The debate about organic farming productivity has often focused on its relative crop yields compared with conventional farming. However, conversion to organic farming not only results in changes in crop yields, but also in changes in the types of crops grown. To date, the effects of such changes on global crop production have never been systematically investigated. Here, we provide a novel, spatially explicit estimation of the distribution of crop types grown, as well as crop production, under a scenario of 100% conversion of current cropland to organic farming. Our analysis shows a decrease of −31% harvested area, with primary cereals (wheat, rice and maize) compensated by an increase in the harvested areas with temporary fodders (+63%), secondary cereals (+27%) and pulses (+26%) compared with the conventional situation. These changes, paired with organic-to-conventional yield gaps, lead to a −27% gap in energy production from croplands compared with current production. We found that ~1/3 of this gap is explained by changes in the types of crops grown (a contribution rising to 50% when focusing on food crops only), and that such changes strongly affect the repartition of total production among different crop types. Feeding the world organically would thus require profound adaptations of human diets and animal husbandry.