What body mass index actually measures
BMI is a ratio of mass to height squared. It answers one narrow question: for someone of your height, is your weight high, low or typical? It says nothing about what that weight is made of, where it sits on your body, or how healthy you are.
The measure comes from the Belgian statistician Adolphe Quetelet, who in the 1830s was looking for a way to describe the average build of a population. He found that across adults, weight scales roughly with the square of height rather than the cube, which is why the denominator is h² and not h³. A cube law would treat every tall person as overweight and every short person as thin, because human bodies do not grow isometrically — we get taller much faster than we get wider.
Life-insurance underwriters picked the ratio up in the mid-twentieth century, and clinical guidance later adopted it for screening. The justification for the cut-offs you see below is epidemiological: across large cohorts, all-cause mortality and the incidence of type 2 diabetes, hypertension and cardiovascular disease rise at both ends of the BMI distribution, with the flattest part of the curve between roughly 18.5 and 25.
That is the whole claim. BMI is a screening flag calibrated on populations, and it is used because height and weight are two of the cheapest, most reliable measurements in medicine. It is not a diagnosis, and no clinician should treat it as one.
The formula, and why the 703 factor exists
In metric units the formula needs no constant: divide kilograms by metres squared and you have BMI directly. A 70 kg adult standing 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9.
The US form looks different but is the same calculation with a unit conversion folded in. One pound per square inch of this ratio equals 703.0696 kg/m², so multiplying pounds-over-inches-squared by 703 lands you on the metric scale. The rounding to 703 introduces a small error: this calculator converts your entry to kilograms and metres and divides exactly, which is why it can return 22.807 where the 703 shortcut gives 22.805. The gap never matters clinically, but it does explain why two BMI tools can disagree in the second decimal.
Two properties of the formula are worth internalising. First, BMI is quadratic in height, so height errors are amplified: for a 70 kg adult at 1.75 m, a 1 cm mis-measurement shifts BMI by 2 × 70 ÷ 1.75³ ÷ 100 = 0.26 point, the same as an 0.8 kg weighing error. Height is measured carelessly far more often than weight is, so it is usually the larger source of noise. Second, because the denominator is fixed for an adult, BMI moves in exact proportion to weight: at 1.75 m, every kilogram is worth 1 ÷ 3.0625 = 0.327 BMI points, so a 3 kg change moves you almost exactly one point.
Worked example: 82 kg at 178 cm
Take an adult who is 178 cm tall and weighs 82 kg.
- Convert height to metres. 178 ÷ 100 = 1.78 m.
- Square it. 1.78 × 1.78 = 3.1684 m².
- Divide. 82 ÷ 3.1684 = 25.88 kg/m².
- Classify. 25.88 is at or above 25.0 and below 30.0, so the WHO category is overweight.
- Find the healthy band. 18.5 × 3.1684 = 58.6 kg at the bottom, 24.9 × 3.1684 = 78.9 kg at the top.
- Find the gap. 82 − 78.9 = 3.1 kg above the upper edge.
Check it in US units: 178 cm is 70.08 in and 82 kg is 180.78 lb, so 703 × 180.78 ÷ (70.08 × 70.08) = 127,088 ÷ 4,911 = 25.88. The two routes agree to the second decimal, as they must.
Three kilograms is a genuinely small target — about six weeks at a modest deficit. If you want that expressed as a daily calorie figure, feed the same body into the TDEE calculator and then the calorie deficit calculator.
How to read your result
Treat the category as a prompt to look further, not as a verdict. The WHO bands are wide, and the risk gradient inside each one is real — a BMI of 25.1 and a BMI of 29.9 sit in the same category and are not the same situation.
The single most useful thing to check next is your waist. A large waist at any BMI signals visceral fat, which is the fat most strongly associated with metabolic disease, and the waist-to-height ratio calculator gives you that in one measurement — the widely used rule is to keep your waist under half your height. If you want an actual body-fat estimate, the Navy body fat calculator uses tape measurements and the lean body mass calculator separates the fat and non-fat components of your weight.
Population matters too. The WHO has published lower action points for adult Asian populations — a public-health action point at 23 kg/m² and a high-risk point at 27.5 kg/m² — because cardiometabolic risk appears at a lower BMI in many Asian groups than in European ones. If that applies to you, read your number against those figures rather than 25 and 30.
Finally, watch the direction of travel rather than the absolute value. A BMI that has moved from 23 to 26 over three years is a more useful signal than a single reading of 26, and it is one you can act on before the category changes at all.
Weight at BMI 18.5, 25 and 30 for common heights
| Height | Height (ft/in) | BMI 18.5 | BMI 25 | BMI 30 |
|---|---|---|---|---|
| 150 cm | 4′11″ | 41.6 kg | 56.3 kg | 67.5 kg |
| 155 cm | 5′1″ | 44.4 kg | 60.1 kg | 72.1 kg |
| 160 cm | 5′3″ | 47.4 kg | 64.0 kg | 76.8 kg |
| 165 cm | 5′5″ | 50.4 kg | 68.1 kg | 81.7 kg |
| 170 cm | 5′7″ | 53.5 kg | 72.3 kg | 86.7 kg |
| 175 cm | 5′9″ | 56.7 kg | 76.6 kg | 91.9 kg |
| 180 cm | 5′11″ | 59.9 kg | 81.0 kg | 97.2 kg |
| 185 cm | 6′1″ | 63.3 kg | 85.6 kg | 102.7 kg |
| 190 cm | 6′3″ | 66.8 kg | 90.3 kg | 108.3 kg |
| 195 cm | 6′5″ | 70.3 kg | 95.1 kg | 114.1 kg |
Rounded to the nearest 0.1 kg. Divide by 0.45359 for pounds — 76.6 kg is 168.9 lb.
Where BMI misleads, specifically
- Muscular bodies. Muscle is denser than fat, so a heavily trained athlete can carry a BMI in the overweight or obese band with a body-fat percentage in single figures. This is the classic false positive, and it is why circumference or skinfold measurement belongs alongside BMI for anyone who lifts.
- Older adults losing muscle. Sarcopenia replaces lean tissue with fat at an unchanged weight, so BMI can sit comfortably in the normal band while body composition deteriorates. The false negative is harder to spot than the false positive because nothing about the number looks wrong.
- Children and teenagers. Below age 20 the same BMI means different things at different ages, so BMI is converted to a percentile against a growth reference. The adult cut-offs are simply not applicable.
- Pregnancy. BMI during pregnancy is not interpretable. Pre-pregnancy BMI is what guides recommended gestational weight gain.
- Very short and very tall people. The square law is an empirical fit to the middle of the height distribution. At the extremes it systematically overstates BMI for tall people and understates it for short people, which is why some researchers prefer a 2.5 exponent.
- Fat distribution. Two people with identical BMI, one carrying weight around the abdomen and one around the hips, have measurably different metabolic risk. BMI cannot see the difference; a tape measure can.
This is a screening number, not a diagnosis
No treatment decision should rest on BMI alone. Clinical guidance uses BMI as one input alongside waist circumference, blood pressure, lipids, glucose and history. If your result surprises you, the useful next step is a conversation with a clinician and a tape measure, not a diet chosen from the number on this page.
What to use instead of, or alongside, BMI
BMI survives because it is free. Every alternative asks for something extra — a tape, a pair of calipers, a scan — and each buys you a different kind of information.
Waist circumference and waist-to-height ratio cost one tape measurement and add the thing BMI most conspicuously lacks: where the mass sits. Circumference-based body fat equations, such as the US Navy method, take three or four tape measurements and return a percentage; they are less accurate than a scan but repeatable enough to track change. Skinfold calipers in trained hands are better still, and DEXA is the practical reference standard, splitting fat, lean tissue and bone mineral regionally.
Fat-free mass index deserves a mention because it fixes BMI's biggest failure directly: it applies the same height-squared denominator to lean mass alone, so a muscular athlete and a sedentary adult of the same weight no longer look identical.
If your reason for calculating BMI is to set a weight target, note that the healthy band at any height is wide — about 20 kg at 1.75 m. The ideal body weight calculator narrows that with the Devine, Robinson, Miller and Hamwi formulas, which is also how hospital pharmacists dose weight-sensitive drugs. And if the goal is fat loss rather than weight loss, the number that matters is body composition, which BMI is structurally unable to report.
Key terms
- BMI prime
- Your BMI divided by 25, the upper limit of the normal range. A BMI prime of 1.10 means you are 10% above the overweight threshold, which makes comparisons across populations with different cut-offs easier.
- Visceral fat
- Fat stored inside the abdominal cavity around the organs, as opposed to subcutaneous fat under the skin. It is the fraction most strongly associated with insulin resistance and cardiovascular risk, and waist circumference is its cheapest proxy.
- Fat-free mass
- Everything in the body that is not fat: muscle, bone, organs, water and glycogen. Weight minus fat mass.
- Quetelet index
- The original name for BMI, after Adolphe Quetelet, who described the weight-to-height-squared relationship in the 1830s as a population statistic.