You can calculate the eligible gyms for ex-raid in an specific area. Let's review what we know about the ex-raids:
- EX Raids are created based on Level 12 S2 cells. Each cell can only host one EX raid per “invitation cycle”
- EX Raids can only happen at gyms that are either sponsored or placed in a park related OpenStreetMap tag
- Gyms that hosted a raid before can and will host an EX Raid again, there is no diminishing returns
- Ex Raids can be triggered and there could be some randomness to it
- An EX Raid is triggered only if the gym reaches a minimum amount of raid activity since the last invitation cycle
EX Raids can only occur once inside each S2 cell that passes over your local area. S2 cells are a simple mathematical way to describe a sphere’s geometry. S2 cells come in different sizes, often denoted as levels, and they are often used by developers to create zones on spherical objects, like the planet Earth.
Niantic uses them to control weather, create gyms, define Pokémon catch / hatch locations, etc… For this step we’ll be using the Level 12 S2 cell geometry.
A company named Sidewalk Labs created an incredible tool that draws Level 12 cells over a Google Map. Head over to their Region Converter and do the following:
- Navigate the map to your local area (our example, Split)
Set the parameters in the top left box to: min level 12, max level 12, max cells 10
- Use one of the drawing tools (square, circle, pin) to select a map area where you want to visualize Level 12 S2 cells
Now that you can see your local S2 cells, remember that an EX Raid can only occur once per cell per cycle. Raiding 100 gyms inside one cell doesn’t benefit you as much as raiding a dozen of eligible gyms in a higher number of cells.
You can see more information in this amazing guide: https://pokemongohub.net/gym-raid-update/comprehensive-guide-trigger-ex-raids/
TL; DR: In order for a gym to be eligible for a EX-Raid:
- It must be in a park (or a featured gym)
- It should reach a minimum raid activity
- Only one gym will be eligible in each area
- The players will be randomly selected to participate between the ones that did at least one raid in that gym