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Body Composition: What Is It and How Do You Assess It?

Knowing how to measure and monitor your body composition is one of the first steps you can take in achieving (and tracking) health and performance progress.

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Assessing body composition is not about getting ripped, let’s get this clear. Your body composition is important for both health and performance reasons. From a health perspective, poor body composition can be associated with a myriad of diseases including diabetes, heart disease, cancer, osteoporosis, sarcopenia, and metabolic syndrome. From a pure performance perspective, improving body composition can be an effective way to improve your power to weight ratios that can help bring about improvements in cycling and running, but it’s also equally important to understand that there can be diminishing returns here, too—that is, get too lean and you could start to lose power and run the risk of injury. 

When first assessing body composition, it is good to establish a baseline measurement and then, from there, objectively assess the effectiveness of a nutritional and/or exercise intervention. By monitoring change over time you can adjust and assess what you need. There are multiple ways in which to assess body composition. These range from girth measurements, bioelectrical impedance, skinfolds, DEXA, and underwater weighing. These varied methods have both pros and cons associated with them that we’ll explore below. The method you choose may be dictated by your budget and also your commitment to assessment—some are more involved than others.

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Body Composition: Methods of Assessment

A Piece of String

Perhaps the most simple place to start is a girth measurement. Take a piece of string and measure your height. Next, take that same piece of string and fold it in half. Place it around the narrowest part of your waist and the two ends should touch or overlap. Essentially, the narrowest part of your waist should be half your height (waist to height ratio =/< 0.5). You can calculate your actual ratio by simply using this formula: Waist to height ratio = waist divided by your height. 

Ideally, you want to see a range of around 0.45-0.55. If it is under 0.45 you may be underweight; if it is over 0.55 you may be overweight. Both carry potential risks: being underweight could leave you open to osteopenia, osteoporosis, or Relative Energy Deficit Syndrome (REDs) while being overweight could leave you at risk of metabolic syndrome, Type II diabetes, heart disease, and stroke. Of course, other factors do contribute to the relative risk of these diseases too.

Conclusion: This is a simple, quick, and effective way to measure body composition. It may be the starting point for investigating further. 


If you are going to get a skinfold measurement done, seek out a practitioner who has completed their International Society for the Advancement of Kinanthropometry (ISAK 1). This will ensure that the assessment is thorough, consistent, and reliable. The assessment will likely be done over seven places on your body and should take 15-30 minutes to complete. The practitioner will mark your body and perform the measurements twice to ensure accuracy and minimize measurement error. If there is an error then a third should be taken. Skinfolds do not relate to hormones and any practitioner who explains your skinfolds are related to cortisol, oestrogen, or your thyroid should make you very wary of them.

Conclusion: Choose an ISAK practitioner to ensure accuracy of measurements.  

Bioelectrical Impedance Analysis (BIA)

These are machines that you stand on, sometimes with a pair of handles to grab, while a current passes through your body to estimate your fat free mass (FFM) and fat mass (FM). The principle is that lean muscle consists of water and electrolytes and acts as a good conductor while fat mass does not and is a poor conductor. Be aware that there are a lot of confounding factors related to BIA including hydration status, nutrition status, menstrual cycle, and the electrodes themselves. Bioelectrical impedance analyzers use equations to describe statistical associations based on a specific population, and as such the equations are only useful if you closely match the specific population in body size and shape. Significant improvements have been made with these machines lately and some do now use multiple frequencies and bioelectrical impedance spectroscopy (BIS) to assess total body water (TBW), even breaking this into extracellular and intracellular water. Despite these improvements, the real use of BIA is for describing the average body composition for groups of individuals rather than individual changes due to the often large measurement error.

Conclusion: FFM is often underestimated in normal-weight individuals while overestimated in obese individuals, especially when compared to DEXA masurements (see below).

Dual Energy X-Ray Absorptiometry (DEXA)

This method assesses body fat (visceral and subcutaneous), muscle, and total body bone mineral (TBBM) density. It is considered the gold standard for bone mineral density measurements, so if you are a female endurance athlete this is certainly worth considering for your body composition assessment. In my opinion, if you are a female endurance athlete with a waist to height ratio </=0.5 and you have also had some irregular eating episodes that have resulted in underfueling your training and performance then a DEXA scan is key. When it comes to body composition measurements it is not 100% accurate as it does assume a constant hydration status which could result in an error of 1-2% in body fat and lean tissue measurements.

Conclusion: DEXA is quick to use, safe, and as reliable as you can get. Ensure you are hydrated before each use and aim to standardize the time of day. Women should aim to get tested at the same time in their cycles, preferably during phase one or two. 

Body Composition Takeaways

No single method is the gold standard in body composition assessment and all have their advantages and disadvantages depending on cost, practicality, and measurement error. Whichever you choose to use, consider attempting to standardize the method as best as possible and stay with that choice for consistency purposes. One other important consideration is that if your goal is improving body composition for performance purposes, be aware that maintaining a low percentage of body fat over a long period of time is not only difficult, it can also result in an increased risk of upper respiratory chest infections and other musculoskeletal injuries. Working alongside a professional practitioner to periodize your plan in order to hit targets when required would be sound advice to follow to ensure you are as healthy as you can be all year round. In short, peak for performance—not to get ripped. 

Scott Tindal is a performance nutrition coach with 20 years of experience working with pro and amateur athletes. He has a Masters degree in sports medicine and a post-graduate diploma in performance nutrition. He is the co-founder of FuelIn, an app-based personalized nutrition coaching program.