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There are three primary methods, all of which measure the age of rock by comparing the ratios of isotopes they contain (radiometric dating). The oldest rocks so far discovered on Earth are around 3.9 billion years old, and some include minerals that are even older (around 4.2 billion years old). This puts a lower limit on the age of the Earth, but not an upper one, since none of the original surface of the Earth still exists, thanks to its molten nature and subsequent processes of erosion and crustal recycling.

More direct means of calculating the age of the Earth are based on the assumption that all the rocky material in the solar system formed at the same time, and from the same pool of material (as a giant disc of dust and gas coalesced into solid matter). Different isotopes of uranium decay into different isotopes of lead and, by measuring the ratios of these isotopes in Earth rock and meteorites, it is possible to plot a graph of the values and calculate from it the amount of time that has elapsed since the original pool of matter became separated into discrete objects. This method, known as lead isochron dating, gives a figure of around 4.55 billion years, as does the other direct method, which is radiometric dating of meteorites (asteroids that have fallen to Earth). Unlike the Earth, asteroids do not undergo geological processes and therefore may date back to the formation of the Solar System. Around 100 meteorites have been dated, and the ages obtained are almost all around 4.5 billion years.