How to Calculate BMI: A Complete Guide
BMI takes two measurements and one division — but knowing what the result does and does not tell you matters more than the arithmetic.
By the HealthMedHub Editorial Team · Published October 2026 · Reviewed for accuracy against WHO, NIH, and NHS sources · Our review process
What BMI Actually Measures
Body mass index is a single number derived from your weight and height. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet, who was studying the distribution of human physical traits across populations rather than diagnosing individuals. That origin explains both what BMI is good at and where it falls short. It describes how heavy a person is relative to their height, nothing more. It does not measure body fat, distinguish muscle from fat, or say anything about where fat is stored.
Despite that narrowness, BMI remains the first-line screening measure in most clinical settings because it is cheap, quick, and reproducible. You need no equipment beyond a scale and a tape measure, and two people measuring the same person will usually get the same answer. The World Health Organization uses BMI to define overweight and obesity at the population level, and its fact sheet notes that worldwide obesity has roughly tripled since 1975. The NIDDK describes BMI as a screening tool that identifies people who may benefit from further assessment.
That distinction is the most important thing to understand before you calculate your own: BMI is a gatekeeper, not a verdict. Its adoption was a matter of convenience rather than principle. Quetelet's ratio had the appeal of being a single dimensionless number that needed no reference tables, and the World Health Organization adopted it as an international classification in the 1990s. The cut-off points in use today derive from the associations observed between BMI and mortality in large cohort studies — associations that are real and consistent across populations, but that say much less about any one individual.
How to Calculate BMI in Metric Units
The metric formula is the simpler of the two and the one most of the world uses: BMI = weight in kilograms divided by height in metres squared.
Worked example. A person weighs 78 kg and is 1.75 m tall. First square the height: 1.75 times 1.75 equals 3.0625. Then divide weight by that figure: 78 divided by 3.0625 equals 25.5. So the BMI is 25.5, which falls just inside the overweight band.
Two practical notes. First, convert centimetres to metres before squaring: 175 cm becomes 1.75 m, not 175. Squaring 175 gives 30,625 and an answer that is meaningless. Second, do not round the height before squaring; keep the decimal places and round only the final BMI, usually to one decimal place. A difference of a centimetre or two can move the result across a category boundary, so measure without shoes and stand tall against a wall.
The order of operations catches people out more often than the arithmetic itself. Squaring happens before dividing, because the formula is weight divided by height squared. On a calculator the sequence is: enter the height in metres, multiply by itself, store that result, then divide weight by it. In a spreadsheet the formula is =weight/(height*height), with the brackets doing the same job. Getting the order wrong usually produces an obviously wrong answer — a BMI in the hundreds or below one — which is a useful signal that something went astray.
How to Calculate BMI in Imperial Units
The imperial formula carries an extra conversion factor: BMI = (weight in pounds divided by height in inches squared) times 703.
Worked example. A person weighs 172 pounds and is 69 inches tall, or 5 ft 9 in. Square the height: 69 times 69 equals 4,761. Divide weight by that: 172 divided by 4,761 equals 0.03613. Multiply by 703, giving 25.4.
The 703 is not arbitrary. It exists to reconcile the metric formula with imperial inputs, converting pounds per square inch into kilograms per square metre. A common alternative uses feet and inches with a different constant, but the inches-only version above is easier to get right because it avoids mixing units. If you would rather not do the arithmetic at all, the BMI Calculator handles both unit systems and returns the category alongside the number.
If you prefer to work in feet and inches, convert to inches first. Multiply the feet by 12 and add the remaining inches: 5 ft 9 in becomes (5 times 12) + 9 = 69 inches. The most common error is forgetting to convert and entering 5.9, which produces a BMI roughly twelve times too high. Because BMI is sensitive to the square of height, height errors distort the result far more than weight errors do: a two-centimetre mistake in height moves BMI by around 0.5 points, while a one-kilogram mistake in weight moves it by only about 0.3.
Understanding Your BMI Category
The World Health Organization's adult categories are the standard reference. A BMI below 18.5 is classified as underweight. From 18.5 to 24.9 is the healthy weight range. From 25.0 to 29.9 is overweight, and 30.0 or above is obesity, subdivided into class I (30.0 to 34.9), class II (35.0 to 39.9) and class III (40.0 and above). These bands are identical for adult men and women, because the calculation already adjusts for height.
Two important qualifications. The thresholds were derived largely from populations of European descent, and the WHO recognises that different cut-off points are more appropriate for some Asian populations, where cardiometabolic risk rises at a lower BMI. Separately, the categories are not designed for children, who are assessed against age- and sex-specific growth percentiles, nor for pregnant women, competitive athletes, or older adults with significant muscle loss.
Being one decimal place above a threshold is not a meaningful change in health status. What matters is the trend over months and the presence of other risk factors such as blood pressure, blood glucose, waist circumference, and family history. If your BMI sits outside the healthy range, the Ideal Weight Calculator can show a weight range associated with that band for your height, which is often a more useful target than a single figure.
It is worth looking at how the number moves as well as where it lands. Because BMI is calculated from two measurements that are each slightly imprecise, a fluctuation of a few tenths of a point between one week and the next is usually noise. Weight alone can shift by one to two kilograms over a day depending on food, fluid, and time of day. Measure under the same conditions each time and compare readings weeks or months apart; a sustained change of one or two BMI points over several months is meaningful, while a single reading that crosses a threshold is not.
When BMI Misleads, and What to Use Instead
BMI fails in predictable ways. Muscle is denser than fat, so a trained athlete can register as overweight while carrying very little fat. The reverse also happens: someone with low muscle mass and a normal BMI may carry a high proportion of body fat, a pattern sometimes called normal-weight obesity. BMI also ignores fat distribution, and it is abdominal fat specifically, measured by waist circumference, that appears most closely associated with cardiovascular and metabolic risk.
This is why clinicians rarely act on BMI alone. The NHLBI assessment combines BMI with waist measurement and an evaluation of other risk factors before recommending any intervention. Risk rises from around 94 cm (37 in) in men and 80 cm (31.5 in) in women, with substantially higher risk at 102 cm (40 in) and 88 cm (35 in) respectively. Body-fat percentage gives a more direct picture of composition, and the Body Fat Calculator estimates it from simple measurements.
A waist-to-height ratio is perhaps the simplest upgrade. Divide your waist measurement by your height in the same units; a ratio below 0.5 is a widely used rule of thumb, meaning your waist should measure less than half your height. Unlike BMI, this captures central adiposity directly, and it needs only a tape measure.
None of this is an argument for ignoring BMI. For all its limits, it remains the most practical way to screen a whole population, and it correlates well enough with body-fatness across large groups to be useful. The mistake is treating a screening result as a diagnosis: a high BMI is a prompt to look further, not a conclusion, and a normal BMI is not a clean bill of health if other measures suggest otherwise. See Why BMI Is Not a Perfect Measure of Health for the wider debate.
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