PokéBase - Pokémon Q&A
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I'm working on a project to create a simple Pokemon emerald clone, and now want to know the formula for a random encounter in tall grass. I found this:

"the formula should be:
1 in (187.5 / ( x )) per step
Let x equal the a value which determines how rare the Pokémon is. The higher the encounter rate, the more common the Pokémon is."

Does anyone know a place where I can find the rarity of each Pokemon? Preferably in JSON but anything is fine.

https://bulbapedia.bulbagarden.net/wiki/Hoenn#Cities.2C_routes.2C_and_other_locations Each page for a location in which you can encounter Pokemon has its encounter rate listed as a percent next to the Pokemon. I'll add this as a full answer, if this is what you're looking for.
Bulbapedia probably has a few errors. I think you should find a dedicated ROM hacking community on another website and ask them how they hack their Emeralds.

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There are many great websites out there, Pokémon Database included, that show you the rarity of the Pokémon found in the route, and even in certain parts out the route. Other great sites are Serebii and Bulbapedia. Most of this is correct. To extract this, simply type in the route, then it will show you items, trainers, and the Pokémon found in the route. It will most likely be separated into grass, water, fishing rods, and talk grass, depending on the route. Next to each Pokémon will be a percentage, which represents the probability that you will encounter that Pokémon in any wild battle in that terrain. For example, if a Magicarp has a 90% next to it under old rod, you have a 90% chance (0.9, 9/10) of finding that Pokémon in any wild encounter. In a perfect world, all the percentages will add to 100%, so you will plug in the percents for x for each Pokémon in each terrain type.

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Thank you, but he also gave these values:
Encounter-rate table
Encounter type Encounter rate
Very common 10
Common 8.5
Semi-rare 6.75
Rare 3.33
Very rare 1.25

Do you know how I can convert the percentages to these values?