The mass and heat balance of Arctic sea ice is affected by the deformation of level ice and the amount of ice stored in sea ice ridges and rubble fields. One important property of sea ice ridges is the macroporosity or void fraction. While macroporosity has been observed in field and laboratory studies, data are sparse and a concise theory of its evolution is lacking. In the present study, the hypothesis is investigated that the initial macroporosity of sea ice ridges is related to random loose packing. When laboratory results on the packing of ice blocks are corrected for boundary effects, good agreement with random loose packing predictions is obtained. The macroporosity then depends mainly on the length-to-thickness ratio of the ice blocks ϵb and it can be expected to fall in the range of $0.4 \lt \phi \lt 0.5$ for typical dimensions of ice blocks in sea ice ridges. In the field, such high macroporosity values are seldom observed., because thermodynamic adjustment, related to the increase in microporosity of submerged cold ice blocks, rapidly decreases the initial macroporosity by 0.1 to 0.15 for typical Arctic freezing conditions. Taking into account this effect, field observations are also consistent with random loose packing of ice blocks. The proposed macroporosity prediction can be useful for modeling the consolidation and property evolution of sea ice ridges and rubble fields and for improving thickness redistribution algorithms in sea ice models.