The Body Mass Index (BMI) is a measurement that compares height to weight. It is often used to determine if a person has a healthy weight for his or her height. According to the National Heart, Lung and Blood Institute, adults with a BMI of below 18.5 are considered underweight. Those with a BMI of 18.5 to 24.9 are normal. Those with a BMI of 25 to 29.9 are overweight. And those with a BMI of over 30 are obese. (Ref.1.) Finding your BMI will help you to determine how much you should weigh. The BMI is used in the same way for both men and women.
Things You'll Need
 Scale
 Calculator
 Pen or pencil
 Tape measure
 Wall or board that can be marked
Measurements

Hold the pen and stand with your back to the wall. Stand up straight, but do not exaggerate your height. Use the pen to make a mark on the wall at the level of your head.

Use the tape measure to calculate the distance between the floor and the mark you have made, ideally in centimeters, although inches will work as well. Write this measurement on a piece of paper, and label it "Height."

Step on the scale and record your weight, preferably in kilograms. If you measured your height in inches, just use pounds; this will prevent further complications later on. Write this measurement on the paper and label it "Weight." In order to insure accuracy, weigh yourself without heavy clothes and without shoes, and place the scale on a flat, hard surface.
Calculations

Use the calculator to square your height measurement. Write this on the paper as "Height Squared."

If you measured yourself in inches and pounds, proceed to step 3. If not, skip step 3 and divide your weight by your height squared. Write this down and label it "BMI."

Multiply your weight by 703 and divide by your height squared. Write the result down as "BMI."

Compare your BMI to the range of indexes above in order to determine if you are underweight, normal, overweight, or obese.