The aim of our study was to estimate methane (CH4) emissions from a dairy cattle farm using a computer application. Emissions of CH4 in the air were forecast for a representative dairy farm raising Holstein-Friesian cows. The cowshed was equipped with a mechanical forced ventilation system with a centrally located ventilation duct. The volume of emissions from the emitter was established, taking into account meteorological conditions. For one year of operation of the emitter, the average annual emission was 1.301 kg/h. The maximum emission of CH4 was estimated at 3.51 kg/h. These data can be helpful in estimating the environmental burden of a dairy farm and in determining the role of ruminants in global warming.