Dairy cow farms produce half of California's (CA) methane (CH₄) emissions. Current CH₄ emission inventories lack regional variation in management and are inadequate to assess CH₄ mitigation measures. We develop a spatial database of CH₄ emissions for CA dairy farms including farm-scale herd demographics and management data. This database is useful to predict CH₄ emission reductions from mitigation efforts, to compare with atmospheric CH₄ observations and to attribute emissions to specific farms.