How accurate are body fat scales? The short answer is that they’re not accurate at all.
In fact, you can’t rely on any of the body fat tests out there to track fat loss and muscle growth over time.
Using the results to guide your training and diet decisions may well send you off in completely the wrong direction.
I’ll explain why in a moment. First, I want to take a step back and explain why I don’t trust any of the body fat tests out there.
What Is the Most Accurate Test to Measure Body Fat?
No body fat test, be it body fat percentage scales, DEXA, or skinfold calipers, can accurately measure how much fat you have. The only way to measure your body fat is to have it stripped out, placed on a scale, and weighed. Although this method is highly accurate, you have to be dead in order for it to happen.
To paraphrase Weightology founder James Krieger, a body fat test is less of a measurement than it is a prediction. And, just like the words of wisdom that come from economists, football pundits and stock market analysts, many of those predictions will turn out to be wrong.
“When a weather reporter gives you a forecast, they don’t measure the weather,” Krieger explains. “They are predicting the weather. And that’s exactly what is happening when you have your body fat tested.
“We are predicting your body fat levels, not measuring them. Just as the weather reporter cannot predict the weather with 100% accuracy, we cannot predict your body fat levels with 100% accuracy.”
How Do Body Fat Percentage Scales Work?
Body fat scales are one of the most popular ways to measure your body fat, mainly because they’re quick and easy to use. Some devices even connect to your computer and churn out numerous graphs, charts and diagrams. But while it might look very scientific and official, much of this information is completely useless.
Body fat percentage scales use a method known as bioelectrical impedance (BIA) to estimate your body composition. They run a light electrical current through your body, measure the degree of resistance (or impedance) to the flow of the current, then use this information to estimate your body fat percentage .
How Accurate Are Body Fat Scales?
Most research shows that body fat scales aren’t accurate enough to track changes in body composition over time. In fact, you could lose fat and gain muscle over a period of several months, but body fat scales might say that your body fat percentage has gone up.
The first and perhaps most obvious problem with body fat scales is that they miss out large segments of your body.
Stand on a set of foot-to-foot body fat scales, for example, and the current will simply go up one leg and down the other . So you’re really only measuring how fat your legs are.
Changes in hydration status will also have a big impact on the results. In fact, BIA seems to interpret a change in body water as a change in fat mass .
Levels of total body water can also be affected by the type of training you do. In a group of men using strength training to lose weight, changes in body fat measured by underwater weighing and BIA agreed reasonably well .
But in those who lost weight using cardiovascular exercise, BIA underestimated fat loss and overestimated the loss of fat-free mass. This discrepancy appears to have been caused by a change in total body water resulting from an increase in plasma volume, which is one of the adaptations to cardiovascular training.
The argument in favor of body fat testing is that even if a given test isn’t accurate, at least it’s consistent.
In other words, it doesn’t matter if a body fat test is “out” by a few percentage points here or there. As long as it’s consistently inaccurate, you can use it to track your progress over time.
The problem with this idea is that a change in weight causes a shift in the density of various tissues. Different types of training also have different effects on the density and composition of fat-free mass .
But body fat scales wrongly assumes that the density of various tissues is the same from person to person, and remains constant over time as you lose weight.
To put it another way, the degree to which a body fat test is “out” by will change over time. Not only is it inaccurate, it’s inconsistently inaccurate.
Do Scales That Measure Body Fat Work?
When researchers have put body fat percentage scales to the test, the results have been less than impressive.
In one study, researchers from Maastricht University looked at changes in body composition in a group of male bodybuilders. They compared several body fat tests — including bioelectrical impedance, the technology used in body fat scales — with something called the 4-compartment model .
The 4-compartment model is currently the gold standard when it comes to predicting body composition, and is the benchmark against which other body fat tests are compared.
Bioelectrical impedance was the least accurate of all the methods. In fact, the error rate got as high as 8%.
So what does that mean exactly?
Let’s say you step on some body fat scales, and your body fat percentage comes out at 20%. You eat right and train hard for a couple of months, and get down to 15% body fat.
But when you go back for another test, the scales say that your body fat percentage is still 20%.
Same thing holds true for muscle growth. You could gain 5-6 pounds (around 2-3 kg) of muscle over a period of several months. But the scales might show that you hadn’t gained any muscle at all.
You’d come away with the impression that whatever you’d been doing to generate those results didn’t work, when actually it did.
And you run the risk of ditching a training and nutrition program that’s working and replacing it with one that’s less effective.
I’ve worked with people who have clearly lost fat and gained strength over a period of several months. They were stronger. Their clothes fitted better. They looked better in the mirror.
But the body fat percentage scales said that their fat percentage had actually gone up.
This left them feeling like all their hard work had been for nothing. Such was their faith in technology that they were more willing to believe a machine than what their eyes were showing them.
Body Fat Percentage Scales: Final Thoughts
A body fat test is meant to let you know when you’ve achieved a specific goal (such as reaching a certain body fat percentage), to let you know if what you’re doing is or isn’t working, as well as appealing to the need that some people have for an “official” estimate of how fat they are.
Body fat scales fail on all three counts, simply because the margin of error is so large.
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ABOUT THE AUTHORChristian Finn is the nation’s leading authority on science-based, joint-friendly ways to build muscle. A former "trainer to the trainers," he holds a masters degree in exercise science, and has been featured in or contributed to major media on two continents, including the BBC and Sunday Times in the U.K. and Men’s Health and Men’s Fitness in the U.S.