What's the chance of a Critical Hit happening while using a move, and what would that percentage be when that Pokemon is holding a Scope Lens/Razor Claw?
Starting in Generation II, the chance of a critical hit is no longer based on the Pokémon's speed, but rather the chance is the same for each Pokémon. Similar to stats, there are temporary in-battle stages used to calculate the probability a particular move will be a critical hit. The stages are as follows:
An attacking move will always start out in stage 1. There are several ways to increase a move's stage (and therefore, chance that it will be a critical hit), as detailed in the table below.
Even though it is possible to get a stage higher than 5, it will still only have a 50% chance of landing in a critical hit. The effects of Focus Energy, the Lansat Berry, and the several versions of Dire Hit cannot stack with themselves or each other. Additionally, if the move used is Storm Throw or Frost Breath, it will always result in a critical hit. On the other hand, if the target Pokémon has the Ability Battle Armor or Shell Armor, or is under the effect of Lucky Chant, a move will never be a critical hit, no matter the stage. When a move scores a critical hit, damage is calculated based on either the modified or unmodified attack and defense stats, whichever is more advantageous to the attacker. For example, an attacker who has used Swords Dance and then makes a critical hit will get the benefit of the increased Attack, while one affected by Charm will use its normal Attack score. Another example would be if a team had Light Screen or Reflect on it and the attacker scored a critical on the team with the certain wall; the defense added by the wall would then be ignored, and the attacker would do double damage. However, it does not ignore modifiers caused by items or Abilities such as Eviolite, Defeatist, or Slow Start. Neither does a critical ignore the attack reduction caused by a burn.
-Bulbapedia
Starting in Generation II, the chance of a critical hit is no longer based on the Pokémon's speed, but rather the chance is the same for each Pokémon. Similar to stats, there are temporary in-battle stages used to calculate the probability a particular move will be a critical hit. The stages are as follows: 1: 6.25% 2: 12.5% 3: 25% 4: 33.3% 5: 50% An attacking move will always start out in stage 1. There are several ways to increase a move's stage (and therefore, chance that it will be a critical hit), as detailed in the table below. II III IV V Way +2 +2 +1 +1 Using a move with a high critical-hit ratio +1 +1 +1 +1 Holding a Scope Lens +1 +1 Holding a Razor Claw +2 +2 +2 +2 Holding a Stick (if the Pokémon is Farfetch'd) +2 +2 +2 +2 Holding a Lucky Punch (if the Pokémon is Chansey) +1 +1 Having Super Luck as its Ability +1 +1 +2 +2 Being under the effect of Focus Energy +1 +1 +1 Consuming a Lansat Berry (or being passed its effect) +1 +1 +1 +1 Using Dire Hit on it +2 Using Dire Hit 2 on it (Wonder Launcher only) +3 Using Dire Hit 3 on it (Wonder Launcher only) Even though it is possible to get a stage higher than 5, it will still only have a 50% chance of landing in a critical hit. The effects of Focus Energy, the Lansat Berry, and the several versions of Dire Hit cannot stack with themselves or each other. Additionally, if the move used is Storm Throw or Frost Breath, it will always result in a critical hit. On the other hand, if the target Pokémon has the Ability Battle Armor or Shell Armor, or is under the effect of Lucky Chant, a move will never be a critical hit, no matter the stage. When a move scores a critical hit, damage is calculated based on either the modified or unmodified attack and defense stats, whichever is more advantageous to the attacker. For example, an attacker who has used Swords Dance and then makes a critical hit will get the benefit of the increased Attack, while one affected by Charm will use its normal Attack score. Another example would be if a team had Light Screen or Reflect on it and the attacker scored a critical on the team with the certain wall; the defense added by the wall would then be ignored, and the attacker would do double damage. However, it does not ignore modifiers caused by items or Abilities such as Eviolite, Defeatist, or Slow Start. Neither does a critical ignore the attack reduction caused by a burn.
Bulbapedia
So, starts at 6.25%, then increases.
http://www.serebii.net/games/criticalhits.shtml
