How this works

Body mass index is calculated from height and weight alone: weight in kilograms divided by height in meters squared. In imperial units, the equivalent formula multiplies weight in pounds by 703, then divides by height in inches squared. The result is a single number that places you into one of four standard WHO categories: underweight (below 18.5), normal weight (18.5–24.9), overweight (25–29.9), or obese (30 and above).

It's worth understanding what BMI actually is: a population-level screening tool developed by a Belgian statistician in the 1830s to study body weight trends across groups, not a medical diagnostic for individuals. It was adopted by health organisations in the 20th century as a cheap, quick screening measure — but it has well-documented limitations that matter for real-world interpretation.

The BMI categories

Underweight — below 18.5

May indicate insufficient caloric intake, malnutrition, or an underlying health condition. Can also be within normal range for naturally slender individuals, particularly some Asian populations.

Normal weight — 18.5 to 24.9

Associated with the lowest all-cause mortality risk at a population level. Does not guarantee individual health.

Overweight — 25.0 to 29.9

Slightly elevated risk for certain conditions at a population level, though research has found this category to have similar or lower mortality risk than the underweight range. Many fit, athletic individuals fall here.

Obese — 30.0 and above

Associated with meaningfully higher risk of Type 2 diabetes, cardiovascular disease, joint problems, and other conditions. Subdivided into Class I (30–34.9), Class II (35–39.9), and Class III (40+) in clinical settings.

Where BMI misleads

BMI uses only height and weight, which means it cannot distinguish between fat mass and lean mass. This creates systematic errors in specific populations:

Muscular or athletic individuals

Muscle is denser than fat. A powerlifter or rugby player with very low body fat may show a BMI in the "overweight" or "obese" range because lean muscle mass is heavy. BMI would classify many elite athletes as overweight.

Older adults

Muscle mass typically declines with age (sarcopenia), meaning an older person can have a "normal" BMI while carrying a higher proportion of body fat than the number suggests.

Different ethnic groups

The original BMI thresholds were calibrated primarily on Western European populations. Research has shown that people of Asian descent tend to carry more health risk at lower BMI values, which has led several health organisations to recommend lower thresholds (e.g., 23 instead of 25 for overweight) for these groups.

Pregnant individuals

BMI is not meaningful during pregnancy. Weight gain is expected and necessary; standard BMI categories do not apply.

Better measures of body composition

For a more complete picture alongside BMI, clinicians often consider waist circumference (a better proxy for visceral fat, which is more metabolically harmful than subcutaneous fat), waist-to-height ratio, or DEXA scan results. Waist circumference above 35 inches (women) or 40 inches (men) is considered a risk marker regardless of BMI.

Frequently asked

Is BMI accurate for everyone?

No — it systematically overestimates body fat in very muscular people and can underestimate it in older adults who've lost muscle mass. It's a population-level screening number, not an individual diagnosis. Use it as one rough data point, not the whole story.

What should I do with this number?

Treat it as one rough indicator among several. A healthcare provider can put it in context alongside waist measurement, blood work, fitness level, and your personal and family health history. A number outside the "normal" range is a reason to have a conversation with a clinician, not a verdict.

Why does BMI use different thresholds for Asian populations?

Research has found that cardiovascular and metabolic health risks emerge at lower BMI values in East and South Asian populations compared to the WHO's original thresholds. Some health bodies now recommend using 23 as the overweight threshold and 27.5 as the obese threshold for these groups.

What's a healthy weight for my height?

The "normal" BMI range of 18.5–24.9 translates to different weight ranges depending on height. For a 5'7" (170cm) person, that range is roughly 118–158 lbs (53–71 kg). Use the calculator in reverse by adjusting the weight input until the BMI lands in the range you're targeting.