BMI calculator

What is BMI?

Body mass index is your weight in kilograms divided by the square of your height in metres. The ratio goes back to the 1830s, when Adolphe Quetelet noticed that across a population, weight rises roughly with the square of height — an observation about the average person, not a diagnosis of any particular one. It got its modern name in 1972, when Ancel Keys and colleagues compared the height-and-weight formulas then in use and found this one tracked body fatness best, calling it the body mass index.

That history matters for how much weight to put on the answer. BMI is a proxy: it is calculated from two numbers that say nothing about what your weight is made of, so it cannot distinguish muscle from fat, or where on the body that fat sits. It is used because height and weight are cheap and universal to measure, not because it is the most accurate thing available.

This calculator applies the World Health Organization's adult classification — underweight below 18.5, normal from 18.5, overweight from 25, and the three obesity classes from 30, 35 and 40. Those boundaries are half-open, which is why a BMI of 24.95 is still in the normal band rather than in a gap between two categories.

How to use the BMI calculator

1. Pick metric or imperial. Metric takes centimetres and kilograms; imperial takes feet, inches and pounds. Both go through the exact conversions — the pound is defined as 0.45359237 kg and the inch as 0.0254 m — so the two systems give the same answer rather than nearly the same one.
2. Type your height and weight. Results update as you type, with no button to press. Type the numbers as precisely as you actually measured them: writing 175.0 rather than 175 tells the calculator your height is known to a tenth, and it will narrow the answer accordingly.
3. Read the range, not just the number. Alongside your BMI and its WHO category you get the range your measurement allows, the weight range that would put you between 18.5 and 25, and how many of the WHO's Asian action points you have reached. If your measurement is not precise enough to land in one category, the page says so instead of guessing.

Why this one shows a range

Almost every BMI page prints a single number to one decimal place. That number is far more precise than the measurement behind it, and the gap is not small. Over all 6,842 whole-inch, whole-pound combinations in the ordinary human range — 58 to 79 inches, 90 to 400 pounds — rounding your height to the nearest inch changes which WHO category you land in for 641 of them, about 9.3%.

Compare that with the arithmetic people actually worry about. The traditional imperial shortcut multiplies pounds over inches squared by 703, but the exact factor is 45359237/64516, or 703.0695796… Over those same 6,842 inputs, using 703 instead of the exact value changes the category exactly once — 0.01%. The ruler outweighs the rounding constant by a factor of 641.

So the honest output is an interval. This calculator reads how precisely you wrote each number, carries that through the arithmetic exactly, and reports the whole span of BMI values consistent with what you typed. Because BMI rises with weight and falls with height, the extremes of that span sit at two opposite corners of your measurement, so no searching is needed to find them.

The two ranges on the page are rounded in deliberately opposite directions. The BMI range is widened — its floor rounded down and its ceiling up — so it never claims more precision than you gave. The healthy weight range is narrowed the other way, so that every weight shown inside it genuinely does fall between BMI 18.5 and 25. Rounding in whichever direction cannot state something false is a small thing that competitors routinely get backwards, printing 25.0 beside the word normal.

Honest limitations

The exponent is a convenience. Dividing by height squared assumes weight scales with the square of height, which is close but not exact: fitted to real measurements, the exponent comes out near 2.09 for young adult men and between 1.94 and 1.99 for women. Because it is not exactly 2, BMI retains a mild dependence on height — it reads slightly high for tall people and slightly low for short ones, in a way no amount of decimal places will fix.

The thresholds are not universal. In 2004 a WHO expert consultation reported that the association between BMI, body fat and health risk differs between Asian and European populations, with substantial risk appearing below the standard overweight cut-off of 25. It kept the international boundaries as the international classification, but recommended additional public-health trigger points at 23.0, 27.5, 32.5 and 37.5 — which is why this page reports those separately rather than quietly moving the categories.

It measures neither fat nor health. A muscular person and a sedentary one of the same height and weight get the same BMI, and the index says nothing about where fat is distributed, which is itself associated with risk. Keys' own comparison found this the best of the available height-weight ratios, which is a statement about the alternatives rather than a claim to accuracy.

This page uses the adult classification only. Children and adolescents are assessed against age-and-sex percentile charts rather than these fixed numbers, so the categories here do not apply to them, and they are not meant for use during pregnancy. A number from this or any BMI calculator is a starting point for a conversation with a clinician, not a substitute for one.

Why is it free?

The whole calculation is a division and a comparison, and both happen in your browser as you type. Your height and weight are never sent anywhere, which is worth saying plainly for a page that asks for exactly the sort of personal detail people do not want stored.

That also means no account, no export limit, and no upsell standing between you and a number you could work out on paper.