BMI Calculator
Body Mass Index (BMI) is the most widely used screening tool for relating a person's weight to their height. It produces a single number that places adults into broad weight categories — underweight, healthy weight, overweight, or obese — and is used by the WHO, the CDC, and the NHS as a first-pass indicator of weight-related health risk. Enter your weight in kilograms and height in centimetres and the calculator returns your BMI in kg/m². The result is meant as a starting point for a conversation with a clinician, not a diagnosis on its own.
About this calculator
The formula is: BMI = weight (kg) ÷ height (m)². Because this calculator takes height in centimetres, it internally divides height by 100 first, then squares it. For someone 175 cm tall weighing 70 kg, the math is 70 ÷ (1.75)² = 70 ÷ 3.0625 ≈ 22.86 kg/m². The standard WHO adult categories are: 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, and 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 — not the cube — making BMI roughly height-independent for the 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 extremely lean, while frail elderly people with low muscle mass can score "healthy" while having unhealthy body composition. Edge cases: BMI is 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 about 150 cm. Some populations (notably people of South Asian descent) face elevated cardiometabolic risk at lower BMI thresholds, and many clinicians use 23 instead of 25 as the overweight cutoff for them.
How to use
Example 1 — Average adult. A 35-year-old who weighs 70 kg and is 175 cm tall enters 70 in the Weight field and 175 in the Height field. Result: 22.86. This falls within the 18.5–24.9 healthy range. Verify: 175 cm = 1.75 m, 1.75² = 3.0625, and 70 ÷ 3.0625 ≈ 22.86. ✓ Example 2 — Higher weight. Someone 90 kg at 168 cm enters 90 and 168. Result: 31.89, which falls in the obesity class I range (30.0–34.9). Verify: 168 cm = 1.68 m, 1.68² = 2.8224, and 90 ÷ 2.8224 ≈ 31.89. ✓ A result in this range warrants a conversation with a clinician about cardiometabolic risk factors, but it does not diagnose obesity on its own — body composition, waist circumference, and clinical context all matter.
Frequently asked questions
What is a healthy BMI range for adults?
The WHO classifies a BMI between 18.5 and 24.9 kg/m² as a healthy weight for most adults aged 20 and over. Below 18.5 is considered underweight, 25.0–29.9 is overweight, and 30.0 or above 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. The "healthy" range is a population average — it does not mean any specific individual within it is in optimal health, only that their weight-for-height is unlikely to be a major risk factor on its own.
Is BMI accurate for athletes and very muscular people?
No, and this is BMI's best-known limitation. The formula treats all body mass the same, so it cannot distinguish 10 kg of fat from 10 kg of muscle. 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 having low body fat percentages and excellent metabolic health. The reverse problem appears in frail older adults or people with chronic illness: low muscle mass can keep BMI in the "healthy" range while body composition is actually poor (a phenomenon called sarcopenic obesity). For anyone with a body-composition profile far from average, use a body fat percentage calculator, waist-to-height ratio, or DEXA scan instead of relying on BMI alone.
Does BMI work the same way for children?
No — BMI is calculated with the same formula for children, but the 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 does not apply. Pediatric BMI is interpreted using age- and sex-specific percentile charts (such as the CDC growth charts in the US or 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 shown by this calculator to assess a child or teenager — consult a pediatrician or a dedicated child BMI percentile calculator.
What are the most common mistakes people make when checking their BMI?
The first is using inconsistent units — entering weight in pounds while the calculator expects kilograms, or height in metres when it expects centimetres, easily produces a number ten or a hundred times off. The second is treating BMI as a diagnosis: a number above 25 does not 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. Finally, people often ignore waist circumference, which independently predicts cardiometabolic risk even when BMI looks normal.
When should I not use BMI?
Skip BMI if you are pregnant or breastfeeding (your weight reflects the pregnancy, not your baseline body composition), if you are under 20 (use pediatric BMI percentiles instead), or if you are an elite athlete or bodybuilder with very high lean mass. 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 like skinfold calipers, bioelectrical impedance, or DEXA scans give a far more meaningful picture, and a clinician should interpret the results in context.