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 Way to Measure Body Fat?
No body fat test, be it body fat 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, known as carcass analysis, is highly accurate, you have to be dead in order for it to happen.
A body fat test is less of a measurement than it is an estimate. A rough guess about what your body fat percentage really is.
While this guess often comes dressed up in complex equations and fancy charts, it’s still a guess, one that’s often a lot less accurate than many believe.
How Do Body Fat 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. They use a method known as bioelectrical impedance (BIA) to estimate your body composition.
You take your socks off and step on the scales, which then send out a weak electrical current. This current runs up one leg and down the other. The body fat scales then measure the degree of resistance (or impedance) to the flow of the current .
Different tissues provide varying levels of resistance, with fat-free mass providing less resistance than fat due to its higher water content.
The idea is that by determining the level of resistance to the current, the scale will know how much lean tissue and fat mass you have, which it then uses to estimate your body fat percentage.
Some body fat scales also connect to your smartphone and churn out numerous graphs, charts and diagrams. But while it might look very scientific and official, much of this information is completely useless.
How Accurate Are Body Fat Scales?
Most research shows that body fat scales aren’t accurate at tracking individual 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.
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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 the device is really only gathering data from your legs, and making assumptions about what’s going on in the rest of your body.
Changes in hydration status will also have a big impact on the results. In fact, bioelectrical impedance analysis 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 bioelectrical impedance analysis agreed reasonably well .
But in those who lost weight using cardiovascular exercise, bioelectrical impedance analysis 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 bio-electrical impedance analysis wrongly assumes that the density of various tissues is the same from person to person, and remains constant over time as you lose weight.
In fact, foot-to-foot bio-electrical impedance analysis has been shown to overestimate body fat percentage in lean subjects, while underestimating it in obese subjects .
That is, the more fat you have, the more likely it is that body fat scales will underestimate your body fat percentage.
On the flip side, the leaner you are, the more likely it is that you’ll end up with an estimate of your body fat percentage that’s higher than it really is.
Not only is it inaccurate, it’s inconsistently inaccurate.
Consistency vs Accuracy
I often come across forum posts from people who own a body fat scale and swear blind that it’s accurate, mainly on the basis that it’s consistent.
That is, when they take back-to-back readings (i.e. they get on the scales, get a reading, get off the scales, then step back on two minutes later) the scales show similar results.
It’s giving me much the same number from one reading to the next, they think to themselves, so it must be right.
Problem is, they’re confusing consistency with accuracy. Multiple back-to-back readings will determine test-retest reliability — how close the numbers are when repeated measurements are made.
However, test-retest reliability isn’t the same thing as accuracy.
To determine accuracy, you need to compare the readings you get from the body fat scales to some kind of reference method.
As I mentioned earlier, the most accurate way, and indeed the only way, to measure your body fat (as opposed to estimating it) is carcass analysis (i.e. stripping the fat from a dead body and weighing it).
When it comes to estimating body fat, the current state of the art is something called the 4-compartment model, or 4C model for short.
The 4C model is an expensive method of measuring body composition that divides the body into four components (mineral, water, fat, and protein) and measures each one independently.
Short of killing someone, stripping off their fat and weighing it, the 4C model is as good as you’re going to get. It’s currently the benchmark test for body composition and the gold standard against which other body composition tests are measured.
Do Scales That Measure Body Fat Work?
When researchers have compared body fat scales with the 4C model, 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 analysis, the technology used in body fat scales — with the 4C model .
Bioelectrical impedance was the least accurate of all the methods. In fact, the error 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 might 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.
Researchers from Texas also tested a number of bioelectrical impedance analysis units, and found an equally large error when compared to the 4-compartment model .
Of the three bioelectrical impedance analysis devices they tested, the Omron HBF-306 (single-frequency hand-to-hand electrodes) had the largest error for estimating body fat percentage, clocking in at 7.9%.
That is, if the Omron puts you at 15% body fat, it could really be anywhere between 22.9% and 7.1%.
Both the single-frequency (Tanita TBF-300A, which has foot-to-foot electrodes) and multi-frequency (Seca mBCA 514/515, which has electrodes for both the hands and feet) units didn’t fare much better, with an error of 7.6%.
Assuming they give you a body fat reading of 15%, your true body fat could be as low as 7.4% or as high as 22.6%.
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.
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.