Coalescence of secondary dendrite arms in terms of the evolution of specific surface area for a dendritic envelope with respect to the fraction solid during directional solidification is studied analytically in 2-D and 3-D frame of reference. For 2D case study, the dendritic growth has been captured by gradual penetration of a parabola with fixed tip radius (ρ) inside a square domain, while for 3D case study, we replace the parabola and square domain for 2-D case with a paraboloid of revolution and cubic domain, respectively. The dimensions of the enclosing volumes are characterized by the secondary dendrite arm-spacing. Coalescence obtained by the analytical solution is compared with existing dendrite growth simulation results obtained by phase-field method.