Most body fat scales work by using a technology called bioelectrical impedance analysis. What this does is send a small electrical current up through one foot, up the leg and then down the other leg. Fat is a poor electrical conductor while muscle is a good electrical conductor. Therefore, the more resistance the electrical current encounters, the higher the fat volume.
While this is the best technique we have for consumer body fat scales, they are not always 100% accurate. Several variables can affect the results, including when you last ate, how hydrated you are, your skin temperature, body position, whether your feet are dirty or highly calloused, and the quality of the scale itself. In order to obtain the most accurate results, users should always use the scale at the same time of the day and never on a full stomach. You should also ensure you are centered on the scale and your feet are clean and preferably slightly damp.
For step on scales, the fact that the current only runs from foot to foot means that only the fat content in the lower part of the body is measured. Some studies show that body fat scales that use only foot electrodes underestimated the amount of body fat in subjects with lots of body fat and overestimated the amount in leaner subjects. This can vary depending on the quality of the scale and if there are user profile settings that can help adjust for age, gender, and body type.
Generally, scales may be less accurate for elderly people, children, highly trained athletes, and those with osteoporosis. However, it should be noted that in spite of any inaccuracies, the scales can still be used to monitor progress if you standardize the way you perform each test. This means consistently testing your body fat percentage under the same conditions over time. Newer scales often include fitness monitoring apps that connect via bluetooth or Wi-fi and help to track readings automatically. This makes the scales much more useful, even if the readings aren’t 100% accurate.
The Top 5 Most Accurate Digital Body Fat Scales
Across my team we tested over a dozen body fat scales using hydrostatic weighing as the baseline measurement for each participant. Each scale was tested once per day over 7 days so that consistency could be taken into account. While none of the devices were 100% accurate there were certainly big differences in accuracy in the initial measurements and also fluctuations over the course of the week. The top 5 (based purely on accuracy) are listed below:
The fact that this scale takes readings from both the hands and the feet eliminates some of the inaccuracies that occur with feet only measurements, since some people carry more fat in different parts of the body.This scale is very directionally accurate (it can track small changes in body fat up or down), but there are no options for configuring your specific body type, and it uses assumptions based on your age and gender to do the calculations. This means some users, particularly those with less common body types, may see inaccurate results depending on a number of factors. Unfortunately, you cannot change the settings to accommodate for this. This body composition monitor is really meant for fitness athletes who need a high degree of accuracy when tracking body fat changes.
In our testing, after the initial setup is complete this scale provides very consistent results over time. There are some limitations as always, but it is one of the more accurate body fat scales on the market today. That said, the price is also quite a bit higher than many other scales, especially considering it does not have bluetooth connectivity or a mobile app to help track progress. This makes it a good fit for athletes or fitness competitors, but possibly not for more casual users.
The readings are at the same accuracy level you’d get at a professional fitness center, making it a good scale for professional fitness athletes. Nonetheless, amateurs might not value this level of accuracy considering the high price tag.
Like other Tanita body composition monitors, it uses Bioelectrical Impedance Analysis (BIA), which is acknowledged as one of the most accessible and reliable methods of screening body fat. The difference between this device and some of the lower cost scales on the market is the ability to measure different segments of the body independently. Typically body fat scales only take readings from the feet, which can lead to inaccurate results depending on a number of factors. That is not the case with the Tanita BC-558.
Overall, this is one of the most accurate body composition monitors you can buy and is best suited for professionals or those who are willing to spend 10x for that accuracy. For recreational users your money may be better spent on a lower cost scale that offers features like bluetooth connectivity and fitness tracking.
Pairing the machine with the companion app is fairly effortless. iOS users should have no trouble, but for Android 6.0 users make sure your location is turned on in order to link the scale to the app. This is not mentioned in the instructions.
After the scale is connected to the app, you setup your profile, and transferring data is simple from there. All you have to do is turn on Bluetooth, pair, and step on the scale. The scale will automatically transfer your information to the app privately.
Our measurements were consistent for weight and body fat, although some other customers have stated that the scale doesn’t zero out properly unless you first put some weight on it, step off, and then step back on for a second reading. This appears to be isolated to a small number of users though.
The app, which can be downloaded from both iTunes and Google Play, is easy to use. Entries in the Weight Gurus app can be saved, deleted or transferred to other services like Fitbit or Google Fit. Attractive graphs are available within the app to help track progress over time, in addition to the many fitness tracking features available in the other compatible services. Some users report issues with Fitbit compatibility, but it seems this has been resolved by an update to the app.
Overall, this is a very attractive and accurate scale with a feature set that dwarfs most others in this price range. Our testing showed consistent results, and the multiple different fitness tracking apps make it extremely easy to monitor progress and spot potential outliers or bad readings and remove them from the data set. Combine that with excellent customer service, and you have yourself a winner.
The weighing process takes no more than five seconds which is better than most, but it can be finicky. In our testing the results are consistent over time, but there are a number of things that can throw it off. A couple of things to keep an eye on: 1) the surface its on; 2) your distribution of weight (must be perfectly centered); 3) how much water you've consumed that day and when; 4) if you are wearing any metallic jewelry.
To get the most accurate readings be sure the scale is on a hard flat surface, your weight is perfectly centered with your feet lined up on the sensors, and any metal jewelry is removed. As with all BIA scales, you should weight yourself at the same time of day every day, preferably on an empty stomach. If you drink a lot of water before weighing, this can throw off the reading. All these variables remaining constant, we received mostly accurate readings over time with this scale.
The best part about Fitbit tracking is that even if you get an inaccurate reading from time to time, the data is all stored online and outliers can be removed to maintain a consistent trend. At the end of the day, it’s the long term trend that matters the most and not day to day fluctuations. Aria automatically syncs to your Fitbit dashboard at each weigh-in making it very easy to stay on track and keep yourself motivated.
On a whole, this is a very well designed scale with a nice feature set, but ease of use and the Fitbit fitness tracking apps are where it excels. While it is relatively expensive compared to competing scales, this is still a good value for those who value simplicity.
In our testing, this scale does get close in terms of accuracy when it is used properly. As with other BIA scales it is recommended to use it at the same time of the day. An ideal time of day is at night, at least 2 hours after your last meal. Readings do fluctuate, but fortunately it synchs all data to a mobile app making it easy to keep track of the overall trend, even if you occassionally get a bad reading. The weight readings were very accurate, body fat slightly less. Our muscle mass readings did seem a little off, and unfortunately there are no settings that allow you to adjust based on body type, age, etc.
For a casual user looking to improve their body composition over time, this scale is a very affordable option at under $40. This is a great price point considering it offers fast and simple synching of your data via bluetooth to your mobile phone. The app is fairly simplistic and could use some UI improvements, but overall, this scale is very easy to use and looks great on your bathroom floor.
Other Ways to Measure Body Fat
A quick Google search will show dozens of ways to measure body fat ranging from the simple and quick to the incredibly detailed. Such techniques can help in setting baseline values for body composition and certain goals. However, the variations in techniques entail a fluctuation in accuracy. One technique may nail down your body fat percentage to within a few decimals, while others tend to leave a huge discrepancy. Here are five of the best techniques for measuring body fat.
Skinfold Calipers – This is probably the most accessible technique for measuring body fat. A skin fold evaluation can be performed in three, four or seven sites, which typically include the arm, chest, abdominals and thigh. The technician pinches the skin and then uses a caliper to measure the thickness of the skin fold. The technician then uses a formula to calculate body fat percentage.
Bioelectrical Impedance – This has been previously mentioned. Bioelectrical impedance scales range from a scale with electrodes under each foot to a scale with handholds and additional electrodes. Regardless of the device used, they work by sending electrical impulses through the lower part of the body and then measuring how fast those impulses return. The rationale behind this technique is that lean tissue conducts electrical impulses faster than fatty tissue. Hence, a quicker response time is associated with a leaner physique and lower body fat percentage.
Hydrostatic Weighing – This is also known as underwater weighing. It compares the normal bodyweight of the subject to his/her bodyweight while fully submerged. Using the two measurements and the density of water, technicians can accurately calculate the subject’s density. The resulting figure is then used to estimate body fat percentage.
DEXA (Dual-Energy X-Ray Absorptiometry) – This is considered as the gold standard for measuring body fat percentage. A DEXA scan exposes the subject to X-ray beams of varying intensities to measure bone mineral density as well as body fat percentage. The subject lies on a table while a machine arm passes over the entire body while emitting both high and low-energy X-ray beams. The technician can obtain readings for bone mineral density and fat mass by measuring the absorption of each beam into different parts of the body.
Air-Displacement Plethysmography – This is actually similar to hydrostatic weighing. The subject sits in a small machine (like a Bod Pod) and the amount of air displaced is measured. The technician can then determine body density and compute body fat percentage.