Ruminants are the main bio-converters of fibre-rich biomass. Although animal feed and feed efficiency are key components of the global food system and land use, they are very difficult to assess. The aim of this study was to estimate the feed conversion ratios (FCR) of ruminants for milk and meat production combining statistical data and modelling at the world level divided into 21 regions including 8 regions in Europe. The FCRs (expressed in kg DM/kg product) necessary to produce one litre of milk or one kg of carcass were calculated using the IPCC’s TIER2 methodology which allowed us to estimate dry matter intake (DMI) based on energy needs and diet digestibility. DMI of dairy cows (after their first calving) was attributed almost entirely to milk while DMI of all other ruminants was attributed to meat, the growth of dairy heifers corresponding more or less to the carcass meat of the culled cows. To calculate feed consumption more precisely 8 categories of animals were considered, namely dairy cows, beef cows females 0-1 year, 1-2 years and + 2 years, males 0-1 year, 1-2 years and + 2 years. A demographic model made it possible to calculate the numbers of animals for each category from omprehensive FAO statistical and technical data. A formulation model was used to allocate feedstuffs to the different productions. The digestibility of the ration was finally estimated for feedstuffs (using feed tables) and for grasslands according to climate data. Results show that FCRs of milk and meat vary greatly between the different regions of the world. They also show that the two FCRs are positively correlated only for very low productivity levels of animals. When the productivity of dairy cows exceeds 4,000-5,000 kg of milk/cow/year, there is a trade-off between milk and meat efficiencies. The increase in milk productivity improves milk FCR but degrades that of meat because more beef cows are needed to compensate for the drop in meat production from the dairy herd. For the same reason, a higher ratio of milk/meat production in a region generally improves the FCR of meat due to a smaller proportion of beef cows.