Article # 190. Difference between BMI & Body Fat Percentage

Difference between BMI & Body Fat Percentage

While your jeans will let you know when you're getting too fat, your doctor or trainer will want to quantify it by measurement. The most commonly used method in the medical community is BMI, or body mass index. In the fitness community, body fat percentage is preferred. Knowing the difference will help you understand why the two methods don't always sync up.

BMI Defined

Body mass index is a mathematical calculation which divides your weight in kilograms by your height in meters. In some ways, BMI is similar to height and weight charts which list desirable weight ranges for your height, frame size and gender. BMI also uses height and weight, but does not consider gender or frame size.

Advantage of BMI

The advantage of BMI is that it is simple to calculate. If you know your height and weight, you can quickly do the math. Divide your weight in pounds by 2.2 to get kilograms, then multiply your height in inches by 0.0254 to get meters. Divide kilograms by meters to get your BMI. Less than 18.5 percent is considered underweight, 18.5-24.9 percent, normal, 25-25.9 percent, overweight, and over 30 percent, obese.

Drawbacks of BMI

While BMI is simple to calculate, it does not allow for different body types. Body types have long been categorized as ectomorphic, mesomorphic and endomorphic, and their varying characteristics affect overall body weight. BMI also falls short when applied to athletic populations because muscular individuals weigh more than non-athletic individuals of the same height. Using BMI, many athletes and body builders would be categorized as overweight or obese.

Body Fat Percentage Defined

Body fat percentage is a ratio of your fat to your lean mass. Lean mass consists of lean muscle, bones and anything else that is not fat. Fat mass includes adipose stores such as belly fat, subcutaneous fat which lies directly beneath your skin, and intramuscular fat, which is swirled into your muscle. You may also have fat surrounding your vital organs. Humans need some fat for essential metabolic functions, but too much can be deadly.

Advantage of Body Fat Percentage

When calculated using hydrostatic, or underwater, weighing or a skin-fold caliper, body fat percentage allows for characteristics of individual body types, and does not factor in height or weight. Because excessive body fat predisposes an individual to a plethora of metabolic disorders, body fat percentage is a useful tool for gauging overall health. Electronic methods such as hand-held devices and smart weight scales use bioimpedance to measure body fat, but they also factor in height and weight, and may be less accurate for athletes.

Drawbacks of Body Fat Percentage

Calculating body fat is more complicated than BMI, which is likely why it is not widely used by doctors. Hydrostatic weighing, considered the "gold standard" of body fat calculation, requires expensive equipment and a pool or tank. Skin-fold calipers are less expensive, and can be used anywhere. However, both hydrostatic weighing and skin-fold methods require skilled individuals to make the calculations.

Norms for Body Fat Percentage

According to the American Council on Exercise, athletic women range from 14 to 20 percent fat, and athletic men 6 to 13 percent. For fit females the range is 21 to 24 percent and fit males 14 to 17 percent. Average women range from 25 to 31 percent and average men 18 to 24 percent. Women over 32 percent and men over 25 percent are considered obese.

Body Fat

The percentage body fat is calculated for males as 100*(-98.42 + 4.15*waist - 0.082*weight)/weight and for females as 100*(-76.76 + 4.15*waist - 0.082*weight)/weight.

The body fat calculation is based both on weight and waist size, and is exquisitely sensitive to the value for waist. If you lower the value for the weight without changing the waist, the calculation considers this as a loss of muscle mass, and hence the percent body fat goes up. Since it’s difficult to measure waist size accurately, this calculation is not useful when evaluating small changes in body weight.

The percentage of body weight considered "essential fat" is around 4% for men and 10% for women. The American Dietetic Association recommends that men have 15-18% body fat and women have 20-25% body fat. Healthy male athletes might be as low as 5-12% body fat, and healthy female athletes could be as low as 10-20%. The American Council on Exercise recommends men's body fat should be 6-25%, and women's should be 14-31%.

From BMI

Body fat can be estimated from body mass index (BMI), a person's weight in kilograms divided by the square of the height in meters; if weight is measured in pounds and height in inches, the result can be converted to BMI by multiplying by 703.[17] There are a number of proposed formulae that relate body fat to BMI. These formulae are based on work by researchers published in peer-reviewed journals, but their correlation with body fat is only estimates; body fat cannot be deduced accurately from BMI.

Body fat may be estimated from the body mass index by formulae derived by Deurenberg and co-workers. When making calculations, the relationship between densitometrically determined body fat percentage (BF%) and BMI must take age and sex into account. Internal and external cross-validation of the prediction formulas showed that they gave valid estimates of body fat in males and females at all ages. In obese subjects, however, the prediction formulas slightly overestimated the BF%. The prediction error is comparable to the prediction error obtained with other methods of estimating BF%, such as skinfold thickness measurements and bioelectrical impedance. The formula for children is different; the relationship between BMI and BF% in children was found to differ from that in adults due to the height-related increase in BMI in children.

Child body fat % = (1.51 × BMI) − (0.70 × Age) − (3.6 × sex) + 1.4

Adult body fat % = (1.20 × BMI) + (0.23 × Age) − (10.8 × sex) − 5.4

Where sex is 1 for males and 0 for females.

References:

National Heart, Lung and Blood Institute: Calculate Your BMI

Health Check Systems: Height and Weight Charts

American Council on Exercise: Percent Body Fat Calculator

The Fitness Lab: Understanding Body Fat