Procs per minute (PPM) describes how a given proc mechanic functions. It is often used to contrast a game mechanic with one that uses "procs per hit" (PPH).
In World of Warcraft
PPM vs. PPH
In World of Warcraft, PPH is the percent chance of a proc occurring each hit. PPH benefits greatly from faster hits.
PPM scales to PPH according to weapon speed. Here is the formula:
- PPH = (Weapon speed) / (60 / PPM)
which is the same as:
- PPH = Weapon speed * PPM / 60
This gives the percentage chance that the proc will be triggered each hit. Note that PPM is an average, not a guarantee.
PPM & Haste
The speed used for calculating the PPH (Proc per Hit) chance is the basic weapon speed, not the characters Haste-affected hit speed. This means the probability of a proc per hit is not affected by Haste. But since Haste increases the number of hits per minute, it also increases the total number of procs.
The PPM rate of a given effect is based on auto-attacks ("white damage" attacks) made with that weapon. However, an effect on the main-hand weapon can proc more often than its PPM rating, as instant attacks (Sinister Strike, Overpower, etc.) are all assumed to be made with the main hand, and therefore can trigger the proc.
In addition, because instant attacks use the calculated PPH rate based on the weapon speed, yet are not themselves restricted by the weapon speed, a slower weapon will result in more total procs.
For example, a Warrior has Crusader (PPM of 1) on a 2.0-speed and a 2.5-speed weapon. These calculations assume ample Rage and no misses.
- Weapon 1 (speed 2.0): 30 white hits per minute, so PPH is 1/30.
- Weapon 2 (speed 2.5): 24 white hits per minute, so PPH is 1/24.
- Weapon 1 (PPH 1/30): 50 hits (20 instant + 30 white), so actual PPM is 50/30 = 1.67.
- Weapon 2 (PPH 1/24): 44 hits (20 instant + 24 white), so actual PPM is 44/24 = 1.83.
Therefore, the slower the weapon you wield in the main hand, the more often any PPM-rated ability on it will proc, assuming you use any special attacks that can trigger the proc.