NASM's Body Fat Calculator

There are many ways to assess body composition more accurately among individuals than taking BMI measurements. These ways to measure body fat include using skinfold calipers, body circumference measures, hydrostatic weighing, bioelectrical impedance, air-displacement plethysmography, and 3D body scans.

It is important to note that, while useful, all these methods of assessment are estimates, not true measures. The only true way to accurately measure, not estimate, body composition is through autopsy. For example, a body circumenference measurement will give a low-to-moderate level of accuracy, but it is extremely convenient and cost effective.

Below, you can calculate your body fat percentage, understand the the relationship between BFP and BMI, and learn more context about those specific measurements. Be sure to check out our other calculators to get an even clearer picture of your body composition: The NASM Calorie Calculator, One Rep Max Calculator, and BMI Calculator.

As mentioned earlier, there are many ways to estimate body fat. These methods use a wide variety of technologies, ranging from very simple (a tape measure) to very complicated (three-dimensional laser scanning technology. Each method has pros and cons and different levels of accuracy. Here is a quick review of those major technologies.

1. Skinfold Calipers: One of the most readily accessible ways to estimate body fat is to measure the physical thickness of the adipose (fat) layer directly under the skin. This layer is known as the subcutaneous fat layer. This method relies on precisely built and calibrated calipers that measure skin thickness in millimeters. While easy to perform, this method requires a substantial amount of training and has a pretty high error rate, with up to 10-15% error rates reported in some studies. As such, it is difficult to be certain of changes of <5% of body fat.
2. Body Circumference: Another very accessible and easy to perform method of estimating body composition is through measuring body circumference. More specifically things such as waist-to-hip ratios, waist-to-height ratios, and neck circumferences are ways to estimate body composition. There are good data to show how well different measures correlate to body fat percentages. Interestingly, waist to weight ratio appears to be the most accurate (Dong et al., 2018).
3. Hydrostatic Weighing: Hydrostatic weighing, also known as underwater weighing, is one of the more accurate forms of body composition assessment with error rates being between 2-5% for most apparatuses. However, this approach is expensive and requires very specialized equipment and training. For most people, this form of body composition assessment is impractical.
4. Bioelectrical Impedance: Bioelectrical impedance analysis, often referred to as BIA, utilizes electrical currents to assess body composition based on how electrical currents flow through the body. This method is very readily available as it can be put into handheld devices and even at home digital weight scales. However, many of the low-cost commercial models are imprecise with large error rates (greater than 10%). Research grade devices can have error rates as low as 5%.
5. Air-Displacement Plethysmography: Air-displacement plethysmography utilizes a similar principle as hydrostatic weighing, displacement. However, instead of displacing water, this method displaces air. It is roughly similar in terms of accuracy as hydrostatic weighing (hydrostatic weighing is slightly more accurate) but is substantially cheaper and easier to perform. The Bod Pod® is the most used device.
6. 3D Body Scan: A new, emerging technology that is becoming more readily available are three-dimensional (3D) body scans. These technologies utilize laser capture technology to measure anthropometrics and estimate body composition. These technologies show relatively similar accuracy to other measures such as bioelectrical impedance, hydrostatic weighing, and air-displacement plethysmography with error rates between 3-10% depending on the instrument used.
7. Dual Energy X-ray Absorptiometry: Dual Energy X-ray Absorptiometry, commonly referred to as DEXA (DEXA) utilizes specialized machines that have different types of X-rays which can measure differences in density of tissue. This approach requires very specialized equipment and training, and some-states require licensure to use DEXA technology due to the radiographic nature of the equipment. While often believed to be the most accurate method of estimating body fat percentage, DEXA is roughly equal to air-displacement plethysmography, hydrostatic weighing, and 3D body scanning technology.

There are specific protocols for how to accurately measure waist circumferences for assessing body composition (Lung National Heart and Nih 2003). Based on published literature, this is the specific protocol:

• Palpate the iliac crest.
• The measuring tape is then placed in a horizontal plane, parallel to the ground around the abdomen.
• The measuring tape is wrapped around the person until the tape is snug but does not compress the skin.
• The measurement should be taken at normal, minimal inspiration.

It is important to reiterate that aside from autopsy, all methods of body composition assessment produce estimates and can have error rates of as low as 2-3% or as high as 10-15% depending on the technology and how it is utilized.

Each technology has pros and cons to its utilization, but there are three main dimensions to consider: accuracy, cost, and difficulty to utilize. From an accuracy perspective, hydrostatic weighing has perhaps the highest level of accuracy, while bioelectrical impedance and BMI/circumference measures have the lowest. The other metrics are important to consider as well and are listed in the table below.

The relationship between BMI and body fat is that it really depends on the population and the method of assessing body fat.

Regarding populations, if you compare BMI and body fat among people who are several standard deviations away from average height, the BMI and body fat relationship is very poor; BMI tends to underestimate body fat among taller people and overestimate it among shorter people.

Furthermore, some populations of people tend to have higher body fat percentages at a given BMI than other populations. As such, there is poor correlation between BMI and body fat percentage across different racial, ethnic, and cultural dimensions (Heymsfield et al., 2016).

The relationship between BMI and body fat is affected by the method of assessment, with larger discrepancies being seen among more accurate assessments of body fat. The corollary is also true, with BMI being more correlated with body fat percentage when less accurate methods of assessment are used.

For example, BMI shares a relatively high correlation to body fat percentage when assessed by waist circumference (~r=0.78), but there was a high level of discrepancy observed when body fat was assessed by more precise measures such as DEXA or hydrostatic weighing (Gierach et al., 2014; Shah & Braverman, 2012)