Approximate but simple expressions for the rate of isothermal coarsening of secondary dendrite arms are presented. Their derivation differs from previous work in that (a) focus is placed on growing dendrite arms, which simplifies the derivation, and (b) the effect of a finite volume fraction solid is taken into account. Expressions for isothermal coarsening are then integrated to predict the final dendrite arm spacing in a cast dendritic microstructure. Coarsening rates predicted for Al-Cu alloys, succinonitrile, and Fe-26 wt pct Ni agree with published experimental data within a factor of 2 which is within the range of variation of experimental data.