China began counting noses today for the first time in nearly two decades, using everything from the abacus to advanced computers to tally the world's largest population.

No less than 5 million census workers--more than the population of many small countries--will spend the next 10 days collecting data from the cramped tenements of Shanghai to the Mongolian yurts, or tents, of China's mountainous far west.

Canvassing this vast land where more than 40,000 babies are born daily surely is history's greatest head count. Nearly three years will be needed to digest the billions of figures that census experts must juggle to reach conclusive results.

Although China is believed to have 1 billion people--a population total that is nearly five times that of the United States--there has been no census taken since 1964, when 723 million were recorded. For this census, a more detailed questionnaire will help to form an expanded socioeconomic profile of one-quarter of mankind.

Communist leaders, convinced that a sound statistical base is essential for economic planning, have promoted the census with the intensity of past political campaigns. For weeks, the propaganda machine has churned out speeches, posters, matchbooks, fans and bus tickets calling for cooperation.

By 7 a.m. today, bands of well-prepared census takers had fanned out to factories and rice fields, carrying the green-and-white form that asks for information on 19 items ranging from occupation to number of child-bearing women in a family.

At Marco Polo commune outside Peking, census enumerators squatted in vegetable plots and canvassed door-to-door, filling in responses for the normally reticent and often illiterate peasants.

"I was too poor to go to school when I was young," explained Wen Yougui, 56, who signed her questionnaire with a name chop, or seal, dipped in red ink because she never learned how to write.

In crowded south Peking, Ma Zhanyu, 63, thumbed through the pocket-sized household registration book that lists the vital statistics of his family as he responded to questions at a makeshift census office in Tianqiao district.

"Has there been any increase or decrease in your household this year?" asked the enumerator, careful to avoid using the unlucky word "death."

"Fortunately, there have been no changes," replied the old man.

After initial survey work ends July 10, census takers will compute their responses with abacuses and hand them up to the next level of a five-tiered hierarchy that ends with a giant IBM computer in Peking.

Along the way, the information will be calculated and recalculated, analyzed and checked for accuracy by village elders, neighborhood officials and finally smaller computers located in almost every province and major municipality.

In all, 29 U.S.-made computers will provide China with detail and precision it was unable to achieve in its previous censuses. The computers are the mostly costly item in the $200 million venture.

Even IBM technology, however, cannot guarantee truthful replies from Chinese citizens who fear reprisals if they should reveal such information as their residences--mobility is tightly controlled here--or the ages of married couples who are younger than the legal minimum.

Although every Chinese is required to register, officials acknowledge wide discrepancies. Families who do not want to lose cotton or grain rations are slow to report deaths of relatives. Those who fear fines for having too many children are slow to record births.

Census officials have emphasized that their intentions are not to snoop but to provide the government with the population statistics it needs to plan for schools, jobs, recreational facilities and transportation.

The Communist Party People's Daily warned that "One is one; two is two. Conditions must be reported exactly as they are. No hiding the truth or false reporting is allowed."

Chinese wary of their ubiquitous government worry about the uses to which the census could be put.

"Who knows when the party may want to turn the information against you?" asked an intellectual.

Little slips through the party's tight organizational net under normal circumstances and the census structure has been built to guarantee accurate reporting. Officials say they expect 100 percent compliance.

In Peking's Tianqiao district, the household committees that regulate life for every 500 families provided canvassers for the census. With their intimate involvement in the community, they are unlikely to be fooled by untrue responses.

Each enumerator is responsible for 100 households. After he tabulates the results, the pollster will invite the community's elderly residents--those best informed--to look for discrepancies.

"We will read aloud the entries and ask if there are any falsehoods," said the district census leader, Xie Ruibin. "If we find anything wrong, the enumerator will go back to the household and get accurate replies."

Once confirmed at the grass-roots level, the data will be sent to the household committee for further checking and tabulating. Then, it goes to the district office and finally to the city census bureau to be coded for the municipal computer.

Eventually, provincial and municipal results will be fed to the national census center in downtown Peking where the mother computer sits behind glass partitions in an antiseptic, temperature-controlled room.

Although the national computer will not render detailed findings until 1985, demographers should have raw population figures--those manually tabulated--by the end of October, according to officials.

China was one of the first countries to conduct a census. According to records from the Han dynasty, there were 13.5 million Chinese in 2200 B.C.