BMI Calculator

The formula is BMI = weight ÷ height², with weight in kilograms and height in meters. At 70 kg and 1.70 m that is 70 ÷ (1.70 × 1.70) ≈ 24.2. If you think in pounds and inches, use the unit toggle — the arithmetic is identical, only the display changes. The index came out of nineteenth-century social statistics, built to describe the distribution of weight across a population rather than to assess one person. That origin explains most of what follows: BMI compares groups and tracks trends well, and is a blunt instrument when pointed at an individual. Squaring the height is an approximation, not a law of biology. Bodies grow in three dimensions, yet weight does not scale with the cube of height either, because proportions shift as people get taller. The exponent 2 was simply the one that best cancelled height out of large populations — which is not the same as working equally well at the extremes of the height range.

The classification used here is the WHO range for adults, the same one applied in the United States: below 18.5 is underweight, 18.5 to 24.9 is the normal range, 25 to 29.9 is overweight, and 30 or above is obesity, split into class I (30 to 34.9), class II (35 to 39.9) and class III (40 or above). These cut-offs are not biological boundaries. They were drawn because, across large populations, the frequency of certain health outcomes starts climbing past certain levels. The underlying risk is continuous — nobody changes state between 24.9 and 25.1. The bands exist to organize screening, not to label a person. The page also shows the weight range that corresponds to 18.5–24.9 at your height. Read that pair for what it is, the formula run backwards, rather than a target to chase. Weight is not the only thing that moves when body composition changes.

BMI knows two numbers. Everything not contained in weight and height is invisible to it. It does not separate lean mass from fat. Muscle is denser than fat tissue, so someone carrying a lot of it can land in the overweight or obese band with very little fat — the classic case for strength athletes and contact-sport players. On its own, BMI is not a useful assessment for them. It also does not know where the fat sits, and location matters. Fat stored in the abdomen, around the organs, is more strongly associated with cardiovascular and metabolic problems than fat carried on the hips and limbs. Two people with the same BMI can be in quite different situations, which is why waist circumference is usually measured alongside it. The reverse case exists too: someone with little muscle and a high proportion of fat can sit comfortably inside the "normal" band. A BMI in range certifies nothing by itself. The relationship between BMI and risk is also not identical in every population. The WHO has discussed additional action points for groups in which the same band maps to a different risk profile — a question for the WHO documentation and a clinician, not for a generic table.

The bands on this page are for non-pregnant adults. Outside that group, the reading changes. Children and teenagers: the calculation is the same, but the result must be placed on BMI-for-age percentile charts by age and sex. A BMI of 19 can be low, fine or high depending on age, because body composition shifts throughout growth. Adult bands applied to a child produce the wrong conclusion; the CDC publishes the growth charts used for ages 2 to 19. Older adults: the literature discusses separate cut-offs past roughly 65, generally with the healthy band shifted upward, because of how body reserve, muscle loss and frailty interact in later life. The height loss that often comes with age also shrinks the denominator and inflates BMI even when nothing about the tissue has changed. Pregnancy: ordinary BMI does not apply. Prenatal care uses pre-pregnancy BMI together with gestational weight-gain ranges, tracked by a provider.

BMI is a cheap, fast starting point, and that is how it should be treated: a signal to look more closely, never a diagnosis. A result outside the reference range — especially at the higher classes, or alongside other markers — is a reason to talk to a physician or a registered dietitian, who can weigh lab work, medical history, current medication, fat distribution, strength and the whole clinical picture. None of that fits inside weight ÷ height². The same applies in the other direction: weight loss you did not intend, or a low BMI with no explanation, is a finding that deserves assessment rather than congratulation. This page calculates and explains. It does not tell you how much to eat, how much to train, or what weight to reach — those are clinical, individual decisions made with a health professional.