Re: Stopped watching One Piece so episodes can add up.
Here, to help you:
It has all the Sagas and Arcs, aswell as the correspondence between Manga Chapter and Anime Episode, for any given Arc. You can check which episode you left off and see if it's worth watching the remaining episodes of that arc, or just leave it alone for a while.
It will also help you know which manga chapter to pick up in case you want to do the transition (I strongly advise reading the Manga from the start and then just continue through, it's better than the Anime.(I do read and watch though xD))