How much water does Macbeth say he needs to clean up his mess?

Answer 1

He says, "Will all great Neptune's ocean wash this blood clean from my hand? No, this my hand will rather the multitudinous seas incarnadine, making the green one red." Basically, if he tried to use all the water in the ocean, he would make the ocean red with the blood and still be bloody himself. There are two figures of speech at work here. One is called hyperbole--extreme exaggeration. Of course all the water in the ocean will be more than enough. The other is metaphor. The blood is a metaphor for Macbeth's guilt, which cannot be washed away no matter how hard he tries. It's not about the real blood--that is easy enough to wash away.

Answer 2

Macbeth says he needs "a great ocean" of water to clean his hands and wash away the guilt of King Duncan's murder, emphasizing the depth of his remorse and the magnitude of his crime.