SAN JOSE, Calif.—China and Indonesia are likely the top sources of plastic reaching the oceans, accounting for more than a third of the plastic bottles, bags and other detritus washed out to sea, an international research team of environmental scientists reported Thursday.

Marine biologists and ocean activists have grown alarmed about the seaborne plastic that fouls shorelines and clogs currents from the Arctic to the South Pacific. But the actual amount and source of it hasn’t been known because consumer habits and pollution-control practices vary so widely world-wide.

In a new accounting of global garbage, researchers in the U.S. and Australia led by Jenna Jambeck, an environmental engineer at the University of Georgia, calculated the share that each of 192 countries could have contributed to plastic waste in the oceans. Their study is based on consumer data and waste-management information covering coastal populations around the world. The U.S. ranked 20th by the researchers’ estimates, deemed responsible for just under 1% of the mismanaged plastic waste.

They reported their calculations in the journal Science on Thursday and presented them at the annual meeting of the American Association for the Advancement of Science here.

All told, Dr. Jambeck and her colleagues calculated that people living within 50 kilometers (30 miles)of the coast in these countries generated a total of 275 million metric tons of plastic waste in 2010. A small but significant fraction of it—between 4.8 million and 12.7 million tons of discarded bottles, bags, straws, packaging and other items—ended up in the world’s oceans.