# Body Analysing Scales - the maths?

Wondering if anyone has bothered to gather the data and do the maths necessary to determine which of the common: Total body Water / Fat / Lean Mass regression equations are lurking in their Bioelectrical impedance analysis (BIA) scales ?

I know it’s a relatively trivial exercise but before I spend an hour or two playing with the dozens of published formula eg.

TBW = (0.372 * HeightCM * HeightCM / Resistance) + (0.142 * massKg) - (0.069 * ageYears) + (3.05 * ifMale)

Matias et al. (2015):

TBW = 0.286 + (0.195 * HeightCM * HeightCM / Resistance) + (0.385 * massKg) + (5.086* ifMale)

I was wondering if anyone has been sufficiently curious to have a play, and compile a list of which manufacturers use which formula(s), and the number of variable in their regression calculations?

FYI: A five min play on my Salter 9141 WH3R has partially revealed :-

TBW % ~= X * massKg + (Y * HeightCM * HeightCM / ResistanceOhm) - 0.2 * ageYears - 7.6 * ifMale

Similarly a 5 min play on the Withings website reveals switching between their Normal and Japanese 2011 formula: simply adds / subtracts 5% to the reported Body Fat % figures, and that their normal Fat equation incorporates: - 0.12 * ageYears

Would be nice if manufacturers documented the formulas they use. You may be able to reverse-engineer the formula in some cases, but that gets complicated for devices that use multiple frequencies…