It is no secret that the world’s oceans are swimming with plastic debris — the first floating masses of trash were discovered in the 1990s. But researchers are starting to get a better sense of the size and scope of the problem.

A study published Wednesday in the journal PLOS One estimated that 5.25 trillion pieces of plastic, large and small, weighing 269,000 tons, could be found throughout the world’s oceans, even in the most remote reaches.

The ships conducting the research traveled the seas collecting small bits of plastic with nets and estimated worldwide figures from their samples using computer models. The largest source of plastic by weight comes from discarded fishing nets and buoys, said Marcus Eriksen, the leader of the effort and co-founder of the 5 Gyres Institute, a nonprofit group that combines scientific research with antipollution activism.

Dr. Eriksen suggested that an international program that paid fishing vessels for reclaimed nets could help address that issue. But that would do nothing to solve the problem of bottles, toothbrushes, bags, toys and other debris that float across the seas and gather at “gyres” where currents converge. The pieces of garbage collide against one another because of the currents and wave action, and sunlight makes them brittle, turning these floating junkyards into “shredders,” he said, producing smaller and smaller bits of plastic that spread far and wide.