Accuracy of the main Body Composition Testing Methods available in NZ

I have made it my mission to understand body composition testing and offer my clients the best available equipment. My research has included hours of reading online journal articles, listening to podcasts, and reviewing video evidence of the various body composition testing methods.

At the time of writing this article, I have tested myself over 100 times on various bioimpedance and infrared scanning body scanners, had 12 x DEXA Body Scans and over 400 Fit3d Body Scans (myself) and administered over 300 x 8-site ISAK skinfolds tests and over 8000 Fit3d Body Scans.

In this particular article (and associated video) I will present some of my main findings. If you are interested in discussing this information further, please send me an email 


This article will address:

4 Main Body Composition Testing Methods used in NZ

Pros and Cons of these methods

Why I have chosen to offer clients skinfolds, DEXA, and Fit3d but NOT Bioelectrical Impedance Testing at Health and Fitness Testing NZ

Key points:

  • Body Composition Testing methods are not perfect
  • ALL Body Fat % measures have some element of error
  • DEXA may produce the least amount of error (e.g. up to 5%) however it takes longer (up to 10 minutes + results explanation), machines are less mobile, and machines require specialised training
  • Skinfolds, using an ISAK 8-site method, take the longest (approx. 15 min + results explanation), are very invasive, require you to be in a room with a client with their clothes off, and have large measurement errors (up to 20%) as soon as the trainer/therapist taking the skinfolds changes to a different person
  • Bioimpedance Scanners are quick, clients can keep their clothes on however results can be easily manipulated by time of day, exercise/eating/drinking beforehand, sleep, the time of month (for women), the type of food eaten beforehand.  ALL results (including circumference measurements) are affected by water/hydration levels in your body. To decrease error in bioimpedance scanning, clients would need to scan 1st thing in the AM every time they scan, whilst also ensuring water levels are the same every time and that they scan at the same time of the month every time. Women would also need to ensure they scan outside of their period every time - logistically this makes testing difficult for groups of clients.
  • Fit3d uses infrared cameras to scan clients. Within a Fit3d Scan over 1200 images are taken. Fit3d are the only brand that uses 3 (not 1) infrared camera and fixed handles. Both of these are a requirement to gain an accurate 3d image during the scan. Scans are quick (35 seconds). Clients must scan in a private room (by themselves) and ensure they stand in the proper scan position every time. Minimal clothing e.g. bra and underwear is essential every time the client scans. Water/hydration levels, and time of month should have less of an impact on results. Clients can scan at all times of day and gain accurate, reliable results.
  • Fit3d girth measurements (esp waist and hips) and Body Shape Rating, combined with Body Fat % and weight, are a better way to track body composition progress (for health) than purely focusing on Body Fat %. Fit3d offers other metrics for those with other goals e.g. left versus right limb volume comparison, limb circumference measurements, Lean Mass, Posture Analysis, 3d images, Posture images, Leg Length, Arm Length etc. 
  • Fit3d overlay images and photos in underwear are a good way to supplement your other Fit3d Body Scan results 

There are over 50 ways to obtain a body fat % reading. The only way to truly measure your body fat % would be to cut out all the internal and external fat from your body and weigh it. You would need to be dead for this to occur. Every other measurement method in the world is some kind of best estimate. Some estimates/machines are significantly better than others.

Below, I have reviewed the 4 most common types of Body Composition Testing in NZ. I have purposely left out full body MRI scans, Bod Pods, and underwater weighing as these methods are all very time consuming, require specialist expertise, and are not (and will not) be readily available to the public in NZ. 


Full Body DEXA Scan

Aside from MRI (or the unrealistic full body autopsy), DEXA is said to be the Gold Standard for body composition testing in the world.

DEXA has proven itself to be accurate for assessing bone health however its use for body composition measurement is more recent. A DEXA scan works by passing low-radiation x-ray beams through tissues, with the varying energy absorption. It gains measurement of Bone/Muscle/Fat by assuming that these metrics absorb energy at different levels. 

Like other body composition methods, DEXA does estimate body fat %, lean mass, fat mass etc. The difference is that DEXA algorithms have been compared and aligned to human cadaver body composition results whereas other methods of body composition testing have used DEXA as their reference. 

Studies have noted that although DEXA results can be close to that of a human cadaver (dead body autopsy), metrics do not 100% align. For example, Fat Mass has been shown to vary by up to 5 kg when comparing DEXA algorithms to cadaver studies. 


  • Relatively good accuracy - usually the main results e.g. Body Fat % is within 1-2% between scans on same day however can be up to 5% if you change machine types don't control your pre test routine
  • Ability to break results down into muscle/bone/fat and assess those metrics in different segments of the body e.g. left v right leg, trunk etc
  • Ability to assess visceral fat
  • Non-invasive


  • Expensive, >$100,000 for equipment and set up
  • Requires specialised training to administer and interpret scans
  • Difficult and time-consuming to transport it to different locations
  • Unable to test really tall people or really obese people due to limited bed size
  • One DEXA scan takes up to 10 min + results discussion time
  • Each DEXA scan has a small amount of radiation 
  • Online results systems and apps are not easy to use 
  • Results can be slightly impacted by eating and drinking fluids before a scan. For example, eating a meal beforehand or drinking a lot of water before scanning could increase lean mass by up to 1kg. This is significantly less than the errors that can occur with BIA or skinfolds (see below) however it should be noted when deciding on what pre-test instructions clients should adhere to.



Skinfolds 8-site ISAK method

Until 2010, skinfolds were the most popular method of estimating body fat % in the world.

Therapists use a set of calipers to take multiple 'pinches' throughout the body. These pinches are measured in mm, added up to a total, and converted into a body fat % via one of various algorithms. 

Obviously, the pinches are not seeing or measuring fat deeper inside your body. The mm pinches are actually just tracking subcutaneous fat. Therefore, a few assumptions are made, including that:

1. Subcutaneous fat levels are correlated with deeper fat (adipose tissue) throughout the body, including visceral fat,

2. That this correlation is the same for people of different ethnicities and body shapes,

3. That subcutaneous fat is always representative of total body fat,

4. That Fat Free Mass (e.g. Bone and/or muscle) is relatively constant for a given body size and skinfold/pinch thickness


  • Minimal equipment is required
  • Minimal risk is required
  • Equipment is portable


  • The person taking the skinfolds must have an understanding of bony and muscle anatomy 
  • To gain 'reliable' results therapists are required to complete a 3-day ISAK International Skinfold course and have practiced on at least 100 different clients
  • As soon as you change the person taking your skinfolds, your results can vary by up to 15-20%
  • Testing is quite invasive - you need to be in your bra and underwear in front of the therapist every time
  • Clients are marked with pen and touched all throughout their body - the client experience may not be comfortable
  • There are over 10 different formulas for converting the skinfold measurements (in mm) into body fat %
  • There are no online results websites, apps, graphs for tracking results and thus results commonly get lost each time you are tested or whenever you change trainer/gyms 
  • There are multiple different types of skinfold calipers (equipment used to take the skinfolds). All of these can produce different measurements 

Full Body Bioimpedance (BIA) Body Scan

BIA scanning involves the client standing on some metal pads and holding metal hand grips for approx. 1 minute.

BIA involves running an electrical current through the body and estimating body composition through the resistance of tissues (bone/muscle/fat) to the electrical current. It based on the assumption that electric current flows through the body at different rates depending on the composition it is flowing through.

Another assumption is made - that muscle/bone/fat etc travel at the same speed in EVERYONE or when there is an option to choose ethnicity, an assumption is made the these currents travel at the same speed in everyone from this particular ethnicity. The equations that provide the results are also assuming that the clients hydration status and water levels are the same each time - this is where it all goes wrong!

From here, Body Fat %, Fat Mass, Bone Mass, Muscle Mass, muscle girth measurements (e.g. waist circumference etc) and all results are estimated via various algorithms. Each bioimpedance brand will have developed their own algorithms, some more prone to error than others.

Water levels and hydration status in one's body can have a large effect on individual results.



  • Quick to perform, clients remain in their clothes
  • Do not have to be in a private room
  • Portable 
  • Results are usually available on a print out
  • Most recent 5 results can be viewed online using an app (some brands)



  • Most bioimpedance scanners are NOT safe for those with a pacemaker or if someone is pregnant - the electrical currents may cause risk
  • Although you can gain reasonable accuracy on test-retest of the same client a couple of minutes apart, results can be too easily manipulated when trying to track progress over time e.g. over days/weeks
  • Changing your hydration status, eating (or not) prior to your scan, exercising (or not) before your scan can all vary results significantly. These manipulations, if not controlled thoroughly, can affect the main results e.g. Body Fat % or 'visceral fat' when scanning the same person over time. For example, on the trials we've done, it is normal to see day to day variations in Body Fat % of about 5% alone. We were easily able to manipulate these to be as high as 15% on some BIA machine brands. 
  • Carbohydrate and sodium loading over 2-3 days can significantly increase the Fat Free Mass (Lean Mass). Theoretically, we believe this is solely because of short term water retention rather than a true increase in Lean Mass. The opposite may also be true when using BIA - if you are purposely (or not) dehydrated, this can cause Lean Mass to be less. 
  • Bioimpedance companies openly suggest females do not test themselves whilst menstruating because results will not be as accurate which means up to 1/5th of every month women will not be able to use the bioimpedance machine
  • In studies where results have been "accurate" they have controlled hydration levels, sleep, exercise, phase in a woman's cycle, meals prior to the scan in great detail. This is the only way to gain a reasonably reliable re-scan result.
  • In the real world, it is very difficult to scan every client first thing in the AM, at the same time, with the same amount of hours' sleep every time. It is difficult to standardise hydration status and meal status prior to a scan. 
  • If you are completing a gym challenge or a specific time-bound training phase it is difficult (for women) to re-test every time at the same time of the month/in their menstrual cycle.
  • With BIA, ALL measurements, including girth measurements and visceral fat are gained through currents that travel through your body. Why should your chest circumference or waist circumference or visceral fat levels change just because you are under or over hydrated?
  • You cannot view 3d images, a posture report, an overlay image of your results when using BIA
  • Printing out reports requires paper, print cartridges and a general waste on the environment. Printed reports are only able to show the last 5-10 results on a piece of paper, NOT results over a lifetime
  • Some brands now offer apps however I'm not aware that any are 100% cloud-based. Without a cloud-based system, as soon as a BIA machine is upgraded or a client moves cities/gyms, their results are lost 

How can you manipulate your BIA Results?

Research, combined with our own in-house experiments have found the following patterns when using BIA:

If you want to significantly increase your Body Fat % and decrease your Skeletal Muscle Mass (Lean Mass), you just need to:

  • Dehydrate yourself prior to your scan,
  • In the 2-3 days prior to your scan, minimise the amount of carbohydrates you are consuming (e.g. carb depletion)
  • Have your BIA test 2 hours after waking up and do not eat anything on the day of your scan

If you want to significantly decrease your Body Fat % and increase your Skeletal Muscle Mass (Lean Mass), you just need to:

  • Increase the amount of carbohydrate and sodium you are eating/drinking for 2-3 days prior to your scan,
  • Eat before your BIA scan, 
  • Have your re-scan in the evening not morning, ideally 1-3 hours after a meal
  • Drink more water and electrolyte drinks leading up to your re-scan 

I chose to add in the section above, not to help people 'cheat their BIA Scan' however to helps trainers and gyms understand how easy it is to manipulate results when using BIA. Gyms/trainers are recommended to take these errors into consideration when finding challenge winners, giving out prizes etc and help educate clients who may become despondent when they receive their BIA results.


Fit3d Body Scan

Fit3d Body scanning is quick, low-risk, mobile (e.g. machines can be easily transported).

Clients stand in a private room (by themselves), usually in their underwear whilst 3 infrared cameras take over 1200 accurate photos of the client using the most advanced infrared cameras available. 

Within minutes private online 3d images are available and clients (and administrators) are able to view their results privately online, or via an app.

Unlike skinfolds and BIA, Fit3d uses volume-based metrics in their Body Fat % algorithm. Through a formal partnership with DEXA in USA Fit3d have compared hundreds of DEXA scans with Fit3d scans and managed to create a Body Fat %/Lean Mass/Fat Mass algorithm that closely resembles DEXA scan results. 


  • Fit3d is SAFE for those who are pregnant, have artificial limbs, or have a pacemaker which can decrease your health and safety risk and save you time screening clients for these things
  • Portable - can set up within 5 minutes
  • Results are not formed by impedance and thus water levels, hydration, menstrual cycle, time of day will have less of an effect on results - a real plus in the real world (outside of a lab) when testing large groups of people
  • Fit3d has an official partnership with DEXA
  • For accuracy, Fit3d uses 3 (not 1) infrared cameras, clients hold FIXED handles during a scan, and clients always stand at the same length away from the infrared cameras every time. These things are essential for accuracy. There are a couple of knock-off brands that have 1 infrared camera, no fixed handles, and the client stands at random lengths away from the camera however if a client cannot stand still during the scan process you cannot get an accurate 3d image. With 3d scanning via infrared cameras, volume metrics play a large role in results so if people cannot stand still in the same position each time you will not be able to gain reliable results. Try holding your arms still out to your sides for 1 minute without fixed handles - it is impossible!

Other benefits I’ve found from using Fit3d regularly

  • Fit3d offers a more holistic set of results – some clients just don’t understand number on a piece of paper. Fit3d clients are able to gain visual feedback through 3d images, graphs, overlay images AND numbers.
  • Quick (only 35 seconds) 
  • Clients can easily self-scan if required - perfect for offering unlimited scans to clients
  • Results are available online, 24/7 using a private login
  • Results can be viewed by using a free app online
  • Clients receive a full posture analysis - perfect visuals for getting clients to buy in to the rehab process
  • ALL results are tracked EVERY time a client scans, not just the last 5 on a piece of paper that the client will lose 
  • Colour-coded graphs (e.g. green = less health risk, red = more risk) enable the client to easily understand not only where they are compared to themselves but how they track when being compared to others of their age/sex world-wide – perfect for goal setting and health awareness
  • Fit3d is the most popular 3d body scanner in the world making results more meaningful in years to come
  • Results are stored on the cloud, meaning that clients can track their results world-wide on other Fit3d machines and gain comparisons over a lifetime 
  • Additional results can be downloaded such as muscle limb volume (Left v Right), arm length, leg length etc. 


  • Results are based on volume so clients must scan in their underwear or tight clothing every time
  • Clients are required to stand still during the 35-second scan


Comparison between various machines and testing modalities

It is impossible to accurately compare results from one testing method to another.

Clients trying to compare their results using one method to results from another method are only going to become disappointed. Body Fat % (for example) between Fit3d and BIA consistently differs by about 10% and can vary up to 25% on the same person, tested on the same day. Skinfold testing also differs from DEXA and Fit3d by, on average, 10% and can again vary up to 20%. 


  • All Body Composition testing modalities have error
  • All Body Composition testing modalities will give you different results 
  • The key is to understand the errors that can occur with the particular method you are using and control these in great detail
  • Outside of the lab, it is very had to control the errors that can occur with bioelectrical impedance testing
  • Fit3d offers a reliable, quick, non-invasive way to track body composition and errors can be controlled easily in the real world



Peter Halstead

MPhty, PGDipSportMed, BSpEx


Health and Fitness Testing NZ,,