a drop in the ocean

Từ điển Oxford Advanced Learner 8th

a ˌdrop in the ˈocean idiom
(BrE) (NAmE a ˌdrop in the ˈbucket)an amount of sth that is too small or unimportant to make any real difference to a situation
•The amount of money raised was a drop in the ocean compared to what we needed.
Main entry: ↑drop_idiom