⚖️ BMI calculator — what your height and weight come to
Put your height and weight in whatever units you use — centimetres, metres or feet and inches; kilograms, pounds or stone — and see where the two land on the body mass index scale, what the healthy weight range for your height is, and how far off it you are.
Your result
Your weight, and the weight the band asks for
What each band weighs at your height
The World Health Organization's adult bands, and the weight each one runs between for somebody your height. The band you are in is marked.
BMI is a screening figure, not a diagnosis. It is two numbers turned into one, and the thing it throws away is what the weight is made of: muscle and fat weigh the same, so a muscular athlete and somebody who has never exercised can read the same, and it says nothing about where the weight sits — a tape measure round the waist is the better guide to that. The bands here are for adults: children and teenagers are read against age- and sex-specific percentile charts instead, pregnancy changes what is normal, and the thresholds themselves differ between populations, with the WHO's Asia-Pacific cut-offs treating 23 as overweight and 25 as obese. None of this is medical advice, and a decision about your weight belongs with a doctor or dietitian who can see you rather than with a page that only knows your height.
Why use this calculator?
BMI is the figure on the form at the doctor's, and it is the same division for a sprinter and for somebody who has not moved since March — which is both what makes it useful as a first look and what makes it worth understanding before you read anything into it. This page does the division in whatever units you happen to think in, and then does the parts that make the number mean something: the band it lands in, the weight range that band covers at your height, how far your weight is from it, and what a single BMI point is worth in kilograms. It also says plainly what the figure cannot tell you.
Where the number comes from
The figure is a good deal older than its name, and both are worth knowing:
- It was described in the 1830s by Adolphe Quetelet, a Belgian astronomer and statistician who was interested in what an average person looked like. Weight over height squared was his way of comparing populations, and for over a century it was called Quetelet's index. It only became the body mass index in 1972, in a paper by the American physiologist Ancel Keys weighing up the height-and-weight ratios then in use; the one this page uses is the one he judged best of them.
- It was never built to judge one person. At any given BMI there is a wide spread of body fat — the same figure covers a light-boned person carrying fat and a heavy-boned one carrying muscle — because muscle and fat weigh exactly the same. That is why a doctor who cares about the risks will usually reach for a tape measure as well: fat carried round the middle is the part that matters most, and BMI cannot see where yours is.
- The scale is a square of your height, so a point of it is worth more the taller you are: about 2.6 kg at 1.6 m and about 3.6 kg at 1.9 m. At 1.75 m the healthy band runs from 56.7 kg to 76.6 kg, which is a range of about 20 kg — the honest width behind a figure people quote to one decimal place.
How does this calculator work?
One division, with both sides turned into kilograms and metres first:
BMI = weight in kilograms ÷ (height in metres × height in metres)
The rest of the page is that figure read from another angle:
- Height: centimetres ÷ 100 gives metres; feet × 12 plus the inches, × 2.54, gives centimetres; metres are already metres. The 2.54 is exact — an inch has been 2.54 cm by definition since 1959.
- Weight: pounds × 0.45359237 gives kilograms, and stone × 14 plus the leftover pounds gives pounds first. That factor is exact by definition too, so the only rounding anywhere in the sum is in what the page prints.
- The bands are the World Health Organization's adult cut-offs — under 18.5, 18.5 to 25, 25 to 30, then 30 to 35, 35 to 40 and 40 and over. The table writes them as 18.5 to 24.9 and 25 to 29.9 because that is how the printed cut-offs round; the arithmetic uses the boundaries themselves.
- The healthy weight range for a height is the same division read backwards: 18.5 × height² up to 25 × height². That is why the range is a band of kilograms rather than one target weight, and why it widens as you get taller.
- One BMI point is your height squared in kilograms — the amount of weight that moves the figure by exactly 1.0 at your height.
- The rules of thumb are four old clinical formulas — Hamwi (1964), Devine (1974), Robinson (1983) and Miller (1983) — that give different answers for men and women. They were derived for medical dosing rather than for judging anybody, which is why the page shows them as the range they are and not as a target.
Frequently asked questions
Is BMI the same for men and for women?
The arithmetic is: the same height and weight give the same figure whoever you are. Where sex comes into it is the reading. At the same BMI, men on average carry more muscle and women more fat, because the cut-offs were drawn from population data rather than from either group on its own. The four rules of thumb under the result are the one place on this page that treats the two differently, and they are old dosing formulas rather than targets.
What is a healthy BMI?
For adults, the World Health Organization puts the healthy band at 18.5 to just under 25, with overweight from 25 and obesity from 30 — split above that into three classes. At 1.75 m that band is 56.7 kg to 76.6 kg. The cut-offs are the same for men and women, and they are a screening tool rather than a line between well and unwell: plenty of people sit just outside a band and are perfectly healthy, and some sit comfortably inside one and are not.
Why does mine say overweight when I am fit?
Because muscle weighs exactly what fat weighs, and the figure cannot tell them apart. A trained athlete with a lot of muscle can read as overweight, or even obese, while carrying very little fat. It works the other way too: somebody who has lost muscle with age can read as healthy while carrying more fat than the figure suggests. Nothing in the division can settle it, which is why a waist measurement and a look at what the weight is made of matter more than one decimal place.
Why is this not used for children?
Because a child's height is still changing, so the same BMI means something different at six than at sixteen. Children and teenagers are read against percentile charts built from children of the same age and sex, not against fixed bands. The adult cut-offs only work once growing has stopped, which is why the form here is the adult one.
What are the Asia-Pacific cut-offs?
A second set of thresholds the World Health Organization published in 2004 for populations of Asian descent: 23 counts as overweight and 25 as obese, rather than 25 and 30. The reason is that at the same BMI, those populations tend on average to carry more fat, and more of it round the middle, so the risks linked to it arrive at a lower figure. If that applies to you, read your number against 23 and 25 as well — the weight table above already shows what those figures mean at your height, so the arithmetic is unchanged.
I lost weight and my BMI barely moved. Why?
Because a point of BMI is a lot of kilograms. It is your height squared, so at 1.75 m one whole point is about 3.1 kg, and at 1.9 m it is about 3.6 kg — a loss that feels like a milestone can be well under a point. The bands are also several points wide: what changes at 25 is a label, not your body, and the figure is a rough map rather than a scoreboard.
Does this tell me what I should weigh?
No, and it would be wrong of a calculator to try. It gives the weight range that the population data links to the lowest risk at your height, which is about 20 kg wide at 1.75 m — and where the right weight sits inside that range, or whether it sits outside it, depends on your build, your muscle, your history and what you can actually keep up. The rules of thumb under the result are not a target either: they were written to work out drug doses, and a person is not a dose.