BMI Calculator
Calculate Body Mass Index (BMI) from weight and height — the most widely used screening number for weight-related nutritional and metabolic risk. A first-pass indicator, not a diagnosis.
Last updated: May 2026
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About this calculator
BMI is body weight in kilograms divided by height in metres squared: BMI = weight_kg / (height_m)². For this calculator, height is entered in centimetres and divided by 100 first. Example: 70 kg at 175 cm → 70 / (1.75)² = 70 / 3.0625 ≈ 22.86 kg/m². The WHO adult categories: 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. The squared-height denominator was chosen by Adolphe Quetelet in the 19th century because, across large populations of average adults, weight tends to scale with the square of height — making BMI roughly height-independent for an average build. This is also BMI's biggest limitation: muscle weighs more than fat, so well-trained athletes and bodybuilders often score in the overweight or obese range despite being lean, while frail elderly people with low muscle mass can score "healthy" while having unhealthy body composition. Variables: weight in kilograms; height in centimetres. Edge cases: not validated for children and teens (use age- and sex-specific BMI percentiles), pregnant or lactating people, people with limb amputations, or very short adults under ~150 cm. Some populations (particularly people of South Asian descent) face elevated cardiometabolic risk at lower BMI thresholds; many clinicians use 23 instead of 25 as the overweight cutoff for them. Always interpret BMI alongside waist circumference, waist-to-height ratio, and clinical context — a BMI in the "healthy" range with high abdominal fat is more concerning than a BMI in the "overweight" range with athletic body composition.
How to use
Example 1 — Average adult. A 35-year-old who weighs 70 kg and is 175 cm tall enters Weight = 70 and Height = 175. BMI = 70 / (1.75)² = 70 / 3.0625 ≈ 22.86. ✓ This falls in the 18.5–24.9 "healthy weight" range — no immediate weight-related risk flag, but doesn't guarantee good metabolic health on its own. Example 2 — Higher BMI. A person weighing 90 kg at 168 cm enters 90 and 168. BMI = 90 / (1.68)² = 90 / 2.8224 ≈ 31.89, which falls in obesity class I (30.0–34.9). ✓ This warrants a conversation with a clinician about cardiometabolic risk factors, but BMI alone doesn't diagnose obesity — body composition (waist circumference, body fat percentage) and clinical context (blood pressure, fasting glucose, lipids) all matter. Many people in this BMI range are metabolically healthy; some at lower BMI are not.
Frequently asked questions
What is a healthy BMI range for adults?
The WHO defines 18.5–24.9 kg/m² as the healthy range for most adults aged 20 and over. Below 18.5 is underweight, 25.0–29.9 is overweight, and 30.0+ falls into one of three obesity classes. These thresholds were set based on large epidemiological studies showing the lowest all-cause mortality for adults of European ancestry tends to fall in this band. For people of South Asian descent, many guidelines lower the overweight cutoff to 23 because cardiometabolic risk rises earlier in that population. "Healthy" is a population average — it doesn't mean any specific individual within it is in optimal health, only that their weight-for-height isn't likely a major risk factor on its own. Always combine BMI with waist circumference (women < 80 cm, men < 94 cm by WHO), blood pressure, lipid panel, and fasting glucose for a real picture of cardiometabolic health.
Is BMI accurate for athletes and very muscular people?
No — this is BMI's most-cited limitation. The formula treats all body mass the same, so it can't distinguish 10 kg of muscle from 10 kg of fat. Well-trained athletes, bodybuilders, and people with naturally high lean mass routinely score in the overweight (25–29.9) or even obese (30+) range despite low body-fat percentages and excellent metabolic health. The reverse problem (called sarcopenic obesity) appears in frail older adults or people with chronic illness: low muscle mass keeps BMI in the "healthy" range while body composition is actually poor. For anyone with a body-composition profile far from average, use a body-fat-percentage calculator, waist-to-height ratio (target < 0.5), or DEXA scan instead of BMI alone. Standard tape-measure tests and bioelectrical-impedance scales are good cheap proxies; DEXA and BodPod are clinical-grade but expensive.
Does BMI work the same way for children and teens?
No — BMI is calculated with the same formula for children, but interpretation is completely different. Children's body composition changes rapidly with age and differs between boys and girls, so a single set of adult cutoffs doesn't apply. Paediatric BMI is interpreted using age- and sex-specific percentile charts (CDC growth charts in the US, WHO growth standards internationally). A child is considered underweight below the 5th percentile, healthy from the 5th to the 85th, overweight from the 85th to the 95th, and obese at or above the 95th. Never use the adult BMI categories on a child or teenager — consult a paediatrician or a dedicated child BMI percentile calculator. The category thresholds also depend on which growth reference you use (CDC vs WHO can produce slightly different classifications for the same child).
What are the most common mistakes people make checking BMI?
The first is using inconsistent units — entering weight in pounds while the calculator expects kilograms, or height in metres when it expects centimetres, produces a number ten or a hundred times off. The second is treating BMI as a diagnosis: a number above 25 doesn't mean someone is "unhealthy", it means their weight-for-height warrants further investigation. The third is checking BMI obsessively over short time spans — week-to-week fluctuations are mostly water and gut contents, not real body composition change. The fourth is comparing one's BMI to a partner's or friend's and drawing conclusions; BMI is meaningful only against population norms within your age, sex, and ancestry group. The fifth is ignoring waist circumference, which independently predicts cardiometabolic risk even when BMI looks normal. Finally, people often forget that BMI is one number; combining it with waist measurement, blood markers, fitness, and family history gives a much richer health picture than BMI alone.
When should I not use this calculator?
Skip it if you're pregnant or breastfeeding (your weight reflects the pregnancy, not your baseline body composition). Don't use it under 20 — use pediatric BMI percentiles instead. Avoid it if you're an elite athlete or bodybuilder with very high lean mass — the formula will misclassify you as overweight despite low body fat; use DEXA or body-fat-percentage measurement instead. BMI is also a poor measure for people with limb amputations (the missing limb mass distorts the ratio), people who are very tall or very short (the squared-height model breaks down at the extremes), and people with conditions like ascites or severe oedema where retained fluid inflates body weight without reflecting fat or muscle. In any of these cases, body composition methods (skinfold calipers, bioelectrical impedance, DEXA) give a far more meaningful picture, and a clinician should interpret the results in context. For people of South Asian, Chinese, or Japanese descent, use lower ethnic-specific thresholds (overweight starts at 23, not 25).