For the first time, scientists have estimated how many Tyrannosaurus rex, the so-called king of dinosaurs, once roamed the Earth.

Why it matters: The number is staggering: 2.5 billion Tyrannosaurus rex lived and died during the roughly 2.4 million years the species survived on the planet, according to a new study set to be published in the journal Science on Friday.

The study may help contextualize the fossil record and the rarity of finding certain fossilized prehistoric organisms, according to lead researcher Charles Marshall, director of the University of California Museum of Paleontology.

"I mean, to me, it's just amazing we could have come up with a number," Marshall told Axios. "Some people have asked me, 'How does your number compare to other numbers of the total that have ever lived?' The answer is it doesn't because there weren't any."

How it works: The team of researchers couldn't use the limited fossil record to estimate the species' population, so they instead used Damuth’s Law, which describes a relationship between population density and body mass.

The relationship, used in population ecology, generally states that species with larger body sizes tend to have lower population densities.

The researchers then computed the average body mass of a T. rex, settling on a mean of 5,200 kilograms (roughly 11,460 pounds).

Using the body mass, the team calculated that the species had a population density of around one individual per 40 square miles.

By the numbers: With this information and an estimated geographic area that the species occupied, the researchers were able to approximate that about 20,000 T. rex were alive at any given time that the species lived on the planet.

To find the total number of T. rex that walked the Earth, the team multiplied the species' standing population by the number of generations it spanned (around 127,000), which they determined by dividing how long the species survived by its estimated generation time of 19 years.

The researchers noted that their estimated population density for the species would translate to roughly 3,800 T. rex in an area the size of California and just two in an area the size of Washington, D.C.

Yes, but: Marshall said the precision of the analysis was "low" and this was primarily due to uncertainty about the accuracy of the relationship between living animals' body mass and their population density, rather than the paleontological data the team used.

James Clark, a professor of biology at George Washington University who did not participate in the study, said the research didn't reach a definitive conclusion but showed the difficulties of estimating the lives of extinct animals.

"It's an exercise in what you can and can't tell," Clark said. "It gives you the chance to say, 'Wow, there really were a lot of these things, and we're not getting a lot of them captured in the fossil record.'"