During the early 19th century, the population of Sri Lanka was small and concentrated in the southwestern part of the island and in the Jaffna Peninsula in the north. The first official census, conducted by the British in 1871, recorded a total population of 2.8 million. Between then and the 1980s, the population increased sixfold. Population growth until around 1900 was given impetus by considerable immigration from southern India, as the British brought in hundreds of thousands of Tamils to work the plantation economy. These immigrants accounted for an around 40 to 70% of the population increase during the nineteenth century. Another remarkable factor in the growth of population after 1900 was a decline in mortality rates. The time of fastest growth was the decade after freedom, when the annual rate of increase was 2.8%. The official total in the 1981 census was 14,846,750, and some projections suggested a total of 18 million by 1991 and between 20 and 21 million by 2001. Furthermore, if the 1980s trends continue, the population will double in forty years.

The Sinhalese constitute the majority in the southern, western, central, and north-central parts of the nation. In the rural areas of the Wet Zone lowlands, they account for more than 95 % of the population. The foremost concentration of the Sri Lankan Tamils lies in the Jaffna Peninsula and in the adjacent districts of the northern lowlands. Smaller agglomerations of this group are also found along the eastern littoral where their settlements are juxtaposed with those of the Muslims. The main Muslim concentrations occur in the eastern lowlands. In other areas, such as Colombo, Kandy, Puttalam, and Gampaha, Muslims form a small but valuable portion of the urban and suburban population. The Indian Tamils, the large majority of whom are plantation workers, live in large numbers in the higher areas of the Central Highlands.