Urbanization has appropriated millions of hectares of cropland1, and this trend will persist as cities continue to expand2. Here we estimated the substitution cost by comparing the yield potential between the converted and newly cultivated land as determined by climate and soil properties. To do so, we used robust spatial upscaling techniques, well-validated crop simulation models, and soil, climate, and cropping system databases 3–5, focusing on populous countries exhibiting high rates of land conversion. We found that productivity of new cropland is substantially lower than the land it replaces, which means that projection of food production potential must account for expected cropland loss to urbanization and the lower productivity of new land that replaces it. Policies that protect existing farmland from urbanization would relieve pressure on expansion of agriculture into natural ecosystems and reduce the associated greenhouse gas emissions and biodiversity loss.