BMI is calculated using both height and weight; it is calculated identically for both males and females. While BMI doesn't directly measure your percent body fat, it is a more accurate indicator of being overweight and obese than weight alone.
BMI is calculated by dividing your weight in kilograms by height in meters squared. To do so, multiply your weight in pounds by 704.5, and divide the result by your height in inches. Divide that result by height in inches a second time. (You can also use the BMI calculator at the About.com nutrition site.)
Note: According to the Weight-Control Information Network, the multiplier 704.5 is used by the National Institutes of Health (NIH), but some other organizations (i.e., American Dietetic Association) use a slightly different multiplier, which suggests you multiply by 700. The resulting number, however, only varies by a few tenths making the difference insignificant.
A BMI of 18.5 to 24.9 is considered healthy, a BMI of 25 to 29.9 is considered overweight and a BMI of 30 or more is considered obese. To understand the health risks related to being overweight or obese, see the following FAQ: What Are the Health Risks of Being Overweight?.
Weight-control Information Network. Statistics Related to Overweight and Obesity. May 2007. http://win.niddk.nih.gov/statistics/index.htm