Many different ways exist to estimate body composition, each with its own pros and cons. The only ways to directly measure the amounts of body fat and the different types of lean body mass are advanced imaging methods or direct measurement after death. Advanced imaging is very expensive and time-consuming, so it is rarely used for body-fat assessments outside of research settings. Practical body-composition assessments just need be valid and reliable, which means they measure what they’re intended to measure with reasonable accuracy and their measurements are consistent.
Training of the people who take the measurements is also an important consideration when choosing a method to assess body composition. That’s because most people performing the assessments aren’t medical providers or fitness experts who perform such tests regularly. So, a body-composition assessment method should be easy to learn without special expertise.
Body mass index
Body mass index (BMI) is a quick tool to categorize an individual as underweight, normal, overweight, or obese based on their height and weight. It’s an accepted and reliable method to assess body composition, with some limitations. With a decent scale, a way to measure height, and CDC’s adult BMI calculator, you really can’t get a BMI calculation wrong.
One of the limitations of BMI is that it doesn’t account for variations in body composition. For different people with the same BMI, their percentage of body fat (BF%) can vary based on sex, race, age, and physical fitness. Having an “overweight” BMI doesn’t necessarily mean you’re unhealthy. BMI tends to classify short, muscular people as overweight. If you take additional measurements, such as abdominal circumference measurements, you’ll find they aren’t “overfat,” so they might not have the health risks their BMI suggests.
BMI is a fairly accurate measure of body fat for BMI values over 30. It’s very unlikely that someone with an “obese” BMI is misclassified because they are very muscular with little body fat. Those few people where that is the case are high-level athletes who obviously don’t carry excess body fat. People who have high levels of both lean body mass and fat mass are still likely to be obese and to have the risks that come with carrying excess body fat.
The science behind BMI holds strong. People who are overweight or obese are at greater risk for various preventable chronic diseases. But remember that BMI is a screening tool. It’s a quick-and-easy way to identify people who would benefit from health-behavior improvements, and if you screen outside the “normal” category, you should investigate further to find out why.
Military weight standards
The weight standards used by the military, as set by DoDI 1308.3, are a close approximation of BMI. For each height range, the maximum allowable weights work out to a BMI of 25–27.5, and minimum allowable weights are for a BMI of 19. This allows both men and women to carry a healthy amount of fat mass, which is necessary for both health and performance. It also accounts for people who are very muscular for their height. Similar to BMI, the purpose of military weight-for-height determination is to provide a rapid screen for excess body fat. When people screen outside the allowable range, additional measurements are taken to determine whether they have an unhealthy amount of body fat or if they are just muscular.
DoDI 1308.3 also establishes the method by which body fat and BMI are determined for the military. The Services usually assess body composition only if a member does not meet the weight-for-height standards. Currently, the only approved method is body-circumference measurement, sometimes called the “rope and choke” or “tape test,” unless a specific exemption is granted. A “self-tensioning tape measure” is used to measure body circumference at the neck and navel for males, and at the neck, narrowest portion of the waist, and hips for females.
The Army, Navy, and Marine Corps each use slightly different formulas to calculate BF% from 2- and 3-site circumference measurements, but the results are within about 1% of each other. The Coast Guard’s BF% values are similar to those of the Marine Corps.
The Air Force received an exemption from the 2- and 3-site measurements and currently use a single-site abdominal-circumference (AC) measurement taken at the top of the hip bone. One-site abdominal circumference doesn’t actually measure BF%, but its measurement cutoff values correlate with risk of disease and early death. While the AC measurement is no longer a scored component of the fitness test, it can still be used as a separate body-composition assessment.
The Navy received a similar exemption. If a Sailor doesn’t meet weight-for-height standards, the Navy first performs the 1-site abdominal circumference measurement. If a Sailor also doesn’t meet the standard for abdominal circumference, then they are taped using the 2- or 3-site circumference method.
One of the main reasons the military has adopted circumference measurement as the standard for BF% assessment is because it is very easy to learn. Other cost-effective measurements are also valid and reliable, but they take longer to learn and require raters to be more highly skilled, which affects the reliability of newly trained raters. In fact, 2- and 3-site body-circumference measurement is valid, reliable, and extremely cost-effective.
Some controversy exists about the use of circumference measurements, particularly with regard to women. Women can carry body fat in a variety of ways, but compared to men, women carry more fat in their hips and thighs. Waist and hip measurement standards are based on typical female fat distributions and take into account women who have more “pear” or “hourglass” shapes with narrower waists, as well as “apple” shapes with bigger waists. The controversy is that measuring around the widest part of the hips includes the glute muscles, but strength-and-conditioning programs often train the glutes for muscle hypertrophy, and bigger glute muscles might lead to overestimation of total body fat.
Other (unapproved) methods of measurement
Skinfold measurement is similar to circumference measurement in terms of validity and reliability, but it requires experienced raters. It takes close to 120 practice tests for a rater to become proficient, creating a time burden to effectively train and maintain the capability of raters. It also has inconsistencies related to age, making it complicated for general use. While it can be an excellent tool for trained medical and physical fitness professionals who perform it often, it’s less practical for overall use by military personnel.
Bioelectrical impedance analysis (BIA) calculates body composition based on how quickly a harmless electrical current passes through your body, because bone, muscle, and fat have different electrical-resistance properties. However, its validity is affected by conditions such as hydration status and waist circumference. Overhydration can lead to lower readings, making it fairly easy to “cheat” a BIA test. BIA devices also vary in reliability, and those that perform more accurately and consistently are generally more expensive.
Air-displacement plethysmography (ADP, commonly known by the brand name BOD POD) calculates body composition by measuring air-pressure differences between a chamber in which the test subject is sitting and an empty control chamber. ADP is reliable, but its validity can be hit or miss, and it sometimes underestimates BF%. ADP is becoming more widely available in Army Wellness Centers and Air Force Health and Wellness Centers. It can be useful to track your BF% throughout the year if you’re trying to shed some body fat. However, ADP needs more rigorous scientific testing in military populations, and the equipment is extremely expensive.
Dual-energy X-ray absorptiometry, also called DEXA or DXA, is one of the most accurate measures of body composition. However, it is not time- or cost-effective outside of medical or research settings. A single test can take about 15 minutes, and machines cost tens of thousands of dollars, making it difficult to fund even a single machine for every U.S. military installation.
Many considerations exist with regard to body-composition assessments in the military. Is the measurement valid and reliable? Is it time- and cost-effective? Can anyone easily take the measurements? Whatever method is used, the purpose of the standard is to ensure the health of every Service Member and reduce the risk of chronic disease. In an effort to provide the best tools for Service Members, the Services are conducting studies to improve body-fat assessment. Read HPRC’s article on optimal body fat and body composition for more information about why body fat is important, and what amount of body fat is considered healthy.
For more information about U.S. military body-composition standards, read the regulations for DoD’s and your Service’s specific body-composition program.
- DoD Directive 1308.1: DoD Physical Fitness and Body Fat Program
- DoDI 1308.3: DoD Physical Fitness and Body Fat Programs Procedures
- Army Regulation 600-9: The Army Body Composition Program
- OPNAV Instruction 6110.1J: Physical Readiness Program
- Air Force Manual 36-2905: Air Force Physical Fitness Program
- Marine Corps Order 6110.3A Change-3: Marine Corps Body Composition And Military Appearance Program
- Coast Guard Commandant Instruction M1020.8H: Coast Guard Weight and Body Fat Standards Program Manual