BMI stands for body mass index and is a measurement of weight for height. In other words, it measures how appropriate your weight may be based on how tall you are.
Numbers less than 18.5 and 25 or greater are considered “at risk numbers.” Many websites, including the CDC have online BMI calculators: http://www.cdc.gov/healthyweight/assessing/bmi/index.html
Less than 18.5 (UNDERWEIGHT)
18.5-24.9 (NORMAL WEIGHT)
30 or greater (OBESE)