AbstractColumnar sea ice grows with an interface of tiny parallel ice plates, the distance of which is known as plate spacing. While it has been proposed as a fundamental microstructure scale of sea ice, the physics behind its formation has not been fully understood. Here the problem is analysed on the basis of morphological stability theory to propose a model that results in a physically consistent prediction of the relationship between the plate spacinga0and growth velocityV. The relationship may be divided into two regimes. In the diffusive regime, forVabove ≈2 × 10−4cm s−1one findsa0~V−2/3to first order. In the convective regime, the extent of diffusive boundary layer is controlled by solutal convection near the interface, which leads to the proportionalitya0~V−1/3. From a comparison to observations it is evident that the plate spacing is predictable over 5 orders of magnitude in the growth velocity, covering the range from fast laboratory ice growth to slow accretion at the bottom of marine ice shelves. The predictability opens new paths towards concise modelling of marine and sea-ice microstructure and physical properties.