BMI Calculator
Body mass index is your weight divided by the square of your height. Enter both in metric or imperial units and this gives you the number, the World Health Organization band it falls in, and the weights that would put you inside the healthy band at your height.
Height and weight
Centimetres, without shoes. 1.75 m is 175.
Kilograms. Weigh yourself at the same time of day for a figure you can compare week to week.
23.5 falls in the WHO normal weight band (18.5 to 24.9). At this height the healthy band spans 56.7 to 76.3 kg.
- WHO category
- Normal weight
- Band this category covers
- BMI 18.5 to 24.9
- Healthy weight at this height
- 56.7 to 76.3 kg
- Distance from that range
- Inside the range
- BMI Prime
- 0.94
- Ponderal Index
- 13.4 kg/m³
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. The advanced figures below are a few more divisions on the same two numbers; everything else on this page is a unit conversion or a lookup table.
BMI = kg ÷ m² • BMI = 703.06958 × lb ÷ in²- kg, m
- Weight in kilograms, height in metres
- lb, in
- Weight in pounds, height in total inches
- 703.06958
- 0.45359237 kg per pound ÷ (0.0254 m per inch)² — the exact conversion the familiar 703 rounds off
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.
The advanced section adds three more figures, none of which need a new measurement beyond the waist field.
BMI Prime = BMI ÷ 25 • Ponderal Index = kg ÷ m³ • Waist-to-height = waist ÷ height- BMI Prime
- Your BMI restated as a ratio to the WHO overweight threshold; 1.00 sits exactly on that line (Gadzik, 2006)
- Ponderal Index
- Mass over height cubed rather than squared, in kg/m³ — the direct fix for BMI’s height bias (Rohrer, 1921)
- Waist-to-height
- Waist and height in the same unit, so the ratio needs no conversion; flagged at 0.5 or above (Ashwell & Gibson, 2014)
Age and sex take no part in any formula on this page. Both only select which sentence of context appears beside the result — never a number, never a band.
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.
The same 72 kg and 175 cm give two more readings for free, no new measurement required. BMI Prime is BMI ÷ 25: 23.5 ÷ 25 = 0.94, four hundredths under the overweight line. The Ponderal Index swaps the square for a cube: 72 ÷ 1.75³ = 72 ÷ 5.359375 = 13.43, published as 13.4 kg/m³.
A waist measurement adds a third figure. At 80 cm, the waist-to-height ratio is 80 ÷ 175 = 0.457, printed as 0.46 — under the published 0.5 boundary, so no flag appears. At 90 cm and the same height, the ratio is 90 ÷ 175 = 0.514, printed as 0.51, at or above that boundary, and the page says so. Entering an age or a sex changes none of the figures above — both only add a sentence of CDC’s own context on how body fat at a given BMI tends to vary by age and by sex.
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 Prime and the Ponderal Index are read off the same height and weight above, not a separate measurement — if those figures are off, both inherit the error.
- Age and sex, when entered, only add a sentence of context. Neither changes the BMI figure, its category, or the healthy-weight range above.
- A waist measurement, when entered, is taken level with the navel, relaxed, at the end of a normal breath out. A looser or tighter tape moves the ratio more than it moves the fat underneath it.
- The 0.5 waist-to-height boundary is a general screening rule, not a diagnosis, and — like the BMI bands above — it was not fitted separately by ancestry or body type.
BMI FAQ
What counts as a healthy BMI?
The World Health Organization puts the healthy band for adults at 18.5 through 24.9. Below 18.5 is classed as underweight, 25.0 to 29.9 as overweight, and 30.0 or more as obese. Those are the bands this calculator reports. They are the same for men and women and do not change with age, which is one of the reasons a single number cannot tell you very much on its own.
Is BMI accurate for muscular or athletic people?
No, and it is not meant to be. BMI has no term for body composition — it divides total weight by height squared and cannot see whether that weight is muscle, fat, bone, or fluid. Muscle is denser than fat, so a well-trained rugby player and a sedentary person of the same height and weight get the same BMI and the same band despite very different bodies. If body composition is the question, a circumference or skinfold method answers it better.
Does BMI work for children and teenagers?
Not with these bands. Children's body composition changes with growth, so BMI under age 20 is read against age-and-sex percentile charts rather than the fixed adult cut-points — the same BMI means something different at 8 than at 17. This calculator is for adults. For anyone younger, use a paediatric percentile chart or ask a clinician.
Why do some countries use lower BMI cut-offs?
Because the risk attached to a given BMI is not the same everywhere. The WHO cut-points were drawn largely from populations of European ancestry, and a WHO expert consultation found that several Asian populations show raised risk of type 2 diabetes and cardiovascular disease at lower BMIs. WHO kept the international bands but recommended additional public-health action points, commonly cited as 23 and 27.5. Several national guidelines have adopted lower thresholds on that basis.
What is the 703 in the imperial BMI formula?
It is the metric formula in disguise. BMI is kilograms over metres squared, so to use pounds and inches you have to convert both — and 0.45359237 kg per pound divided by 0.0254 m per inch squared comes to 703.06958. Rounding that to 703 is the shortcut you see printed everywhere. This calculator uses the exact factors instead; the difference is under 0.005 BMI points, which never shows at one decimal place.
Should I aim for the middle of the healthy range?
There is no evidence that a particular point inside the band is better than another, and chasing a specific number is rarely useful. The healthy range shown here is arithmetic — the weights that put your BMI between 18.5 and 24.9 at your height — not a target anyone has prescribed for you. What to do with it, if anything, is a conversation for you and a clinician.
What is BMI Prime, and how is it different from BMI?
BMI Prime is your BMI divided by 25, the WHO overweight threshold — a rescaling proposed by physician James Gadzik in 2006. A BMI Prime of 1.00 sits exactly on the overweight line; 0.94 means 6% under it, 1.20 means 20% over it. It carries the same information as your BMI, just rescaled so the overweight threshold is always 1.0 regardless of height, which makes it easier to compare how far two people of different heights sit from that same line.
What is the Ponderal Index, and why divide by height cubed instead of squared?
The Ponderal Index is mass divided by height cubed rather than squared — kilograms per cubic metre. Squaring height, as BMI does, is a statistical convenience that reads slightly high for tall people and slightly low for short ones, because a body's mass scales closer to the cube of its height than the square. Cubing height was proposed by Fritz Rohrer in 1921 for exactly that reason, and it remains in occasional clinical use as a height-bias check alongside BMI rather than a replacement for it. Adult values typically fall between 11 and 15 kg/m³.
What does the waist-to-height ratio tell me that BMI does not?
BMI cannot see where weight sits on the body, and fat carried around the waist carries more cardiometabolic risk than the same fat on the hips or thighs. Ashwell and Gibson's 2014 review proposed a single cross-population rule: keep waist circumference under half your height, a boundary of 0.5. NHLBI publishes a related, unit-based guideline — a waist over 40 inches for men or 35 inches for women signals higher risk — which points the same direction without needing a height figure at all. Neither is diagnostic; both are screening signals, same as BMI itself.
Does entering my age or sex change my BMI or its category?
No. Age and sex here add context only — CDC notes that, at the same BMI, women tend to carry more body fat than men, and older adults tend to carry more than younger ones. Neither observation is built into the arithmetic; WHO's bands are the same number for every adult regardless of age or sex, and this calculator keeps them that way. What the fields do is flag when the fixed adult bands do not apply at all: under 20, the right comparison is a BMI-for-age percentile chart, not these categories.
Sources and review notes
- 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
- Centers for Disease Control and Prevention — Adult BMI, for the imperial formula and the screening-not-diagnosis framing
- NIST Special Publication 811 — the exact inch and pound definitions used for the imperial conversion
- CDC — About Body Mass Index, the source of the note that body fat at a given BMI runs higher for women than men and for older adults than younger ones
- Gadzik, “‘How much should I weigh?’ — Quetelet’s equation, upper weight limits, and BMI prime,” Connecticut Medicine, 2006 — the source of BMI Prime
- Flewellen & Bee, “Ponderal index: quantifying obesity,” JAMA, 1979 — the source of the Ponderal Index formula
- Ashwell & Gibson, “A proposal for a primary screening tool: ‘Keep your waist circumference to less than half your height,’” BMC Medicine, 2014 — the source of the waist-to-height ratio and its 0.5 boundary
- NHLBI — Aim for a Healthy Weight, for the complementary waist-circumference guideline (over 40 in for men, 35 in for women)
The arithmetic here is a handful of divisions 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.