An approximate formula for the thermal conductivity of multicomponent gas mixtures is derived from rigorous kinetic theory by well‐defined approximations. Numerical calculations with the formula are relatively simple, and the only data needed are the molecular weights, thermal conductivities, and either viscosities or heat capacities of the pure components at the same temperature as the mixture. The form of the formula is quite similar to the earlier empirical Lindsay‐Bromley equation. The formula is tested by comparison with experimental results on a number of binary and ternary mixtures involving both monatomic and polyatomic nonpolar gases. Agreement is satisfactory, and is nearly as good as obtained with the full rigorous theory.