The point of a body fat analyzer is to give you readings about your body composition. This includes body fat percentage, muscle percentage, BMI and of course weight. For the calculations of weight we can be sure of pretty much every scale out there to perform the same, it’s not a mysterious science. Same goes for calculating BMI, once you have the height and weight of a person, the formula is simple.
Calculating body fat percentage though is somewhat of a mystery and companies that manufacture the devices haven’t been open about how they do this calculation. So there are a lot of questions online about how do they actually work.
Most consumer body fat analyzers use a method called electrical impedance. The way this works is it sends a harmless electrical pulse through your body and calculated the resistance. See the diagram below.
As you can see in the diagram the woman is standing with both feet on the scale on the metal bars. The point of contact for the feet is where the origin of the pulse will be from one foot and the other foot as the receiver. The purpose is to be able to measure resistance.
From your physics lessons you should remember that electricity will try to find the shortest path through a conductor, in this case your body. So the pulse will travel up one leg and down another. A measure of resistance is then taken. Muscle has more water than fat so will conduct the electricity better, meaning less resistance. So the rule is the lower the resistance, the more muscle and less fat you have.
Once you have this value you’ll need to convert it into a body fat percentage. This is done during the manufacturing of the product. A sample of about 100 people with different ages and body fat percentages are measured in a body pod, to get a more accurate figure. They are then measured on the product which gives the resistance reading. The two figures are mapped to each other and interpolated for values in between. What this will do is give a rough guide to a conversion between resistance and body fat percentage.
There are a few issues with this method. First is that the figure you get will be based on the sample taken during the product design phase. The sample size tends to be an “average person”. So if you have above average muscle or fat in your body this will throw the numbers off. Some scales have an athletic mode to compensate for this error.
Another issue is as you can see from the diagram, because of the nature of electricity taking the shortest path through your body it will only measure the body fat in your legs. Every one if different, but we can sat for certain that body fat is not distributed evenly throughout the body. That depends on multiple factors, most being your genetics. When you use a body pod or DEXA scan you get more of a full picture of your overall body composition.
A potential solution for this is using handle bars. The Omron BF511 uses these as part of the measurement process. When you stand on the scales you hold up the handle bars perpendicular to you for a few seconds and you get your reading. What it’s doing is sending a pulse through your legs then a pulse through your arms. The pulse through your legs will measure the body fat (resistance) up one leg and down another. The other one will go up one arm, through your chest and down another. The measurements are then averaged.
Although the method isn’t perfect, it means there are more contact points with your body to be able to give a more accurate reading. From the scales that I have tried, I’ve found it to give the most accurate and more importantly consistent results.
The method for measuring your body composition will always be evolving as we find more ways to derive the value. There are scales being developed currently that will use more contact points, for example 8 or 16 instead of 4. This obviously is inconvenient for every day measurement, but can give you a sense of confidence that you’re getting a good reading.
The other possibility and hope is someone creating an open source database for electric impedance so that all the companies can share their data. Instead of having a sample size of 100 people, we will have thousands. The more data we have, the more accurate we can be.
Of course someone could come up with a whole new may of measuring your body composition that will revolutionize the industry, we’ll be looking out for that.
Body Fat Analyzer Accuracy
This whole article was written to answer the question about the accuracy of a body fat analyzer. Now that you understand the method and the limitations of it, how should you be using your body fat analyzer? From all my tests and trials throughout the years I have found that more important than accuracy is consistency. The device should be a measure of progress than trying to get the absolute value within a 0.1% error. I have found from doing a body pod scan vs the consumer product I get about a 5% error either way. It also depends on the time of day I measure it and whether I’ve exercised or eaten beforehand.
The way I use it is every morning before I eat anything and after I’ve gone to the toilet I will measure myself and log it. This has provided a very consistent way of tracking progress.
If you are interested for me to go into more detail about the methods and science behind it, let me know and I’d be happy to write more about it. If you work in the industry and would like to build a more open source solution for the consumer then we’d be happy to help with that too.