Between 1874 and 1920, the total population increased from about 945,000 to 2.5 million. By 1950 it had increased to between 3,710, 100 and 4,073,960, and in 1962 it had reached 5.8 million. From the 1960s until 1975, the population of Cambodia increased by about 2.21 % yearly, the lowest increase in Southeast Asia. By 1975 when the Khmer Rouge took power, it was around at 7.2 million. Of this total an around 1 million to 2 million accordingly died between 1975 and 1978. In 1981 the PRK gave the official population figure as nearly 6.7 million, although around 6.3 million to 6.4 million is likely a more accurate one. The average annual rate of population growth from 1978 to 1985 was 2.3 %. Life expectancy at birth was 44 years for males and 44 years for females in 1959. By 1970 life expectancy had increased by about 2.5 years since 1945. The greater longevity for females apparently reflected improved health practices during maternity and childbirth.

In 1959 about 44 % of the population was under 15 years of age; by 1962 this figure had increased slightly to 45 %. In 1962 an around 53 % of the population was between 15 and 64 years of age, while 2 % was older than 65. The % age of males and females in the three groups was almost the same.

The population of Cambodia has been fairly homogeneous. In 1962 about 81 % of the population was ethnic Khmer. The remaining 19 % included Chinese, Vietnamese, Cham, Khmer Loeu, Europeans. By 1981, as a result of the Vietnamese repatriation in 1970 to 1971 and the deaths and emigration of large numbers of Cham and Chinese, ethnic Khmer accounted for about 90 % or more of the population.