Being overweight can increase your risk of a number of illnesses:coronary artery disease, high blood pressure, diabetes, and stroke. Doctors use BMI as a simple way to assess whether a patient needs to lose weight. The calculation is based on comparing a person's weight with their body height. To calculate your Body mass index [BMI]:
Divide your weight in kilograms by the square of your height in metres: BMI = W / (Hm x Hm)].

If you are 40 lbs ( 20 kilos) overweight you have significantly increased your risk of:Heart attack +360%; Cancer +80%; diabetes +2,060%; high blood pressure +260%; Arthritis +400%; gall stones +270%; early death +110%.
People with BMIs between 19 and 22 live longest. Death rates are noticeably higher for people with indexes 25 and above. A BMI of 25 to 29.9 is considered overweight and one of 30 or above is considered obese. A maximum healthy weight (based on a BMI of 25) is equal to 25 times the square of one's height in meters, or [25 x (Hm x Hm)].