Body Mass Index suggests desirable weight ranges. The calculation is weight (in kilos) divided by (height (in meters) squared).
For example, I am 1.77 meters tall and weigh 64 kilos. My body mass index is then 64 / (1.77 * 1.77) = approximately 20.5. A healthy body mass index is between 20 and 30 with 20 to 25 being preferable.
Anything over 30 is overweight and over 40 is dangerously so. Of course, the calculation should be taken with a grain of salt and some common sense - it ignores gender, natural body build and so on.