Generally speaking, when there's no filler, (and no endless "characters dramatically staring at the other characters") 2-3 manga chapters fit into a 20 minute episode just fine. 2 if its a lot of talking, and as much as 4 chapters if its all action. (Bleach was able to cleanly fit in SEVEN chapters in a single episode but Bleach had no content in its chapters)
A solid 1 hour episode can cover 7-10 chapters with very little loss, though you obviously have to make adjustments for cliffhanger pacing, character count, how often action scenes happen, and the medium and whatnot.
Specific examples
[hide]The Drum arc, which was 25 chapters long, and 14 anime episodes, fit quite nicely into a 113 minute movie (under 2 hours) with no significant losses, even though it was nearly 5 hours in its original anime form. And in fact had some additions. That's the good way to do it.
The Alabasta arc, which was 63 chapters, fit into 39 episodes and was great. A few of those like the bonus adventures with Ace were pure filler, but aside from the entirely bonus episodes, it was pretty much the last time the series didn't have filler padding in the anime. They turned that into a 90 minute movie, and that did not work AT ALL. That was too much cutting, and you had to have already seen the long version to make sense of it. That's the bad way to do it.
It's really, REALLY easy to cut and trim and make these things work and pace well if you approach it the right way. They should be able to do roughly 100 chapters per 10 episode block, since the show will be an hour long. (Though obviously its not an exact science, Alabasta didn't end on chapter 200, it ended on 217, so you adjust accordingly.) It sounds crazy and like a lot, until you realize that an average chapter really is only about 7 minutes of screentime unless its a talky one. Of course the final manga is probably going to be 1200-1300 chapters or so, so that's too long at 13 seasons and they'll have to cut even more still, but thats when you do things like dramatically cut down the length of Water 7 or take out Skypeia.
The Baratie arc in particular, was 27 chapters. So that should parse to about three episodes. But if you cut back on stuff like Pearl and don't make the Krieg fight quite as extensive, it should be doable in 2. Syrup village arc which was 20 chapters, should similarly be doable in 2 episodes, but with trimming cut down to 1 and a half. (and that half can then go to other content.)
A normal full length feature film is 90 minutes and that has to introduce all the characters and world from scratch and resolve everything. A series can do as much storytelling in that time… if it wants to.
So.
Episode 1-Prequel setup with Shanks, Alvida
Episode 2- Zoro and Axe Hand, farewell to Coby
Episode 3-Nami and Buggy
Episode 4-Syrup village
Episode 5-Syrup village and Baratie
Episode 6-Baratie and Mihawk
Episode 7-Finish Baratie, head to Arlong Park
Episode 8-Arlong
Episode 9-Arlong
Episode 10- Arlong and Loguetown
And that's just if you're doing a basically straight clean cut 10 chapters per episode ratio. Possibly move some things around to get Nami in slightly sooner, or avoid having beating up a woman be the first fight, push the opening flashback to later, give longer flashbacks to Usopp than he originally had, get Buggy's cover story in as C plot, etc... whatever the format demands that isn't strict fidelity but it fits pretty cleanly.
Skip Gaimon and his land of wacky animals. That's just a real easy cut that removes a double length chapter out of the equation.
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