A measure of a person’s weight in relation to his or her height — a way to gauge whether a person needs to lose weight. A person’s BMI is expressed as weight in kilograms divided by height in meters squared (kg/m2).
To determine your body-mass index without converting your weight to kilograms and your height to meters, first multiply your weight in pounds by 703. Then square your height by multiplying your height in inches by your height in inches. Divide the answer you get in your first calculation (weight × 703) by the answer you get in your second calculation (height squared). The result is your body-mass index.
A study in The New England Journal of Medicine showed that people with a body-mass index below 19 have the lowest death rates. Those with an index between 19 and 24.9 have a 20% higher risk of dying than people with the lowest risk; those with an index between 25 and 26.9 have a 30% higher risk; those with an index between 27 and 28.9 have a 60% higher risk; and those with an index over 29 have double (100%) the risk of dying as compared with people with a BMI less than 19. The U.S. Department of Health and Human Services considers a person overweight if he has a body-mass index of 25 to 29.9 and obese if he has a body mass-index of 30 or higher. People with a body-mass index below 18.5 are considered underweight.