BMI Calculator

This is general information, not medical advice

This page exists to do arithmetic and to explain where the arithmetic came from. It is general educational information about a public-health measure. It is not medical advice, it is not a diagnosis, and it cannot take account of anything about you that a height and a weight do not capture — which is nearly everything.

A BMI figure on its own says nothing about whether you are well. If you are worried about your weight, or thinking about changing how you eat or train, or you have been given a number by a form or an insurer and do not know what to make of it, talk to a doctor, nurse, or registered dietitian. They can look at the things this page cannot: your history, your bloods, your medications, your build, and what you actually want.

If you are pregnant, under 20, an older adult, recovering from illness, or living with an eating disorder, the adult bands below were not drawn with you in mind and are especially likely to mislead. Please do not use this page as a reason to act alone.

How the number is worked out

BMI is one division. Everything else on this page is either a unit conversion or a lookup table.

The healthy weight range is the same formula run backwards: the weights that produce a BMI of 18.5 and 24.9 at the height you entered. The bands themselves are continuous, not the gapped 18.5–24.9 and 25.0–29.9 you see on posters. A BMI of exactly 25 is overweight; the printed 24.9 is a rounded upper bound, not a boundary with a gap after it.

One deliberate choice: the band is decided from the figure rounded to one decimal place, the same figure shown on screen. A raw 24.96 displayed as 25.0 but labelled “normal weight” would read as a bug, so the number and its label always agree here.

175 cm and 72 kg, step by step

Convert the height to metres: 175 cm is 1.75 m. Square it: 1.75 × 1.75 = 3.0625. Divide the weight by that: 72 ÷ 3.0625 = 23.51, which prints as 23.5. That sits between 18.5 and 25, so the band is normal weight.

For the healthy range, run it the other way. The lightest healthy weight at 1.75 m is 18.5 × 3.0625 = 56.66 kg, and the heaviest is 24.9 × 3.0625 = 76.26 kg — 56.7 kg to 76.3 kg once rounded. A person at 92 kg and the same height has a BMI of 30.0 and sits 15.7 kg above the top of that range.

The imperial path reaches the same place. 5 ft 9 in is 69 inches and 159 lb is 72.12 kg, so 69 × 0.0254 = 1.7526 m, squared is 3.0716, and 72.12 ÷ 3.0716 = 23.48 — again 23.5. The healthy range at that height is 125.3 lb to 168.6 lb.

What BMI cannot see

The index is older than the use it is put to. Adolphe Quetelet, a Belgian astronomer and statistician, described the ratio of weight to height squared in the 1830s while trying to characterise “the average man” — a statistical composite of a population. He was describing groups, not assessing individuals, and he never proposed it as a measure of anyone’s health. The name “body mass index” came much later, from Ancel Keys and colleagues in 1972, who tested the available height-weight indices and found Quetelet’s the best of a mediocre set for population studies. They said so explicitly. The move from describing populations to labelling individuals happened afterwards, and the index was never redesigned for it.

That history shows up in three concrete ways.

• It has no term for body composition. Weight is weight. A muscular person and a sedentary person of the same height and mass get the same BMI and the same band, and the two bodies are not comparable in any way that matters clinically. The same blindness runs the other way: someone within the healthy band can carry a lot of fat and little muscle. BMI cannot distinguish any of it.
• The cut-points came largely from European-ancestry populations. They are not a law of biology; they are where risk curves started to bend in the data available when the bands were set. A WHO expert consultation found that several Asian populations show raised risk of type 2 diabetes and cardiovascular disease at lower BMIs, and WHO recommended additional public-health action points — often cited as 23 and 27.5 — while keeping the international bands for comparability. This calculator reports the international bands, because deciding which population someone belongs to is not a choice a web form should make.
• It squares height, and people are not squares. Dividing by height squared was a convenience that made the numbers behave across an average population. It systematically reads slightly high for tall people and slightly low for short ones, because mass in a solid body scales closer to the cube of a linear dimension than the square.

None of that makes BMI useless. It is cheap, it needs no equipment, it is reproducible, and across large groups it tracks population-level risk well enough to be worth measuring. It is a screening signal, not a verdict on a person.

What this calculator assumes

• You are an adult. The fixed WHO bands do not apply under age 20, where BMI is read against age-and-sex percentile charts instead.
• You are not pregnant. Pregnancy changes weight for reasons BMI cannot interpret, and pre-pregnancy BMI is the figure clinicians use.
• The international WHO cut-points are the right ones for you. If your clinician or national guideline uses lower thresholds, take theirs over these.
• Height is measured without shoes and weight without heavy clothing. A 2 cm error in height moves a BMI of 23.5 by roughly 0.5 — height matters more than most people expect, because it is squared.
• Imperial figures are converted with the exact definitions — 1 inch = 0.0254 m, 1 pound = 0.45359237 kg — rather than the rounded 703 multiplier.
• The healthy weight range is the arithmetic span of BMI 18.5 to 24.9 at your height. It is not a target and nobody has prescribed it for you.

BMI FAQ

Sources and review notes

1. World Health Organization — Obesity and overweight fact sheet, the source of the 18.5 / 25 / 30 adult cut-points and of the note that risk at a given BMI varies between populations
2. Centers for Disease Control and Prevention — Adult BMI, for the imperial formula and the screening-not-diagnosis framing
3. NIST Special Publication 811 — the exact inch and pound definitions used for the imperial conversion

The arithmetic here is a single division and has nothing behind it to go stale. The bands do change: they are a public-health convention, revised by committee, and the WHO fact sheet above is the version this page reports. Where a national guideline differs from it, the national guideline is the one your clinician will use.