Surpassing the 1 billion mark for the first time, China's population swelled almost by half in the last 18 years while becoming only slightly more educated and urbanized, according to census figures released today.

Officials counted 1,008,175,288 people on the Chinese mainland, which is about one-quarter of mankind.

Including the breakaway island of Taiwan and the European colonies of Hong Kong and Macao -- which Peking claims to be part of China -- the figure surges to 1,031,882,511.

One of every four Chinese is illiterate -- a staggering total of 235.8 million -- and 80 percent of the population still lives in the vast countryside, according to the statistics released today.

The final mainland headcount is a little higher than foreign estimates because of omissions in past Chinese reporting.

The secretive Communist regime previously has failed to include the number of military personnel in its overall tally, thus clouding true figures.

The last official estimate given, in December 1981, put the mainland's population at 996.2 million.

That figure excluded the nation's huge armed forces, China's Statistical Bureau acknowledged today.

This census, however, was supervised by U.N. experts and includes a figure for China's uniformed men -- 4.23 million.

Peking's pragmatic leaders commissioned the poll in July -- the first since 1964 -- to get an accurate picture of China's development needs.

Figures released today are only part of a complicated sociological survey, which will be revealed in full at the end of 1984.

Another unexpected finding in the initial results is the 1.45 percent population growth rate for last year, which is higher than previously reported.

It marks the second straight year of rising growth rates despite the government's strict birth-control program.

Chinese officials say this trend must be reversed if the nation is to achieve its goal of limiting population at 1.2 billion by the end of this century.

The first batch of census figures dramatizes the difficulty China faces in that task if the recent past is any guide.

Since 1964, China has added 313.6 million people -- almost the entire population of North America -- to an already bulging aggregation that squeezes into a geographic space that is the size of the United States.

Every year, the population increased by an average of 17.4 million people -- the size of many countries -- for an overall rise of 45.1 percent.

No less than 18 of the mainland's 21 provinces have populations exceeding Canada's and the three largest -- Sichuan, Henan and Shandong -- contain almost as many people as live in the Soviet Union.

China's cities and towns have felt the brunt of this surge, absorbing about 80 million new inhabitants. Unprepared for this explosion of humanity, they are a tangle of housing, sanitation and transportation problems today.

Overall, the number of urban dwellers has grown from 18.4 to 20.6 percent of the population.

Still, the vast majority of Chinese, 801.6 million, lives in the rural hinterlands where facilities taken for granted in modern societies -- running water, flush toilets, heat and electricity -- are almost unheard of.

The countryside that is too populous in parts to feed itself is most resistant to Peking's birth-control policies. Bound by tradition and inspired by Communist Party chairman Mao Tse-tung who advocated a big population as a national strength, they continue to want many children.

Mao's successors, however, blame the runaway population for the party's poor showing in improving life styles, and the census' findings on education help to support their argument.

Although the number of Chinese who have received some secondary education has quadrupled to 245 million since 1964, the nation still has almost as many people who cannot read or barely so.

Some progress has been made in reducing the illiteracy rate -- the figure dropped from 38.1 percent of the population in 1964 to 23.5 percent today -- but foreign demographers judge the current number high considering Communist efforts to universalize education since 1949.

At the top of the educational structure, China boasts only 4.4 million university graduates, which is regarded as low by foreign experts. Another 1.6 million have received some college training or are now attending a university.

Ethnic minorities who traditionally have received the fewest social services like education are the fastest growing members of the population, according to the statistics.

Spared from the government's sporadic birth-control campaigns, China's 55 minority groups now number 67 million, or 6.7 percent of the population.

Over the past 18 years, they have grown by 68 percent, compared to 43.8 percent for the majority Han race.