A study, led by the UK's National Oceanography Centre, scooped through layers of the upper 200m (650ft) of the ocean during a research expedition through the middle of the Atlantic.
Such an amount of plastic - 21 million tonnes - would be enough to fully load almost 1,000 container ships.
The findings are published in the journal Nature Communications.
Dr Katsia Pabortsava, from the National Oceanography Centre, who led the study, said by measuring the mass of very small plastic particles in the top 5% of the ocean, she and her colleagues could estimate "the load of plastic in the entire Atlantic" which is "much larger" than the previous figure.
"Previously, we haven't been able to balance the amount of plastic we found in the ocean with the amount we thought we had put in," she said.
"That's because we weren't measuring the very smallest particles."
On their expedition - from the UK to the Falkland Islands - she and her colleagues detected up to 7,000 particles per cubic metre of seawater.
They analysed their samples for the three most commonly used, and most commonly discarded, polymers - polyethylene, polypropylene and polystyrene - all often used in packaging.
The findings, the team hopes, will help future efforts to measure the ecological and environmental damage that might be caused by these plastic fragments, by providing a more "robust measure" of its accumulation in remote parts of the ocean.