Some 8.7 million different species exist on Earth, though a tiny amount of those have actually been discovered and catalogued, researchers said Tuesday.
The count, described by the open access journal PLoS Biology in which it is presented as "the most precise calculation ever offered," replaces previous estimates that swung between three million and 100 million.
About 1.25 million species have been discovered and classified since Swedish scientist Carl Linnaeus came up in the mid-1700s with the taxonomy system still used today.
The 8.7 million figure is a projection based on a mathematical analysis of currently known species.
Around 86 percent of land species and 91 percent of creatures in the ocean have yet to be discovered, said the findings by scientists at Dalhousie University in Canada and the University of Hawaii.
"The question of how many species exist has intrigued scientists for centuries and the answer, coupled with research by others into species' distribution and abundance, is particularly important now because a host of human activities and influences are accelerating the rate of extinctions," said lead author Camilo Mora of the University of Hawaii.
"Many species may vanish before we even know of their existence, of their unique niche and function in ecosystems, and of their potential contribution to improved human well-being."
The study estimated that there are 7.77 million species of animals, of which 953,434 have been described and catalogued, and 298,000 species of plants, with 215,644 of them described and catalogued so far.
Researchers also said there are likely 611,000 species of fungi, such as mold and mushrooms, of which 43,271 are known to science.
Some 36,400 species of protozoa, or single-cell organisms such as amoebas and 27,500 species of chromista, such as brown algae and water molds, were also included in the projected count.
"Humanity has committed itself to saving species from extinction, but until now we have had little real idea of even how many there are," said co-author Boris Worm of Dalhousie University.
The Red List issued by the International Union for Conservation of Nature monitors 59,508 species, of which 19,625 are classified as threatened.