Body mass index, or BMI is an easy way to estimate how much fat your body contains. A healthy BMI falls between 18.5 and 24.9, while a BMI that's too high or too low may mean you're at risk for health issues related to your size. The measure isn't infallible, however, and it can overestimate fatness in muscular people and underestimate fatness in some sedentary individuals. Your BMI may give you insight into the health of your body size, but it isn't a diagnostic tool.
Determining Your BMI
Figure your BMI using an online program or a calculator. The formula requires that you divide your weight in kilograms by the square of your height in meters. To adjust the formula for pounds and inches, use a conversion factor of 703. The formula thus becomes your weight in pounds divided by your height in inches squared; multiply the results by 703. The equation using pounds and inches is: BMI = weight / (height x height) x 703.
Bioelectrical impedance -- used in body fat scales -- body fat calipers, DEXA X-ray scans and hydrostatic weighing provide more accurate readings of body fat levels, but they aren't always convenient, available or affordable. Although BMI doesn't measure your fat directly, as do these other methods, it does correlate moderately with fat levels determined using these more precise screenings.
Assessing Your BMI
If your BMI falls below 18.5, you're considered underweight, and your energy, immunity and stamina might improve if you gain weight. A BMI between 18.5 and 24.9 places you in a normal category and at no increased risk for disease. A BMI from 25 to 29.9 qualifies you as overweight and at elevated risk of medical complications. A BMI above 30 is considered obese, which puts you at an extreme risk for health problems, such as high blood pressure, sleep apnea, type 2 diabetes, joint pain, heart disease, stroke and possible early death.
Examples of people who fall into the range of "normal" BMI are a 5-foot, 4-inch person who weighs 110 to 144; a 5-foot, 10-inch person weighing 132 to 173 pounds; and a 6-foot person who weighs 140 to 183. If the 5-foot, 4-inch person weighs 145 to 173, she is classified as having an overweight BMI, if she weighs 174 to 227 pounds, she qualifies as obese. In the same manner, the 5-foot, 10-inch person has an overweight BMI at weights between 174 and 208 pounds or obese at 209 to 271 pounds. An overweight BMI at 6 foot tall occurs at 184 to 220 pounds and obese, 220 to 287 pounds.
BMI is not age- or gender-specific for adults. The BMI of children and teens is measured using the same formula as for adults, but the results are interpreted differently. The results take into account growth charts that factor in age and gender, because body fat changes with age and puberty.
BMI Has Limitations
Athletes and bodybuilders may register a high BMI because they have extra muscle mass, not extra fat, that makes them weigh more than an average person. A pound of muscle and a pound of fat weigh the same, but muscle is denser and more compact, so a muscular person often looks as if he weighs less than the scale reveals. Athletes whose BMI is above 25 probably aren't overweight, but a healthcare provider can determine if additional screenings are necessary to rule out possible health concerns.
BMI may also fail to detect over-fatness in certain people whose weight is normal for their height. For example, some older adults and those who are sedentary may weigh in at a level that seems healthy for their height, but too much of that weight comes from fat. A fat percentage of over 20 percent in men and over 30 percent in women can be as dangerous to your health as being overtly overweight.
Reducing Your BMI
If you have a high BMI, your doctor may recommend you lose weight to reduce it. Losing even 5 to 10 percent of your total weight can help improve health markers, including blood pressure and cholesterol levels.
Changing your eating and exercise habits helps you lose pounds. Use an online calculator to help you determine how many calories you need to maintain your weight; then subtract 500 to 1,000 calories per day to have a deficit that helps you lose 1 to 2 pounds per week -- a safe and sustainable rate.
Regularly consuming inadequate calories -- fewer than 1,200 per day for women and fewer than 1,800 for men -- can leave you vulnerable to nutrient deficiencies and muscle loss, which won't help you reach a healthier weight. If your computation results in a calorie-intake level that's too low, add in more physical activity to burn calories and create a calorie deficit without dropping your food intake to dangerously low levels.