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I'm currently considering getting a body fat scale that uses electrical impedance for my home. I understand about how body fat scales are a better measure of progress between measurements than absolute measurements, and how one needs to measure with consistent conditions for meaningful comparisons. I've been using a body fat scale at my gym for years.

I've noticed that while prices vary widely, there are essentially three levels of price. The very low end ones you just stand on, and have simple LCD displays. Next up from that are the ones that have a retractable handle to hold onto so that the electrical signal goes through your arms as well as your legs. The third level is for a full stand unit, with large video screens and the ability to connect to printers and other peripherals. There are also some other features that can be found at any level, like Bluetooth connectivity to one's phone.

At their core, if you strip away the screens and peripherals, they're all essentially the same thing: a set of electrically conductive pads along with a chip for measuring the electrical signals transmitted through one's body.

From that perspective, is there any reason to believe that these devices get meaningfully better as you go up in price? Or are you just paying for the housing and fancy screens? Is there anything about the strength of signal or the quality of the conductive surfaces that would be important in terms of quality?

It seems to me that it would be good to get a unit with hand sensors as well, but beyond that, is there anything else about one of these devices that I should be looking for in order to get the most accuracy?

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    The best ones show the lowest %! – DMoore Sep 20 '14 at 17:42
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    This question appears to be off-topic because it is about purchasing electronics and not fitness. – user2861 Sep 30 '14 at 5:17
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    This fellrnr.com site reviews a few of the models – arober11 Jun 2 '15 at 8:02
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It's difficult to get any sort of professional review because the general agreement among the consumer review sites is that body fat scales are inaccurate as a rule.

.... our results proved that it is probably inaccurate no matter what the manufacturers say.

And in general, higher-priced scales are more likely to look nicer than actually perform better although they do mention offhand that "Devices that also have hand electrodes tend to fare somewhat better."

One problem with body-fat scales is that they are often inaccurate .... Studies have found that different body-fat scales produce widely varying readings and that these often differ from standard methods of fat measurement. (Devices that also have hand electrodes tend to fare somewhat better.) In a study published in Obesity Facts in 2008, scales with only foot electrodes underestimated body fat in people with lots of body fat and overestimated it in leaner people. Even the manuals say the devices may be less accurate for elderly people, highly trained athletes, children and people with osteoporosis, among others. Consumer Reports no longer tests body-fat scales because of their inaccuracies.

  • The increase in price doesn't necessarily mean an increase in accuracy. It's still just an approximation based on an approximation. Good enough for general use, but don't put too much confidence in the answers you get. – Berin Loritsch Sep 22 '14 at 13:13
  • This answer would be better if you put some of the relevant sections of those articles in as quotes. – user10169 Jun 16 '15 at 9:22
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For just about any body fat scale what you want to judge is the progress over time. It isn't giving you a very accurate number, but it will be able to show you a trend. I don't believe buying a high-end Fat % scale is going to give you more than 1 or 2 % more accuracy then a mid-tier Fat % scale.

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It's not the price of the scales, it's the maths you should be looking out for!

Body fat scales use a process called Bioelectrical Impedance Analysis (BIA) to measure the impedance to a small electrical current being passed between two or more points on your body. Most domestic models simply measure the resistance / reactance to a 50Hz current being passed up one leg and down the other. The electronics / components to do this are trivial and inexpensive, so the raw data the scales have to work with will be very similar.

The scales will plug the Resistance / Reactance readings obtained into a series of regression algorithms, to first estimate the percentage / volume of water encountered between the two electrodes, then this result into another equation to estimate your body fat percentage, from found ratios of water, to fat, mineral content (bone) and muscle mass obtained from DEXA scans of a sample population.

There are at least a dozen academic papers available that suggest different algorithms for this process, with stated Standard Error rates and details of the sample population the regression algorithms are based on.

The algorithms incorporate a number of other variables such as: Sex (females have some additional fat stores), Race (Africans have 10-15% more bone mineral content and 3% less fat per kg, than Europeans, who have 3-5% less body fat, than Asians), similarly the ratio of fat to muscle in athletes differs to that of the population at large, limb length / height (effects fat deposition), age (your ratio of fat to muscle increases over 40), BMI (many formula don't work for bodies with a BMI <17 or >40)......

Essentially you want a device that uses a regression formula that was developed for a sample population that stereotypically describes you, or alternatively applies some post calculation adjustment to compensate for known differences eg. the Withings scales will add 5% to the calculated body fat reading if you claim to be Japanese. The other alternative is to seek out a device that displays or can be hacked to display the found resistance and reactance values, so you can have a play with the maths and work out a model that suits.

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Body fat scales estimate your body composition using a lookup table. It measures your body electrical impedance, and uses a statistically generated lookup table of the general population in order to produce a fat % number. As such it will never be good for measuring extreme cases (super fat, or super skinny). The impedance measurement itself is affected by hydration level and by recent physical activity as explained in this article. These devices are best use to get a trend over time rather than an absolute fat %.

You might expect higher price devices to last longer and/or give more repeatable results. As far as their accuracy is concerned, they are all based on the same lookup principle and so work best (least badly?) on people with average body composition.

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Waste of time and money.

If you're really that interested in your body fat % and want something that is going to be as accurate as possible, have a DEXA scan done or hydrostatic weighing.

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