Given the available inputs of
- Height (cm)
- Weight (kg)
- Fat (%)
- Muscle Mass (%)
I know the Watson Formula calculates Total Body Water (in Litres) as
- TBW(Male) = 2.447 - 0.09156 X age + 0.1074 X height + 0.3362 X weight
- TBW(Female) = -2.097 + 0.1069 X height + 0.2466 X weight
But how do you calculate Body Water Percentage?
Are the additional input of Fat and Muscle enough to make the calculation possible?
Bill Oertell Makes a very good ( and on hindsight obvious :-) ) point. I did a little more digging and most references to healthy water percentage ranges are referring to percentage of body water by mass. So given that the Watson Formula is returning water in Litres and 1L = 1Kg we simply need
- (Simplistic) Body Water Percent by Mass = TBW/Weight * 100%
However this does give us a very simplistic view on water percentage. The original formula I found is not taking into account variances caused by Body Fat and Muscle Mass that the modern electronic fitness scales record using bioelectrical impedance. When I run the data from one such device ( Salter MiBody ) through the given formulas I get results that are close ( +/- 1.5% ) but there is clearly a variance caused by other factors. I think what I need is a more modern Total Body Water formula taking into account Fat and Muscle numbers.