The article presents the methods of computing the size of urban population in 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 had to be carried out on the basis of indirect sources, which cannot be precise. The most popular method of computing the size of urban population consists in counting the buildings situated in the town in question, which had been registered in inventory or tax sources. The authors analysing the date of the whole early modern period, especially the 18th-century Austrian registers have come to the conclusion that the value of the conversion factor should be related to the type of buildings. In the case of most Polish towns dominated by wooden buildings the conversion factor should be six people per one house. If we have the information on the number of houses, the calculations of the size of population should be carried out on the basis of the data contained in the land tax collection registers, which register the number of urban mansi and the heads of households of the population composed of craftsmen, tradesmen and landless tenants.