BMI is a calculated value and approximates the body’s fat percentage. Actually measuring a person’s body fat percentage is not easy and is often inaccurate without careful monitoring of the methods. The following methods require special equipment, trained personnel, can be costly, and some are only available in certain research facilities.
Underwater weighing (hydrostatic weighing): This method weighs a person underwater and then calculates lean body mass (muscle) and body fat. This method is one of the most accurate ones; however, the equipment is costly.
BOD POD: The BOD POD is a computerized, egg-shaped chamber. Using the same whole-body measurement principle as hydrostatic weighing, the BOD POD measures a subject’s mass and volume, from which their whole-body density is determined. Using this data, body fat and lean muscle mass can then be calculated.
DEXA: Dual-energy X-ray absorptiometry (DEXA) measures bone density. It uses X-rays to determine not only the percentage of body fat but also where and how much fat is located in the body.
The following two methods are simple and straightforward:
Skin calipers: This method measures the skinfold thickness of the layer of fat just under the skin in several parts of the body with calipers (a metal tool similar to forceps); the results are then used to calculate the percentage of body fat.
Bioelectric impedance analysis (BIA): There are two methods of the BIA. One involves standing on a special scale with footpads. A harmless amount of electrical current is sent through the body, and then percentage of body fat is calculated. The other type of BIA involves electrodes that are typically placed on a wrist and an ankle and on the back of the right hand and on the top of the foot. The change in voltage between the electrodes is measured. The person’s body fat percentage is then calculated from the results of the BIA. Early on, this method showed variable results. Newer equipment and methods of analysis seem to have improved this method.
Health clubs and weight-loss centers often use the skin caliper or bioelectric impedance analysis method; however, these can yield inaccurate results if an inexperienced person performs them or they are used on someone with significant obesity.
It is important to understand what “healthy weight” means. Healthy weight is defined as a body mass index (BMI) equal to or greater than 19 and less than 25 among all people 20 years of age or over. Generally, obesity is defined as a body mass index (BMI) equal to or greater than 30, which approximates 30 pounds of excess weight.
The World Health Organization uses a classification system using the BMI to define overweight and obesity.
A BMI of 25 to 29.9 is defined as a “pre-obese.”
A BMI of 30 to 34.99 is defined as “obese class I.”
A BMI of 35 to 39.99 is defined as “obese class II.”
A BMI of or greater than 40.00 is defined as “obese class III.”
- Below is a table identifying the risk of associated disease according to BMI and waist size.
Disease Risk* Relative to Normal Weight and Waist Circumference
BMI (kg/m2) Obesity Class Men 102cm (40 in) or less
Women 88cm (35 in) or less Man > 102cm (40 in)
Women > 88cm (35 in)
Underweight < 18.5
Normal weight 18.5 – 24.9
Overweight 25.0 – 29.9 Increased High
Obesity 30.0 – 34.9 I High Very High
Obesity 35.0 – 39.9 II Very High Very High
Extreme Obesity 40.0 + III Extremely High Extremely High
* Disease risk for type 2 diabetes, hypertension, and CVD.
+ Increased waist circumference can also be a marker for increased risk even in persons of normal weight.