This page will outline how things like crit, hit, etc work. It’s still being modified.

Initial content shamelessly swiped from Blizzard posts on the WoW forums.

Agility
- Increases Armor Class by 2 for every point of AGI.
- Increases the chance of a critical hit with melee and ranged attacks. The amount of the increase is dependant on both class and level. For most level 60 character classes, approximately 20 points of AGI will increase your critical hit chance by approximately 1%. Rogues require 29 AGI for an additional 1% critical hit chance, and Hunters require 53 AGI for an additional 1% critical hit chance, but both of these classes also gain attack power from agility and the items available to them typically have much higher amounts of AGI.
- Increases the chance to dodge an attack. The amount increased is dependant on both class and level. For most level 60 character classes, approximately 20 points of AGI will increase your chance to dodge by approximately 1%. Rogues only require 14.5 AGI for an additional 1% dodge chance. Hunters require 26.5 AGI for an additional 1% dodge chance, but Hunters typically have a high amount of agility, as well as an Aspect spell that further increases their chance to dodge attacks.

Defense
- Increases the chance of being missed by an attack.
- Increases the chance to dodge, parry, and block(-.04% per defense above the mob’s attack skill, ie, 315 for a 63 mob).
- Decreases the chance of being affected by a critical hit (-.04% per defense above the mob’s attack skill, ie, 315 for a 63 mob).
- Decreases the chance of being affected by a “crushing blow”. Creatures that are higher level than your character can land crushing blows that deal increased melee damage. The chance of a crushing blow increases as the level difference between you and the opposing creature increases. Players never deal “crushing blows”, only creatures. When calculating the chance of a Crushing Blow the Defense rating used is limited to 5 times your level… so +defense only affects this if you do not yet have a natural 300 defense at 60.
-Each point of defense increases dodge, parry and block by 0.04%. Thus 100 defense is +4% dodge, +4% parry and +4% block, as well as the above mentioned reduction in crit/crush chance

Attack Power
- Attack power increases your base melee DPS by 1 for every 14 attack power.
- Strength increases your melee attack power by 2, except for rogues and Hunters. Rogues and Hunters gain 1 attack power per point of Strength, and 1 per point of Agility.

Crit vs Hit

The way WoW calculates crit rate is over ALL attacks. Crit rate is not based on hits only. In other words, if you have a 5% crit rate, that 5% chance includes misses.

All crit rate adjusting abilities, items, and talents add the flat % to the base % crit rate. So if I have a 5% base crit rate and then use an item or talent that increases that crit rate (let’s use Improved Backstab talent for example - +30% crit), my new crit with backstab is 35%.

Regarding how defense decreases the rate of critical strikes, each point of defense that a target has over the attacker, the attacker loses 0.04% chance to crit. So, for example, if a level 60 Rogue is attacking a level 60 Warrior who has 25 defense, the rogue’s crit rate will be decreased by 1%.

+toHit items subtract from your miss%.

So, ignoring all defensive actions (Block/Parry/Dodge/etc..) if I have 20% crit chance, 20% miss chance, and 60% hit chance and I equip an item that gives me +5% toHit and +5% crit, my stats become 25% crit, 15% miss chance, 60% hit chance.

New hit chance = (Original hit%) + (toHit modifiers) - (crit modifiers)
60% + 5% - 5% = 60%

New crit chance = (Original crit%) + (crit modifiers)
20% + 5% = 25%

New miss chance - (Original miss%) - (toHit modifiers)
20% - 5% = 15%

Spell Hit Chance

+1% spell hit chance increases the base chance for a spell to land against a target by 1%

If your target is the same level as you, a spell has a base chance to hit of 96%.

If the target is +1 level compared to you: 95%

+2 levels: 94%

+3 levels: 83% if the target is a mob, 87% if the target is a player.

+4 levels: mob: 72% player: 80%

+5 levels: mob: 61% player: 73%

Etc…

If you wear a +1% spell hit chance item, the above percentages will increase by 1. +2% gives +2.

Etc…

For binary spells only, there is an additional modifier for the resistance of the victim to your particular spell school: fire, frost, shadow, nature, arcane. That modifier is multiplied by your hit chance to get your actual chance to land. This is done with binary spells only, because they never do partial damage.

Example:

A Mage (level 60) fires a frost bolt at Yeti of Doom (level 63). The Mage is also wearing a total of +6% spell hit gear. Yeti of Doom has frost resistance such that he takes 50% from level 60 frost attacks. So, here’s the hit calculation:

0.83 (83% for +3 levels mob) + 0.06 (+6% spell hit) = 0.89

0.89*0.5 (50% damage from frost) = 0.445.

The game will roll a number between 0 and 1, and if it’s less than 0.445, the frost bolt will hit for full damage. Otherwise, a resist message will appear.

2nd Example:

After the resist, the Mage decides to fire a fireball at Yeti of Doom. The Mage still has +6% spell hit. Fireball is not a binary spell. Here’s the calculation:

0.83+0.06= 0.89

The game will roll a number between 0 and 1, and if its less than 0.89, the fireball will hit. Otherwise, a resist message will appear. After the fireball lands, the game will then apply spell resistance to determine a partial resist, if any. Assuming the yeti also has 50% fire resistance, on average, 50% of the damage will be resisted.

+Damage/Healing

There is some confusion about how these effects work and how effective they are, so I wanted to explain what they do.

When an item has the effect “Increases damage and healing done by magical spells and effects by up to 20″, the damage dealt or hit points healed by one of your spells will be increased by a fixed amount up to 20. The actual amount that the spell is increased by is not determined randomly, but by which spell you are casting. You are not receiving a random bonus value between 1 and 20 whenever you cast a spell. Some random items, such as those with the property “of Shadow Wrath”, indicate that they provide “+X Shadow Spell Damage”. The actual amount of the bonus is again determined by the spell being cast. Once again, the bonus received could be reduced depending on the spell being cast.

Spells with longer casting times such as Fireball and Greater Heal will receive the full bonus listed every time they cast the spell. Spells with instant or short cast times receive a fraction of the bonus listed. As an example of such a spell, Flash Heal (Rank 7) only receives approximately 43% of the item bonus. Talents or other effects that might change the casting time of the spell will not change the bonus; the amount received is a property of the spell being cast. [Instant cast spells activate the global 1.5 second cooldown and are treated as 1.5 second spells for purposes of calculation. AoE spells get only a small fraction of the +damage)

Most long duration Damage and Heal over time spells have the bonus divided amongst every tick. So Renew, which lasts 15 seconds and heals an amount every 3 seconds, has 20% of the bonus applied each tick. In a similar fashion, channeled spells also have the item bonus divided amongst every tick of the spell’s duration.

Lower level spells receive a greatly reduced benefit from these effects.

These effects all stack. So if a Mage is wearing 3 items that provide up to +20 Fire damage, you will have a combined +60 to Fire damage. This means every high level Fireball cast will inflict an extra 60 damage to the target.

These bonuses are also added to the effect of the spell before the 50% bonus from a critical spell hit is applied.

To get the full damage or heal bonus, a direct damage spell or direct heal spell must have a casting time of at least 3.5 seconds. So spells such as Fireball and Greater Heal gain the full effect from +damage or +heal items. Frostbolt has a 3.0 second casting time and an additional secondary frost effect, so it also has a reduced effect from +damage or +heal items. Each Frostbolt gains 81.4% of the listed damage bonus. Casting time reduction talents such as Improved Frostbolt will not effect the amount of bonus applied. The percentage amount of the bonus decreases as the base casting time of a spell decreases.

Here are some other examples of damage or heal bonus coefficients for direct damage and healspells.

Shadow Bolt (3 second casting time): 0.857
Smite (2.5 second casting time): 0.714
Wrath (2.0 second casting time: 0.571
Flash Heal (1.5 second casting time): 0.429
Scorch (1.5 second casting time): 0.429
Fire Blast (instant casting time): 0.429