AbstractOn the basis of the experimental data reported in literature, the contributions of cation mass (m) and molar volume (V) to lattice heat capacity (C) were analyzed. The volumetric‐mass formula,Cx=(l —f)·C1+f·C2+Cm·(mx—mx′), was presented for estimating the heat capacities of rare‐earth compounds. In the formulaC1andC2represent the lattice heat capacities of two reference substances respectively,f=Vx—V1/V2—V1andCmrepresents the lattice heat capacity variation with the variation 1 g of cation mass. The equation relating theCmwith temperatures was derived as follows:Cm= 0.084e−0.0074T−0.27e−0.045T, andmxandmx′ (= (1 ‐f)m1+fm2) represent the practical and “assumed” cation masses of the subs