BMI Calculator
Instantly compute your Body Mass Index from weight and height to screen which WHO weight category you fall in. Returns a single kg/m² number classified as underweight, normal, overweight, or obese.
Last updated: May 2026
Compare with similar
About this calculator
Body Mass Index (BMI) relates body weight to height squared and is the most widely used population-level screen for under- and over-weight. The formula is BMI = weight (kg) / (height (m))². In this calculator height is entered in centimetres, so it is first divided by 100 to convert to metres before squaring, and the result is reported in kg/m². The World Health Organization classifies adults as underweight at BMI < 18.5, normal weight 18.5–24.9, overweight 25–29.9, obese class I 30–34.9, obese class II 35–39.9 and obese class III ≥ 40. Some Asian health authorities use a lower obesity cut-off of 27.5 because cardiometabolic risk rises at lower BMIs in those populations. Inputs assume both numbers are positive; entering 0 or negative values returns an undefined/infinite result rather than a meaningful BMI. The metric does not measure body composition: muscular athletes, pregnant women, very tall or very short people, children, the elderly with sarcopenia, and people with significant oedema are routinely misclassified. Treat BMI as a rough screen, not a diagnosis, and complement it with waist circumference, body-fat percentage or clinical assessment when accuracy matters.
How to use
Example 1 — a normal-weight adult. You weigh 70 kg and are 175 cm tall. Step 1: 175 ÷ 100 = 1.75 m. Step 2: 1.75² = 3.0625 m². Step 3: 70 ÷ 3.0625 ≈ 22.86 kg/m². Verify: this sits inside the WHO normal range (18.5–24.9), so the classification 'Normal weight' is correct. ✓ Example 2 — an overweight adult. You weigh 92 kg and are 178 cm tall. Step 1: 178 ÷ 100 = 1.78 m. Step 2: 1.78² = 3.1684 m². Step 3: 92 ÷ 3.1684 ≈ 29.04 kg/m². Verify: 29.04 falls between 25 and 29.9, so this person is classified as 'Overweight' just below the obesity threshold of 30. A 3-kg weight loss to 89 kg would drop the BMI to roughly 28.1 — still overweight, but moving in the right direction. ✓
Frequently asked questions
What is a healthy BMI range for adults?
For adults aged 18 and over the WHO defines a healthy BMI as 18.5 to 24.9 kg/m². Values below 18.5 indicate underweight, 25 to 29.9 indicate overweight, and 30 or above indicate obesity, with three further class subdivisions. These thresholds are population-level guidelines and may not apply equally to every ethnic group — for example, several Asian health authorities use a lower obesity cut-off of 27.5 because cardiometabolic risk rises at lower BMIs in those populations. The ranges were designed for non-pregnant adults of average build and are not validated in children, adolescents or older adults with significant muscle loss. Always interpret your number alongside other markers such as waist circumference, blood pressure, lipids and family history rather than reading the category in isolation.
Why does BMI use height in metres squared rather than just height?
Squaring the height accounts for the fact that body volume — and therefore mass — scales in three dimensions while height is one-dimensional. Dividing weight by height alone would systematically make taller people look heavier relative to their actual build. Using height² produces a dimensionally consistent ratio (kg/m²) that is roughly comparable across people of different statures and was the empirical relationship Adolphe Quetelet found best fit population data in the 19th century, which is why BMI is also called the Quetelet Index. More recent work (such as Trefethen's BMI) has proposed exponents closer to 2.5 because the square slightly under-corrects for very tall people and over-corrects for very short ones. For everyday screening, however, the square remains the global standard and the small bias does not change clinical categorisation in most adults.
When is BMI not an accurate indicator of body fat?
BMI cannot distinguish fat mass from lean mass, so it routinely misclassifies highly muscular individuals — bodybuilders, rugby players, sprinters — as overweight or obese despite very low body-fat percentages. Conversely, older adults who have lost muscle (sarcopenia) may sit inside the 'normal' band while carrying an unhealthy proportion of body fat, a pattern sometimes called 'normal-weight obesity'. BMI is also unreliable for pregnant women, people with significant oedema or ascites, those who have undergone limb amputation, and patients with extreme heights at the tails of the distribution. In children and adolescents you should use age- and sex-specific BMI percentiles, not the adult cut-offs. When BMI gives a confusing result, follow up with waist circumference, waist-to-height ratio, DEXA, or a clinical assessment rather than acting on the number alone.
What are the most common mistakes when calculating BMI?
The single most common mistake is mixing units — entering weight in pounds with height in inches into a metric formula, or forgetting to convert centimetres to metres before squaring, both of which produce results off by factors of two to ten. Another frequent error is rounding height too aggressively (e.g. typing 170 when you are actually 173 cm) which shifts the result by 0.5–1 kg/m² near category borders. People also weigh themselves in heavy clothes, just after a large meal or while well-hydrated and treat the number as their 'true' weight; for tracking, weigh fasted in the morning under consistent conditions. Acting on a single borderline result is another pitfall — BMI naturally fluctuates a unit or so week to week, so trend over weeks matters more than any one reading. Finally, do not apply adult cut-offs to children, adolescents, athletes or the elderly without adjustment; the resulting classification is often misleading.
When should I not use this BMI calculator?
Do not use this calculator for children or adolescents under 18 — use age- and sex-specific BMI-for-age percentiles from the WHO or CDC growth charts instead, because adult cut-offs misclassify normal childhood growth as over- or under-weight. Avoid using it during pregnancy: weight gain in pregnancy is expected and pre-pregnancy BMI should be calculated separately by your maternity team. Bodybuilders, elite athletes and people with significant muscle mass should rely on body-fat percentage or DEXA rather than BMI, since they will frequently appear 'overweight' on BMI while having very low body fat. Patients with limb amputation, scoliosis, oedema, ascites or other conditions that distort weight or measured height need clinical adjustments rather than the standard formula. And if you are using BMI to make a treatment or medication decision — for example bariatric eligibility or dosing — defer to a clinician who can integrate body composition and comorbidities.