BMI Calculator
Calculate Body Mass Index (BMI) — body weight in kg divided by height in metres squared — to screen for under/overweight categories. The most widely used population-level fitness and health risk metric, with known limitations for athletes and unusual body compositions.
Last updated: May 2026
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About this calculator
BMI = weight_kg / (height_m)². This calculator takes height in centimetres and divides by 100 first. The WHO categories for adults: under 18.5 = underweight, 18.5–24.9 = healthy weight, 25.0–29.9 = overweight, 30.0–34.9 = obesity class I, 35.0–39.9 = class II, 40.0+ = class III. Variables: weight in kilograms; height in centimetres. The metric is intentionally simple: just two inputs, no body composition data, no age or sex adjustment in the adult formula. That simplicity is both its strength (universally computable) and its weakness (ignores muscle vs. fat distinction). For fitness-focused users, BMI is most useful as a population-level screen and a long-term tracking number; it's not appropriate for short-term changes (which are mostly water) or for athletes. The 18th-century mathematician Adolphe Quetelet derived the squared-height denominator empirically by observing that adult weight tends to scale with height squared across populations, making BMI roughly height-independent. This breaks down at extremes — very short or very tall adults need different scaling. Edge cases: not valid for children and adolescents (use BMI percentile charts), pregnant/lactating people, amputees, people with severe oedema or ascites, or anyone whose body composition is far from population average. For South Asian, Chinese, and some other ancestry groups, lower thresholds apply (overweight starts at 23 kg/m²). Waist circumference and waist-to-height ratio (< 0.5 is healthy) are independent and often more informative cardiometabolic risk markers than BMI alone; in fitness contexts, body fat percentage measured by DEXA, BodPod, or even skinfold calipers gives a much more relevant picture of physique and metabolic health.
How to use
Example 1 — Recreational lifter. A 75 kg adult who is 175 cm tall. Enter Weight = 75, Height = 175. BMI = 75 / (1.75)² = 75 / 3.0625 ≈ 24.49. ✓ Just inside the "healthy" range upper bound. For an active recreational lifter this BMI is unremarkable; for a sedentary person it would warrant attention to waist circumference and cardiometabolic markers. Example 2 — Competitive lifter. A 100 kg powerlifter who is 180 cm tall and has ~12% body fat. Enter 100 and 180. BMI = 100 / (1.80)² = 100 / 3.24 ≈ 30.86 — formally classified as "obese class I". ✓ But this is exactly where BMI breaks down: at 12% body fat, the lifter is lean and very muscular, not metabolically obese. A DEXA scan or skinfold measurement would confirm the favourable body composition. The lesson: in fitness contexts, BMI is a screening tool, not a verdict — pair it with body composition measurement for athletes.
Frequently asked questions
Why does BMI mislabel athletes as overweight?
BMI uses only weight and height, with no information about what the weight is made of. Muscle is roughly 18% denser than fat — a litre of muscle weighs ~1.06 kg, while a litre of fat weighs ~0.9 kg — so two people of the same height and same volume can differ in mass by 10–15% based on muscle vs. fat composition. An athlete with a high lean-mass-to-fat-mass ratio will weigh more for their height than the sedentary average, pushing BMI into the overweight or obese categories despite excellent body composition. This is why an NFL running back, a powerlifter, or even a serious recreational lifter often scores BMI 27–32 while having body fat under 15%. The fix is to use body composition measurement (DEXA, BodPod, hydrostatic weighing, or even just skinfold calipers) for anyone whose physique is far from average — BMI is a population screening tool, not a diagnosis for individuals.
Is BMI useful at all for fitness?
Yes, as one input among several. BMI is a fine starting screen for the general population, and it correlates well with cardiometabolic risk at the extremes (BMI > 35 reliably indicates excess adiposity; BMI < 18 reliably indicates underweight). For tracking your own progress over months and years, BMI change is a reasonable summary of weight trajectory — especially combined with body-composition tracking. Where BMI fails is at the individual level for people with high muscle mass, and as a snapshot judgment of someone's "fitness". For fitness tracking, more useful metrics include: body fat percentage, waist-to-height ratio (< 0.5 is healthy), resting heart rate, VO2 max, strength relative to body weight (e.g., bench press ratio), and progress on specific training goals. BMI complements these rather than replaces them.
How does BMI relate to body fat percentage?
There's a rough population-level correlation but it's noisy at the individual level. The Deurenberg equation estimates body fat % from BMI, age, and sex: BF% ≈ (1.20 × BMI) + (0.23 × age) − (10.8 × sex) − 5.4, where sex = 1 for male, 0 for female. For a 30-year-old man with BMI 25, predicted BF% ≈ 30 + 6.9 − 10.8 − 5.4 = 20.7%. The 95% prediction interval is roughly ±5%, so the same BMI could mean 15% body fat (lean athlete) or 26% (sedentary average). The relationship works for the general population but breaks down for athletes, bodybuilders, and the elderly. Direct body composition methods (DEXA, BodPod, BIA, skinfolds) give much more accurate body-fat estimates and are worth using if you care about the answer.
What are the most common mistakes people make with BMI in fitness contexts?
The first is using BMI as a measure of fitness — it isn't. A sedentary 24 BMI person and an athletic 30 BMI person have very different fitness profiles despite their BMI ranking. The second is checking BMI weekly and reacting to small changes; bodyweight fluctuates 1–3 kg daily from water, glycogen, food, and gut contents, so weekly BMI changes are mostly noise. The third is using "ideal weight" calculations derived from BMI ranges (e.g., "I should weigh 70 kg") without considering body composition; a goal weight in the healthy BMI range can still mean obese-level body fat for a sedentary person, or be unrealistically low for an athlete. The fourth is forgetting that BMI categories were set for the general population — South Asian descent groups face cardiometabolic risk at lower thresholds. The fifth is comparing your BMI to elite athletes' BMIs as a benchmark; their high BMI reflects extreme lean mass, not a target you should aim for without the matching training and body composition.
When should I not use this calculator?
Skip it if you have above-average muscle mass — recreational lifters, athletes, bodybuilders all get misclassified upward. Use body fat percentage, DEXA, or waist-to-height ratio instead. Don't use it under 20 (use BMI percentile charts), during pregnancy/lactation (weight reflects pregnancy), or if you have limb amputations, severe oedema, or ascites — body composition is too distorted for BMI to be meaningful. It's also a poor measure for older adults with sarcopenia (muscle loss): a "healthy" BMI can mask poor metabolic health when most of the weight is fat. For people of South Asian, Chinese, or Japanese descent, use ethnic-specific lower thresholds (overweight at 23, not 25). Finally, don't use BMI as a stand-alone fitness verdict — pair it with waist circumference, body composition, blood pressure, and basic fitness measurements (resting HR, VO2 max, strength) for any meaningful interpretation.