AbstractHere we ask, what controls the horizontal scale of the Madden‐Julian Oscillation, i.e., what controls its zonal wave number k? We present a new one‐dimensional (1D) β‐plane model that successfully simulates the MJO with the same governing mechanism as the 2D shallow water model of Yang and Ingersoll (2013). Convection is parameterized as a short‐duration localized mass source that is triggered when the layer thickness falls below a critical value. Radiation is parameterized as a steady uniform mass sink. Both models tend toward a statistically steady state—a state of radiative‐convective equilibrium, not just on a global scale but also on the scale of each MJO event. This gives k ~ (Sc/c)1/2, where Sc is the spatial‐temporal frequency of convection events and c is the Kelvin wave speed. We offer this scaling as a prediction of how the MJO would respond to climate change.