Each species of Pokémon has a catch rate that applies to all its members. Higher catch rates mean that the Pokémon is easier to catch, up to a maximum of 255. When a 
Poké Ball is thrown at a wild Pokémon, the game uses that Pokémon's catch rate in a formula to determine the chances of catching that Pokémon. The formula also takes into account the following factors:
- The health of the Pokémon (relative to its full health)
- The type of Poké Ball
- Any status condition of the wild Pokémon
- Any active Pass Powers (in Generation V) or O-Power (in Generation VI)
The rarity of the Pokémon (i.e. how often it appears in the wild) is independent of its catch rate after it is encountered. Even rare Pokémon may have high catch rates, making them relatively easy to catch in battle, such as Nosepass, who has a catch rate of 255.
Poké Balls that guarantee capture (the 
Master Ball and 
Park Ball, and any Poké Ball used on Kalos Route 2XY, Route 101ORAS, or in the Entree Forest) bypass the catching formula entirely.
Critical capture[]
Since Generation V, when a Poké Ball is thrown, a special type of capture that is much more likely to catch a Pokémon, called a critical capture, may occur. When a critical capture occurs, the Poké Ball will make a high-pitched whistling sound as it is thrown, then pause in mid-air, and shudder before it drops down to the ground, shaking only once on the ground before the Pokémon escapes or is caught.
The probability of a critical capture occurring depends of the number of Pokémon that have been caught in the Pokédex. First, compute the value of a (the modified catch rate) as shown in the appropriate section above. Then, multiply a by the appropriate multiplier from the table below.
| Species Caught | Multiplier |
|---|---|
| >600 | 2.5 |
| 451-600 | 2 |
| 301-450 | 1.5 |
| 151-300 | 1 |
| 31-150 | 0.5 |
| ≤30 | 0 |
Finally, divide the result by 6 and round down to the nearest integer. Call this final result c. A random number between 0 and 255 is generated; if it is less than c, a critical capture will occur. Overall, this means the critical capture chance ranges from 0 (if 30 or fewer species have been caught) to about 41.67% of the modified capture rate a (if over 600 species have been caught).
Critical captures only make one shake check instead of three or four, which means they are considerably more likely to successfully capture the Pokémon. In Generation V, the success chance for a critical capture is the cube root of the success chance for a regular capture; thus, for example, if the regular success chance is 5%, the chance of success with a critical capture will be about 37%, while if the regular success chance is 50%, the chance of success with a critical capture will be just over 79%. In Generation VI, meanwhile, the success chance for a critical capture is instead the fourth root of the success chance for a regular capture: if the regular success chance is 5%, the critical success chance will be around 47%, while if the regular success chance is 50%, the critical success chance will be around 84%. Given the rarity of critical captures, however, the Pokémon is always more likely overall to be caught with a regular capture than a critical one.