How Did Scientists Calculate the Age of Earth?

How Did Scientists Calculate the Age of Earth?

The examination and analysis of rocks on Earth’s surface, and of extraterrestrial rocks, have enabled scientists to determine the approximate age of the planet.


3 - 12


Earth Science, Geology, Physics

NGS Resource Carousel Loading Logo
Loading ...
Selected text level

How old is Earth? Scientists know our planet is about 4.5-billion years old. But how did they find out Earth's age?

The First Estimate Was Far Off

In the 1800s, scientists tried to find the age of the planet. One of these scientists was Lord Kelvin. He studied physics and math. Lord Kelvin knew that Earth was very hot when it first formed. He thought about how long it would take to cool down. He said that Earth was between 20-million years old and 400-million years old.

Lord Kelvin's estimate was wrong. That is too young for Earth. But the way he tried to answer the question was right. He used the scientific method.

From Millions to Billions of Years Old

Scientists also tried to compare layers of rock to get the age of the planet. This is called relative dating. They compare one layer to another layer. This can show which layers are older or younger. But it cannot give an actual age. However, it did tell the scientists that Earth was billions of years old.

Advances in science continued. In the early 1900s, scientists found elements that break down into other elements. They are called radioactive elements. The time it takes for half of the element to break down is called its half-life.

Scientists use a certain process to tell the age of a rock. They find radioactive elements in the rocks. They look at how much of the element is left. They look at how much of the new element has formed. They use the half-life to tell how much time has passed. This is called radiometric dating.

Looking Beyond Earth

One problem with radiometric dating is the the rock cycle. During the rock cycle, rocks change forms. Old rocks are destroyed as they slide back into Earth. New rocks form when melted rock cools and becomes solid.

The first rocks that formed on Earth are no longer here. This makes it hard to find Earth's age. The oldest rocks that have been found are about 3.8-billion years old. Some tiny minerals are 4.2-billion years old.

Scientists have looked at rocks from the moon and from meteorites. These rocks have not been changed by the rock cycle. Scientists used radiometric dating on those rocks. Scientists have gathered a lot of data from the rocks on Earth and beyond. They have used the data to estimate that Earth is 4.5 billion years old.

Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

Tyson Brown, National Geographic Society
National Geographic Society
Production Managers
Gina Borgia, National Geographic Society
Jeanna Sullivan, National Geographic Society
Program Specialists
Sarah Appleton, National Geographic Society, National Geographic Society
Margot Willis, National Geographic Society
Clint Parks
Roza Kavak
Last Updated

October 19, 2023

For information on user permissions, please read our Terms of Service. If you have questions about how to cite anything on our website in your project or classroom presentation, please contact your teacher. They will best know the preferred format. When you reach out to them, you will need the page title, URL, and the date you accessed the resource.


If a media asset is downloadable, a download button appears in the corner of the media viewer. If no button appears, you cannot download or save the media.


Text on this page is printable and can be used according to our Terms of Service.


Any interactives on this page can only be played while you are visiting our website. You cannot download interactives.

Related Resources