A simple technique for counting marine debris at sea reveals steep litter gradients between the Straits of Malacca and the Bay of Bengal

► A size and distance-based technique was used to count floating debris. ► Effective detection distance varied more with item size than type of litter. ► Litter densities were much greater in the Straits of Malacca than the Bay of Bengal. ► Within the Bay of Bengal, litter density increased towards the Ganges Delta. ► Future at-sea surveys need to standardise data collection protocols.