Parry

Targets of melee attacks can have a chance to parry each incoming frontal attack. A successful parry nullifies the attack (for the attacker it is like a miss) and reduces the swing timer of the defender. It is a passive ability which does not require any action to be used. Most melee classes get this ability — warriors at level 6, hunters and paladins at level 8 and rogues at level 12. Shamans have a 20 point Enhancement talent called Spirit Weapons which grants them the parry ability in addition to threat reduction on their attacks and spells.

Functionality

 * If a character or mob has a parry chance, it's always active and can take effect on every incoming frontal melee attack.
 * If applicable, the parry chance is added to the attackers attack table.
 * The only stat which increases the chance to parry is defense (0.04% per point).
 * Some classes can increase their chance to parry with talents (either direct or via defense).
 * Similarly items can directly increase the chance to parry or indirectly via defense.
 * Attacks from the rear cannot be parried. Mobs sometimes do so anyways - this can be due to a bug or because they turn very quickly in place.
 * When a warrior's special attack is parried, he does not lose the rage cost associated with that spell (even though the spell fails). Miss, dodge, or block events will consume the full rage cost when the spell fails.

Swing timer
After a successful parry, the defender's "swing timer" is reduced by 40% of your weapon speed (or even reset), unless this would result in a reduction to less than 20% of your swing time remaining. This results in an average of .24 extra swings per parry; thus parry favors slower, higher damage weapons. Note that with a slow weapon and fast incoming attacks, it is possible to gain multiple speed reductions. While tanks like to profit from this effect, melee damage dealers should attack from behind whenever possible, both to increase their own damage and to avoid causing unneeded damage to the tank through the enemy's parries.

Your swing timer is always running any time you are able to parry (you can't parry during spellcasting, stun, etc.), and since it resets on a predictable basis, there are several addons that can display it. Using such a timer and monitoring incoming events, you will note that parry affects only your current swing, and only the time remaining when the parry occurs. In order to display parry events accurately, you must know the correct swing speed (including haste effects) and the elapsed swing time.

Note: When a mob parries an attack, its swing timer also advances in the same manner. This does not apply to certain raid bosses for which parry-haste functionality has been explicitly turned off by the game designers.

Chance to parry
Your chance to parry is based on the formula: % = 5% base chance + contribution from parry rating + contribution from talents + ((Defense skill - attacker's weapon skill) * 0.04)

Your Parry Rating is provided by items that have a Parry Rating bonus.

In combat, you will notice that your Parry percentage match what you see on your tooltips. Miss chance and Critical chance are unmodified by Parry, so you're not "wasting" Parries on misses nor are you able to Parry a Critical. This may seem odd to some folks if they are expecting a "if hit, then check if Parry, then check..." type system. WoW, like many other games, uses a combat results table-based combat scheme (where one roll determines outcome of an attack), so percentages are absolute. Your parsed Parry won't necessarily match your tooltip if you're fighting creatures higher or lower in level to you.