The article presents methods of computing the size of the population in the rural areas of the Kingdom of Poland in the second half of the 16th century. As there are no sources such as censuses the assessments of the population have been carried out on the basis of indirect sources, which cannot be precise, such as registers of land taxes of the years 1578–1581; they give information on the number of mansi (a mansus, laneus, a field measurement equal to 16.8 hectares) in a village that belonged or were used by peasants, village headmen, impoverished gentry, innkeepers or were just empty mansi. In addition to that, the tax was supposed to be paid by landless lodgers (with and without cattle), craftsmen, millers, ore miners and representatives of other proto industrial professions. According to a suggestion made by Irena Gieysztorowa the authors compare the data of the tax collection of 1552 (from peasant’s households) with the chronologically closest ones of the 1560s (the land taxes), which allows to regionally differentiate the conversion factor of the number of peasant households in relation to the taxed mansi, representative also of the turn of the 1570s and the 1580s. Moreover, the authors have used the findings of historical demography of the last decades, among other things the conversion factors of the size of households of several groups of rural population. On the basis of the research on the 16th century economy the authors have established the conversion factors of how big was the demand of the farms for permanent hired workforce.