The Born–Mayer equation is an equation that is used to calculate the lattice energy of a crystalline ionic compound. It is a refinement of the Born–Landé equation by using an improved repulsion term.
where:
*N =  Avogadro constant;
*M = Madelung constant, relating to the geometry of the crystal;
*z = charge number of cation
*z = charge number of anion
*e = elementary charge, 1.6022 C
*&epsilon; = permittivity of free space
*:4&epsilon; = 1.112 C/(J·m)
*r = distance to closest ion
*ρ = a constant dependent on the compressibility of the crystal; 30 pm works well for all alkali metal halides