A simplified confined-chain model for AB diblock copolymers in the lamellar microphase-separated state is presented. Block end-to-end distances and segment distribution functions are calculated by random flight statistics in one dimension without solving the diffusion equations as in the traditional approach. A simple expression to evaluate lamellar domain periods is obtained in terms of polymer pure density data and molecular weights. Numerical results are in close agreement with experimental data on domain sizes. A simple criterion for determining the equilibrium microdomain morphology is presented and compared with other theories and data from the literature.