How big is Great Barrier Reef? | Scale of the Universe

The Great Barrier Reef is a vast system of coral reefs off the northeast coast of Australia. It stretches about 2,300 kilometers (1,400 miles) along the coast of Queensland. It is not one wall of coral but a chain of about 3,000 separate reefs, along with about 1,050 islands. As of 2026, it is the largest coral reef ecosystem in the world.

Size

The Great Barrier Reef Marine Park Authority, the Australian government agency that manages the reef, gives its length as 2,300 kilometers, measured along the coast. National Geographic gives the same figure. The reef spans 14 degrees of latitude, from 10 to 24 degrees south of the equator, which is about 1,550 kilometers from north to south. Its length is greater because it lies at a slant to the lines of latitude.

From the mainland coast, the reef system reaches up to 250 kilometers (155 miles) out to sea. The marine park that protects it covers 344,400 square kilometers (133,000 square miles). Its islands range from small sandy islands on the reefs, called cays, to large, rugged islands, one of which rises more than 1,100 meters (3,600 feet) above the sea.

Size comparisons

The reef is about as long as the dwarf planet Eris is wide, 2,326 kilometers, and a little shorter than the width of Pluto, 2,377 kilometers.
It is about 1.8 times as long as Texas is wide from east to west.
By area, the marine park is about half the size of Texas.
The reef's length is about two-thirds of the Moon's diameter of 3,475 kilometers.
About 55 marathons, each 42.195 kilometers (26.2 miles) long, laid end to end would reach from one end of the reef to the other.

How corals build a reef

Coral reefs are built mainly by hard corals, small animals that lay down skeletons of calcium carbonate, the mineral that makes up limestone. Layer upon layer, the skeletons form a limestone framework covered by a thin skin of living coral. Coralline algae, sponges, mollusks and other living things add to it.

Many reef-building corals have tiny algae living inside them. The algae use sunlight to make food, and they share it with the coral. When the water gets too warm, corals expel the algae and turn white, which is called bleaching. Bleaching does not always kill corals, but severe heat can kill large amounts of coral.

Protecting the reef

The agency has managed the reef since 1975, under a law passed that year, and the marine park was declared in sections between 1979 and 2001. In 1981 the reef became a World Heritage site, the first coral reef ecosystem in the world to be listed. It is home to about 1,625 species of fish and to six of the world's seven species of marine turtle.

As of 2026, the agency names climate change as the reef's biggest threat. Marine heatwaves, spells of unusually warm sea water, have triggered mass bleaching, in which corals across large parts of the reef bleach at once. It happened in 2016, 2017, 2020, 2022 and 2024, and in earlier years.