The classical problem of determining heat transfer performance of extended surfaces is revisited. An improved one-dimensional formulation of the energy equation is proposed, which takes approximately into account the nonuniformity of temperature distributions across the fin, as opposed to the classical approach, which neglects transversal temperature gradients. The resulting modified expressions are as simple as those obtained from the classical solution, but accuracy on heat transfer rates at the base and average temperature profiles along the fin is significantly improved, as demonstrated by numerical results for rectangular longitudinal fins and cylindrical pins. The present analysis extends the applicability range of the very simple one-dimensional formulation to considerably larger values of Biot number at the fin's lateral surface.