more dangerous than subcutaneous fat). And it doesn't factor in bone density, age-related muscle loss, or ethnic differences in body composition.
For a more complete picture of body composition, consider pairing BMI with waist circumference, waist-to-hip ratio, or a body fat percentage measurement.
Is BMI accurate for athletes? Not always. Athletes with significant muscle mass often register as "overweight" or "obese" by BMI despite having low body fat. If you train regularly with weights, a body fat measurement is more informative than BMI alone.
What's the ideal BMI? Research associates the lowest health risks with a BMI between 18.5 and 24.9. Within that range, there's no single "ideal" number — it depends on your body composition, fitness level, and overall health markers.
How often should I check my BMI? BMI is most useful as a periodic screening tool, not a daily metric. Checking once every few months or when you notice significant weight changes is sufficient. Daily weight fluctuations from water retention and food intake make daily BMI tracking misleading.