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I'm looking to improve my body composition. Smart scales might help with that since they claim to measure body-fat percentage and more, but I've read several articles claiming that smart scales aren't very accurate.

Now I'm wondering whether they're at least consistent in their inaccuracy. For example, if a smart scale is consistently off by, say, 5% (in the same direction) when it comes to body-fat percentage, then I can still use it to track my progress.

Are smart scales consistent in their inaccuracy, or are they not even reliable in that regard?

Dennis Hackethal
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  • I asked a similar question about BFP calculations here: https://medicalsciences.stackexchange.com/questions/30772/what-is-the-error-on-various-body-fat-percentages (but not smart scales) – Azor Ahai -him- Jan 22 '24 at 18:58

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No, they aren't. They have numerous sources of error, but one of the big ones is that their measurements are hugely affected by hydration status, causing your claimed body fat percentage to vary based on how much salt and water you have recently consumed.

James Krieger of Weightology has an excellent article series which covers the inaccuracies of these devices.

If your goal is to assess whether weight you are gaining or losing is fat or muscle, far better approaches are:

  • If losing weight, track measurements of your strength, such as how much weight you can lift in various exercises, aiming to not see significant decreases in strength. If you don't have much arm or leg fat, you could also take arm, leg, and waist circumference measurements, hoping to see only the waist measurements decreasing.
  • If gaining weight, take regular waist circumference measurements, and aim to not see much of an increase in these, or alternatively accept some increase, but aim to keep the measurement below the recommended values for your sex and ethnic background.
David Scarlett
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    As an addendum, there are many different methods of skin fold measurements that can be done alone or with a trained partner. A skillful skin fold is second only to scan/underwater weighing for accuracy. – JohnP Jan 22 '24 at 16:05