Body Composition Scales: Are they accurate?

Do body composition scales accurately track fat loss?

Did you really just put on 6% body fat in a couple of days?  Most probably not.  The human body is extremely complicated and accuracy of body composition results require absolute consistent protocols when taking your measurements.  

Here at Wise Women Fitness, we use the Tanita Ironman BC-1500 Segmental Body Composition Analyzer as a way to measure body composition in order for clients to track their progress, namely through muscle mass and body fat %.

These scans are meant to be a benchmark only and not the holy grail. 

This post is a bit long, but I encourage you to read through it in its entirety.  

an understanding of how body composition scales work will help you understand fluctuations you may get in your scan results.

Let’s begin…

Independent research at several major universities (including Columbia University in New York City) has confirmed that in clinical settings, the Tanita Body Composition Analyzer is accurate within +/- 5 percentage of the institutional standard of body composition analysis–Dual Energy X-ray Absorptiometry (DEXA).

While Tanita’s Ironman scale results are repeatable to within +/- 1 percent variation when used under consistent conditions, its findings shouldn’t be taken as gospel. As with all measuring tools, there are surrounding factors that can have an influence on the reading taken at the time.

It should be made clear that there is only one method of calculating body composition that is close to 100% accurate, and that is an autopsy.  Tanita believes its method to be the most convenient and accessible to accurately predict body composition.

So before you freak out when you next step on the scanner, as your results show a different story from what you had expected, this email will give you insight into how it works and the factors that can affect the reading (positively and negatively).

The Tanita scale uses Advanced Bioelectric Impedance Analysis (BIA) technology. When you stand on a Tanita monitor, a very low, safe electrical signal is sent from four metal electrodes through your feet to your legs and abdomen, and the hand-held electrodes will provide extra readings for each leg, arm, and abdominal area.

The electrical signal passes quickly through water that is present in hydrated muscle tissue but meets resistance when it hits fat tissue. This resistance, known as impedance, is measured and input into scientifically validated Tanita equations to calculate body composition measurements.

Therefore, temporary changes in the body can result in temporary changes in body composition results.

Once you understand how the readings can change in such a short space of time, you’ll see why just one snapshot scan doesn’t always tell the full story.

Factors to consider when interpreting your Tanita stats:


Depending on whether you weigh in before or after working out, will have an influence on the results, especially the body fat % reading. During exercise, water is concentrated in the exercised area to supply nutrients and remove waste products. After exercise, impedance decreases and so does body fat – i.e. you will look leaner after you train. Additionally, your body temperature increases when you exercise which again reduces impedance and decreases body fat %.

Body temperature 

If the body is exposed to warmer temperatures, the skin temperature increases, and body fat % decreases. Generally, lower body temperature means an increase in impedance, which means lower fat-free mass (i.e. less muscle, more fat). A study showed that sauna exposure increased body temperature and caused a 1.7-3.2% reduction in body fat. And after being in an icy room, body temperature decreased and body fat increased by 1.3-4.2%. And while body temperature from the cold exposure returned to normal after 30 mins, body fat % did not return to a stable state even an hour after cold exposure.

So bear this in mind if you’ve just trained or been in a warm environment, you’re likely going to have lower body fat and on the flip side, if you’ve just been outside and haven’t had time to warm up, your body fat % might appear increased.

Taking a shower or bath

This increases blood flow and decreases impedance and so body fat % again decreases. A study showed that taking a shower before getting on the scanner resulted in a reduction in weight and body fat. From this study, one lady over a period of 7 days lost overall weight and body fat % (0.1kg-0.7kg and 0.5%-1.9% respectively)

So if you’ve just had a shower, chances are you’ll be lighter and leaner.


Eating decreases impedance through the ingestion and digestion of food. A study showed with meals eaten over the course of the day impedance is impacted and body fat influenced by up to 8.8% for females and 9.9% males.

The more you eat over the course of the day, the more chance you have of the reading not showing a true reflection of the results in front of you.

Toilet, clothing, and poop/urine do not affect impedance (resistance) BUT are considered as fat mass because electricity does not flow through them. So if you haven’t been to the toilet you’ll get a different measurement compared to scanning after going to the toilet.

Body fat decreases immediately after going to the toilet (reduced body weight used as the amount of fat mass). So consider this too as a factor.


Ladies, your results will have a larger variability than men due to periods. Weight is increased in the luteal phase (day 14 onwards in the cycle) due to the accumulation of total body water (increase in progesterone). This results in a decrease in body fat %.

However, diet at this time (increased sweet/salty food cravings) may cause accumulation of fat mass, increasing body fat %.

Therefore, weight and body fat can change quite a bit during the luteal and menstrual phase. Changes are so individual so it’s best not to scan during the luteal/menstrual phase and try to scan when in the follicular phase (day 1-14 of the cycle) if possible.


Weight is a big factor in total body water. For the most accurate results, it’s better to remove clothing and ensure no food is ingested (fasted is best) and you’ve been to the toilet. i.e. with clothes body fat % will increase.

So in an ideal scenario, you would do the scan without any clothes on, fasted and when you’ve been to the toilet – and for women only when you’re in the follicular phase of your cycle.

However, in practical terms, this isn’t always possible. Therefore you just need to have the factors outlined above in your mind as you’re stepping on to be scanned.

In closing, if your results have taken a sudden turn for the worse, it might not be that you’ve put on all that body fat in a short space of time (and equally you probably haven’t gained 3kg of muscle in a week either!)

The best way forward is to try and be as consistent as possible with the scans (time of day / when you’ve eaten / time of the month etc.) in order to get as accurate of a measure as possible.

Follow a consistent measuring schedule and your hydration fluctuations throughout the month will average out.  Then you will then be better able to assess any real change in your body fat percentage over time.

I hope this post helps you to better understand your body composition scan results.

Yours in health,


Click here to read what some of our ladies are saying about WWF.