Whether a move scores a critical hit is determined by comparing a 1-byte random number (0 to 255) against a 1-byte threshold value (also 0 to 255); if the random number is less than the threshold, the Pokémon scores a critical hit. If the threshold value is T, then this means the probability (P) of scoring a critical hit is P = T / 256. (Note that this means it is impossible for a critical hit to be guaranteed; there will always be a 1/256 chance that a critical hit will not be scored.)

The value of T is based on a Pokémon's base Speed. For a normal move, T is half the base Speed:

    T = BaseSpeed / 2 or P = BaseSpeed / 512

Due to a bug, Focus Energy and Dire Hit lower a Pokémon's chance of scoring a critical hit instead of raising it, dividing T by 4:

    TFE = BaseSpeed / 8 or PFE = BaseSpeed / 2048

If the move being used has a high critical-hit ratio (Crabhammer, Karate Chop, Razor Leaf, or Slash), T is multiplied by 8:

    THighCHR = BaseSpeed * 4 or PHighCHR = BaseSpeed / 64

Both effects may also be active at the same time. This would result in:

    TFE+HighCHR = BaseSpeed or PFE+HighCHR = BaseSpeed / 256

At all times, the maximum value for T is 255 (which equally means the maximum value for P is 255/256). All operations that factor into T are also integer operations (meaning divisions are rounded down to the nearest whole number). (Note that the division in P = T / 256 is not rounded.) Due to the process the game uses to compute T, BaseSpeed is also effectively rounded down to the nearest even number.