BMI, or Body Mass Index, was developed by the Belgian statistician and anthropometrist Adolphe Quetelet (1796-1874). It is calculated by dividing the subject's weight in kilograms by the square of his/her height in metres (BMI = kg / m2).

The current definitions commonly in use establish the following values, agreed in 1997 and published in 2000:


anorexia : <17.5
underweight : <19.1 (women) or <20.7 (men)
in normal range ideal weight : 19.1-25.8 (women) or 20.7-26.4 (men)
marginally overweight : 25.8-27.3 (women) or 26.4-27.8 (men)
overweight : 27.3-32.3 (women) or 27.8-31.1 (men)
very overweight or obese : >32.3 (women) or >31.1 (men)
severely obese : 35 - 40
morbidly obese : 40 - 50
super obese : 50 - 60 kg/m2

So if you weigh 65 kg and are 1.70m tall, the calcaulation goes like this :
65 / 1.7 x 1.7 = 22.49 (normal weight)

You can also let this website calculate everything for you (also uses pounds and inches for Americans). The result will show up under "Medical Recommendation" => Your current Body Mass Index is ... kg/m2