The method of determining the economic viability of farms is presented. An ability of a family farms to work out an income that enables to remunerate the labour costs of family members plus 4% of its working assets is adopted as the base for determination of this viability. Apart form determining the economic viability of a farm, the suggested method allows to enrich the classification of farms by making use of a demographic criterion and taking into consideration vocational activity conducted outside a farm. Basing on the assumptions made, the classification of farms forming a sample of the Polish Farm Accountancy Data Network (FADN) in 2004 was done. The results of these calculations were generalized for 81% of the observation area of the Polish FADN which covers 745023 farms.