A drop in the ocean
Definition 1 of 1
Pro Tip 1/3Emphasizing Scale
Used to highlight how small an action is compared to a much larger need or situation. 
Her attempt to fix the issue was a drop in the ocean, given the company's widespread problems.
Pro Tip 2/3Positive or Negative
Can convey hopelessness or the need for greater effort; tone depends on the context. 
Volunteering once is a drop in the ocean, but if everyone does, it adds up.
Pro Tip 3/3Idiomatic Usage
Understand it's an idiom; don't take it literally but grasp the figurative meaning. 
Recycling a single bottle feels like a drop in the ocean, but it's important.