A bottom-up approach is constructed to determine N losses from livestock farming sys- tems and to relate these losses to the supply of fresh milk, pig and beef to Paris. First, the three products are expressed in terms of their nitrogen content; then, their fodder equivalent is determined by modelling feed formulas for swine, beef and dairy cows to meet their energy and protein requirements. Fodder deficits in livestock farms are determined by comparing the nutrient requirements of the livestock with the fodder pro- duction on the livestock farms. This allowed determining the geography of the livestock systems according to the imports of fodder to the livestock farms from external crop farms. Then we assessed the “farm-gate” N budgets in all crop and livestock farms of the entire livestock systems using data on total N fertilization, atmospheric deposition and manure management practices to finally derive N losses in relation to fodder cul- tivation and to manure management. Measured in N, the supply of milk, beef and pig to Paris sum 1.85 kg N/cap and the corresponding N losses from the farming systems total 8.9 kg N/cap. N losses per unit of product differ among the three livestock systems according to where and how the fodder is grown and to what densities the livestock is reared.