BMI Calculator

This BMI calculator turns a height and weight pair into a single body mass index figure and tells you which standard category that figure falls into. You can enter measurements in metric form, using kilograms and centimetres, or in imperial form, using pounds and inches. The result is rounded to one decimal place and shown alongside the category label, together with a scale marking the boundaries at 18.5, 25, 30 and 40. It suits anyone who wants a quick screening number before a health check, a fitness review or an insurance form, and anyone comparing a reading taken at a clinic with one worked out at home. BMI is a population level screening measure rather than a diagnosis, so treat the figure as a starting point for a conversation with a clinician rather than an assessment of your health.

Body mass index is defined as mass in kilograms divided by the square of height in metres, written as kg/m2. The metric path in this calculator does exactly that. It takes the height you enter in centimetres, divides it by 100 to obtain metres, squares that value, then divides your weight in kilograms by the result.

The imperial path uses the standard conversion factor of 703. Pounds and inches are not SI units, so the raw ratio of pounds to squared inches has to be scaled to match the metric figure. The calculation is 703 multiplied by weight in pounds, divided by height in inches squared. The 703 comes from the conversion constants themselves: a kilogram is about 2.2046 pounds and a metre is 39.3701 inches, and combining those two conversions produces a factor very close to 703.

Both paths round the final index to one decimal place. The rounded value is then compared against four fixed thresholds to produce the category label: below 18.5, below 25, below 30, and 30 or above.

Take a metric example first. Someone weighs 72 kg and stands 175 cm tall. The calculator converts 175 cm to 1.75 m, then squares it: 1.75 multiplied by 1.75 gives 3.0625. Dividing 72 by 3.0625 returns 23.5102, which rounds to 23.5. Because 23.5 sits between 18.5 and 25, the category shown is normal weight.

Now an imperial example. Someone weighs 160 lb and measures 68 in, which is five feet eight inches. The calculator multiplies 703 by 160 to get 112,480, then squares 68 to get 4,624. Dividing 112,480 by 4,624 returns 24.3252, which rounds to 24.3, again inside the normal band but noticeably closer to the 25 boundary than the first example.

The gap between those two results shows why precision in the height entry matters. If the second person had entered 67 in rather than 68 in, the denominator would fall to 4,489 and the index would rise to 25.1, crossing into the next category on a one inch difference.

The categories used here are the widely published adult thresholds: under 18.5 is labelled underweight, 18.5 to just under 25 normal weight, 25 to just under 30 overweight, and 30 and above obese. They apply to adults and were never designed for children, who are assessed against age and sex specific growth charts instead.

The formula counts total mass and cannot tell muscle from fat. A heavily trained athlete can register in the overweight band while carrying very little body fat, and an older adult who has lost muscle can register in the normal band while carrying a good deal. Body composition, waist measurement and blood markers all add information that BMI alone does not carry. The index also behaves differently across populations, and some health bodies apply lower thresholds for people of South Asian and East Asian descent.

Common input mistakes are entering height in metres when the field expects centimetres, which produces an absurdly large index, and mixing metric weight with imperial height. This tool is for general information and is not medical advice.