It’s actually impossible to watch chronologically.
Summary:
- The Toaru series can be hard to get into.
- You just watch the anime in the release order.
- The chronological order has multiple arcs happening at the same time, and is not recommended for first-timers.
The Toaru series has been praised for its worldbuilding and complex power system. With it influencing many light novels that came after it, it is definitely iconic and worth watching.
That said, its sheer length might scare people, and it can be hard to figure out the watch order. However, the watch order is actually quite easy — unless you want to watch the series chronologically.
Watching Order
The Toaru series, in anime, consists of A Certain Magical Index (Toaru Majutsu no Index), A Certain Scientific Railgun (Toaru Kagaku no Railgun), and A Certain Scientific Accelerator (Toaru Kagaku no Accelerator). The former two have three seasons, two cours each, while the Accelerator anime only has one 1 cour season.
Watching the series in its release order is how you’re supposed to experience it. This would be: Index S1, Railgun S1, Railgun OVA, Index S2, Index Movie, Railgun S2, Index S3, Accelerator, Railgun S3.
Chronological Order
The chronological order of the anime is a bit more complicated. With Railgun and Accelerator being spin-offs to Index, they take place parallel to its events. With how many characters there are in the series, it’s understandable that all of them will be involved in their own stories — which will often overlap with each other.
It can be very hard to pinpoint the chronological order of watching — some events happen at the same time, and some are shown twice from the perspectives of different protagonists. The most notable of it is the Sisters arc, which is the third arc of Index S1 anime — and also the first arc of Railgun S2.
The first arc of Railgun actually happens before Index. Chronologically, you watch the first 14 episodes of Railgun S1, then Railgun OVA. Then, the main plot of Index starts — you watch the first 6 episodes of Index S1, then Railgun episodes 15-16, Index episodes 7-9, and the rest of Railgun S1 (but the last arc is filler).
The Sisters arc that comes next is covered in both Index and Railgun. The first 9 episodes of Railgun S2 happen before the part that is in both series, and then, Index episodes 10-14 and Railgun S2 episodes 10-16 happen at the same time. Fans prefer Railgun’s treatment of the arc, but both are necessary for understanding it.
Similarly, Railgun S2 episodes 17-24 happen during Index S1 episodes 15-17 — but the arcs are different, and what’s happening in Railgun is filler here. After that, you’d have to watch Index out of order — episodes 18-19 of S1, then episode 1 of S2, then episode 20 of S1 — these form one arc together. Then you finish Index S1 and watch the entirety of Accelerator.
After that, you watch Index S2 up to episode 7, then the movie (although it’s not entirely canon). The next arc is again covered in both Index and Railgun, but the stories are mostly separate — covered by Index S2 episodes 8-13 and Railgun S3 episodes 1-15.
Past that point, it’s straightforward. Index S2 14-16, Railgun S3 16-19, Index S2 17-24, Railgun S3 17-20, Index S3 1-6, Railgun S3 21-25, and the remainder of Index S3. Keep in mind that the chronological watch order is not recommended for a first-timer (and also that Index S3 is often considered very rushed and experienced better in the LN).