Iranian statisticians have started scouring the country to count the population in a nationwide census that is held every 10 years, state television said.
The census, last held in 1996, will last 20 days but preliminary results are not expected until March next year, the television added.
Iran's population is thought to be around 70 million, having doubled since the Islamic revolution in 1979 when clerical leaders championed the idea of population growth.
However from the 1990s onwards the Islamic republic changed track and encouraged families to have no more than two children, as the hordes of young people entering the labour market were unable to find jobs.
The start of this census comes just days after President Mahmoud Ahmadinejad distanced himself from the "two children are enough" policy of his predecessors and said that Iran could sustain a population of 120 million.
Speaking as statisticians symbolically counted him as the first person in the census, Ahmadinejad said that the pace of development in Iran meant the count should be held every five years rather than 10.
"We have to correct the census law as the developments are very fast and our development programme laws should be in accordance with the daily needs," he said.
Mohammad Ali Madad, head of Iranian statistics centre, said that the 40-million dollar operation would be carried out by 90,000 employees.