I don't know the formula it uses, but I have an older Tanata scale that also estimates bodyfat and it is much the same. Remember these scales estimate body fat--they don't directly measure it. On my scale there is a big difference between regular and athletic mode--that is likely assuming more lean mass and perhaps that alters the formula used to estimate. On my scale, changing the age also changes the estimate as I noticed when I updated my age on a birthday that my bodyfat estimate increased. I was weighing every day, so the increase was noticable I think a couple percentage points. The scales use electrical resistance through the lower body (up one leg and down the other) for their estimate. They can be dramatically effected by how hydrated or dehydrated you are or whether you need ot use the rest room as well.
Sam | USA
Fitbit One, Macintosh, IOS
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