The population clock of the U.S. Census Bureau indicates the number of 300 million in Washington D.C., Oct. 17, 2006. The population of the US officially hit 300 million at 7:46 a.m. (1146 GMT) on Tuesday. (Xinhua Photo)

The U.S. population reached the historic milestone of 300 million on Tuesday morning, according to the U.S. Census Bureau's Population Clock.

The "Population Clock" recorded the milestone at 7:46 a.m. EDT (1146 GMT) -- a timing based on calculations that factor birth and death rates and migration.

According to the most recent national population estimates, the United States registers one birth every seven seconds and one death every 13 seconds, while net international migration is expected to add one person every 31 seconds. The result is an increase in the total population of one person every 11 seconds.

The Census Bureau said that with that formula in mind, it is near impossible to determine whether the 300th million resident will be a newborn, an immigrant coming across the border or someone flying into the United States.

This 300-million-milestone comes just 39 years after the 200 million mark was reached on Nov. 20, 1967. The U.S. population hit 100 million 139 years after the country was officially established. It then took 52 years to reach the 200-million milestone in 1967, and only 39 years to hit 300 million on Tuesday.

As the world's third most populous country following China and India, the United States now has five percent of the world's population.

The U.S. adds about 2.8 million people a year, at a growth rate of less than 1 percent. About 40 percent of the growth comes from immigration, which spurs continued growth and diversity. The rest comes from births outnumbering deaths.

Immigration, especially form Latin America and Asia, drives much of the population growth and promises to reshape the United States as a more diverse nation and one where the average age will increase more slowly than in most other industrialized nations. According to the Census Bureau, 14 percent of the current U.S. population is Hispanic, compared to four percent in 1968, and it is projected that 25 percent of the population will be Hispanic in 2050.

The Immigration and Naturalization Services Act of 1965 abolished national-origin quotas fixed in the 1920s and opened the nation's shores to new immigrants. Today, the United States accepts more legal immigrants as permanent residents than the rest of the world combined.

However, illegal immigration nowadays also is a hot-button issue being played up in upcoming legislative elections in November, so no special celebrations were planned for Tuesday's milestone. The federal government chose to let the big number pass quietly.

Editor: Yan