As of 2021, Cambodia's population stood at 16.9 million.Since the first population census in the 60s the Cambodia's population is growing constantly. From the 1960s until 1975, the population of Cambodia increased by about 2.2% yearly, the lowest increase in Southeast Asia. By 1975 when the Khmer Rouge took power, the population was estimated at 7.3 million. Of this total an estimated one to two million reportedly died between 1975 and 1978. In 1981, the PRK gave the official population figure as nearly 6.7 million, although approximately 6.3 million to 6.4 million is probably more accurate. The average annual rate of population growth from 1978 to 1985 was 2.3%. A post-Khmer Rouge baby boom pushed the population above 10 million, although growth has slowed in recent years. The distribution of the population is uneven from geographical point of view. Most of the people leave in the central part of Cambodia, in the Central Plain and the Tonle Sap regions. The Plateau and Mountains region are with the lowest population density.
According to the 2019 census the number of women is greater than the number of men - for all ages is 94.9 males for every 100 females.
The sex ratio differs from province to province. In 2019, there were 8 provinces where there were more men than women.Compared to 2008, the population density in the Central Plain and the Tonle Sap regions increased significantly. In the other regions, the population density has increased slightly and the Plateau and Mountains region remained the lowest in terms of population density.Cambodia’s population is slowly ageing. The age pyramid shows the usual pattern of gradually decreasing population percentages with increasing age. In the last years the relative proportion of children declines. There is a lower proportion of children in the age group 0-4 relative to the age group 5-9 due to the fact that health has been improving and fertility and mortality have been declining over time.There is a noticeable contraction of population in the age group 40-44 years. The conspicuous decline in the proportion of population in the age group 30-34 years in 2008 and eleven years later in the age group 40-44 years may be attributed to the combined effect of low fertility and high child mortality during the Khmer Rouge period (1975-79).Fertility has declined a little since 2008. In average in Cambodia a woman had 2.5 in 2019. In general, fertility in Cambodia is lower in urban than in rural areas but the difference is very smallThe largest of the ethnic groups in Cambodia are the Khmer, who comprise approximately 90% of the total population and primarily inhabit the lowland Mekong sub region and the central plains.The Khmer historically have lived near the lower Mekong River in a contiguous arc that runs from the southern Khorat Plateau where modern-day Thailand, Laos and Cambodia meet in the northeast, stretching southwest through the lands surrounding Tonle Sap lake to the Cardamom Mountains, then continues back southeast to the mouth of the Mekong River in southeastern Vietnam. Ethnic groups in Cambodia other than the politically and socially dominant Khmer are classified as either "indigenous ethnic minorities" or "non-indigenous ethnic minorities". The indigenous ethnic minorities, more commonly collectively referred to as the Khmer Loeu ("upland Khmer"), constitute the majority in the remote mountainous provinces of Ratanakiri, Mondulkiri and Stung Treng and are present in substantial numbers in Kratie Province. The predominant religion in Cambodia is Buddhism (97%), followed by Muslim (2%) and other religions (0.8%).Khmer is the predominant mother tongue in the country (95.8%). Ethnic minority languages constituted 2.9%. 1.3% were people with a foreign language as mother tongue. 22 languages other than Khmer are spoken in Cambodia, most of which are also Austroasiatic languages