BMI Calculator

Body mass index is one division: your weight in kilograms divided by the square of your height in metres. Someone 1.75 m tall weighing 77 kg has a BMI of 77 / (1.75 x 1.75) = 77 / 3.0625, or 25.14. That is the whole definition, published by the National Heart, Lung, and Blood Institute in its Clinical Guidelines on Overweight and Obesity (NIH Publication No. 98-4083), and it is the only equation running underneath this page. If you enter feet, inches and pounds, nothing changes about the maths — your figures are converted first. Most US calculators take a shortcut here: pounds divided by inches squared, multiplied by 703. That 703 is a rounded convenience constant meant for arithmetic done by hand. The exact multiplier is 703.06957964..., because NIST defines a pound as exactly 0.45359237 kilograms and an inch as exactly 0.0254 metres. This calculator converts using those exact factors and then applies the kilograms-per-metre-squared definition, so the same body described in pounds and inches and described in kilograms and centimetres produces an identical answer rather than one that drifts in the second decimal place. The category comes from the NHLBI bands: below 18.5 is underweight, 18.5 up to 25 is healthy weight, 25 up to 30 is overweight, 30 up to 35 is obesity class I, 35 up to 40 is class II, and 40 and above is class III, sometimes called extreme obesity. The World Health Organization uses the same 25 and 30 cut-offs, describing overweight as "a BMI greater than or equal to 25". Note how the boundaries are handled: NHLBI prints the healthy band as "18.5 - 24.9" and overweight as "25.0 - 29.9", but taken literally that leaves a gap no BMI is allowed to occupy. The bands here are half-open intervals, each one ending exactly where the next begins, so a BMI of precisely 25.00 is overweight and every possible value belongs to exactly one row. The displayed BMI is also rounded down rather than to nearest, for the same reason — half-up rounding can print "25.00" beside the word "healthy" for a body whose real BMI is 24.996. From the same height, the page derives the weight range that would put you in the healthy band, reported back in whichever units you typed, plus how far outside it you are if you are outside it. BMI Prime is your BMI divided by 25, so 1.00 sits exactly on the overweight threshold and 1.20 is twenty percent past it. The insight worth carrying away is what BMI cannot see. It receives two numbers, height and weight, and has no access to anything else. It does not know how much of your weight is muscle, bone, water or fat, and it does not know where fat is carried, which is the part most strongly linked to metabolic risk. A trained rugby forward and a sedentary person of the same height and weight get the same BMI and are in entirely different health positions. This is not a precision problem that better arithmetic fixes; it is inherent to a two-variable screening measure designed for describing populations, not diagnosing individuals. Clinicians pair it with waist circumference, blood pressure and blood work for that reason. The bands also apply only to adults aged 20 and over — children and teenagers are read against age- and sex-specific growth percentiles instead. If your goal is weight change rather than classification, the calorie calculator estimates the daily energy intake that moves you, and the percentage calculator is useful for tracking a change as a share of your starting weight.