How does a macroscopic length scale emerge as a result of a collective pattern formation process from microscopic dynamics? This question is of great interest in a broad class of far-from-equilibrium systems, including active matter, granular media, and intracellular phase separation. In our work, we study length scale selection in the important class of reaction—diffusion systems dominated by mass-conserving reactions. Our main finding is that wavelength selection in such systems can be understood as a coarsening process that stops at a certain point. Based on this idea, we present a quantitative theory that reveals the physical mechanism of coarsening in mass-conserving reaction–diffusion systems and provides a simple, intuitive criterion for when it stops.