BMI Calculator

What is BMI and how is it calculated?

Body Mass Index (BMI) is a screening metric that estimates weight-for-height relative to a population average. It was introduced in the 1830s by Belgian mathematician Adolphe Quetelet and later adopted by the World Health Organization as a public-health indicator. The formula is body mass in kilograms divided by height in meters squared: BMI = kg / m². In imperial units, multiply weight in pounds by 703 and divide by height in inches squared. A person weighing 80 kg at 1.80 m tall has a BMI of 24.7 (80 / 1.80²), placing them at the top of the "normal weight" range. BMI requires only a scale and a tape measure, which is why it has become the standard screening tool in primary care and epidemiological studies despite its well-documented limitations at the individual level.

What is a healthy BMI range?

A healthy BMI for adults aged 20 and over falls between 18.5 and 24.9, according to the World Health Organization classification used by the US Centers for Disease Control (CDC) and the UK National Health Service. Values below 18.5 are classified as underweight; 25.0–29.9 as overweight; 30.0–34.9 as class I obesity; 35.0–39.9 as class II obesity; and 40.0 or higher as class III (severe) obesity. These cutoffs were derived from large-scale epidemiological studies correlating BMI with all-cause mortality risk. They do not apply to children, adolescents, pregnant women, elderly adults over 65, or highly muscular individuals. The WHO recommends lower cutoffs for Asian populations (overweight at 23.0, obesity at 27.5) because cardiometabolic risk rises at lower BMIs in East and South Asian ancestry groups.

How do I calculate BMI manually?

To calculate BMI manually, divide your body weight in kilograms by your height in meters squared. For a person weighing 75 kg at 1.75 m tall, the calculation is 75 ÷ (1.75 × 1.75) = 75 ÷ 3.0625 = 24.5 BMI. In imperial units, the formula is weight in pounds divided by height in inches squared, multiplied by 703. For a 165-pound person at 5'9" (69 inches): 165 ÷ (69 × 69) × 703 = 165 ÷ 4761 × 703 = 24.4 BMI. Both methods produce the same result within rounding. Because BMI is a screening tool, a range tells you more than a precise value — normal weight covers BMI 18.5–24.9, a span of over 6 points. Small weight fluctuations from water, food, or time of day routinely shift BMI by 0.5–1.0 points, so weekly averages are more informative than single readings.

Why is BMI inaccurate for athletes and lifters?

BMI systematically overestimates body fat in muscular individuals because it cannot distinguish fat mass from lean mass. An NFL linebacker at 115 kg and 1.88 m tall has a BMI of 32.5 — the "obese" category — despite potentially having single-digit body fat. Analysis of US National Health and Nutrition Examination Survey (NHANES) data found that roughly 16% of adults classified as "overweight" by BMI have body fat percentages in the healthy range, while about 9% of "normal" BMI adults have elevated body fat. Muscle tissue is roughly 18% denser than fat, so resistance training increases BMI without increasing health risk. For lifters and strength athletes, Fat-Free Mass Index (FFMI) is a more informative metric — it divides lean body mass by height squared. FFMI above 25 indicates exceptional muscularity; values above 26–28 are considered the natural untrained upper limit per Kouri et al. (1995).

BMI vs body fat percentage vs FFMI

Different body-composition metrics answer different questions. BMI is a population-level screening tool requiring only a scale and height. Body fat percentage directly quantifies fat mass; the gold standard is a DEXA scan, with bioelectrical impedance (BIA) and skinfold calipers being less accurate. FFMI is a specialized metric for strength athletes that isolates lean mass relative to height. Use BMI for a quick screen, body fat percentage for composition tracking, and FFMI when evaluating muscularity. Most bathroom smart scales use BIA, which can swing by 3–5% based on hydration and is unreliable for week-over-week changes. DEXA scans cost $50–150 at medical imaging centers and provide the most accurate body-composition data outside a research setting.