It is defined as the mass of water that will absorb or lose as same quantity of heat as the substance for the same change in temperature.
Let us consider a body having mass m, specific heat capacity s and change in temperature ∆θ.
Q = ms∆θ è1
If W be the water equivalent of the substance then,
Q = Wsw∆θ è2
Equating equation 1 and 2
ms∆θ = Wsw∆θ
W = ms/sw
In CGS system sw = 1calg-1K-1 then,
W = ms
Note: the water equivalent of a substance is numerically equal to the heat capacity of a substance in CGS system but not dimensionally.