Come and Get it

Attack and Defense Ratings

To actually hit your opponent you need with almost every attack Attack Rating. To prevent yourself from being hit you need Defense.

common forumla

CtH % = (100 * AR / (AR + DR)) * (2 * atLvl / (atLvl + dfLvl))

AR: Attack Rating DR: Defense Rating atLvl: attacker level dfLvl: defender level

You see it's not just AR and DR, but also the ratio of attacker and defender level which determines the Chance to Hit. So if your enemy's defense is zero he might still be too high in level for you to hit.
Also the CtH is never better than 95% and never worse than 5%, this is a hardcoded limit.

If you want to calculate yor chance of being hit instead your CtH the formula stays same, but you've to change perspective.

Walking and Running

A walking character doesn't suffer a defense penalty. Whereas running will skip the entire CtH formula. So even atLvl and dfLvl are irrelevant, and any mLvl=1 Fallen in Normal will hit a cLvl99 character, provided the character hasn't moved away already till the Fallen has ended his swinging animation ;-)
Infight moving doesn't count as 'running', even when run mode is activated. This applies to Barbarian's Whirlwind and Paladin's Charge, also to Leap. Leap Attack, Dragon Flight and Teleport.

autoaim, autotarget, autohit

There's some confusion about this terms, especially since some PvP guys started to call Amazon's Guided Arrow 'autoaim'. So I'll give you common definitions in all major boards and every place where sensible people discuss seriously game mechanisms.

autoaim
causes a missile to choose its way into a direction where an enemy stood at the very moment the missile was started. Most common example is in fact Zon's Strafe (... instead of Guided Arrow ...). An Amazon may look in opposite direction of an enemy, still the Strafe skill will cause her to turn around 180 degrees, so the missile will travel to that enemy.
Some secondary missiles like the triggered bolts of Lightning Fury Fist of the Heavens or some Traps also are autoaiming.
Once it is started the travelling direction of the missile won't change anymore. Thus this is entirely independent from actually hitting anything.
autotarget
causes a missile to vary its travelling direction if the chosen target moves. The character needs to aim the missile at least roughly into the correct direction. Speaking of attacks this is Guided Arrow's behaviour.
Necromancer's Bone Spirit is also autotargetting, but we don't discuss spells here in detail.
A Guided Arrow still should be checked for its Chance to Hit, but there's more to it ...
autohit
is an inbuilt property of some attacks, which therefore are guaranteed to cause a hit, regardless of CtH. Our former example Guided Arrow is not just autotarget, but also autohit, so CtH formula is skipped here.
Another prominent example is Paladin's Smite.
You may read elswhere rumors about Guided Arrow and Smite being subject to minimum and maximum CtH. This is nonsense. You don't get 'a bit of autohit', either an attack is autohit or it isn't, if the CtH formula is skipped its restrictions aren't applied, too.
Other autohit attacks are Zon's Lightning Bolt and Lightning Fury, even Fury's physical javelin. This is not common knowledge, because both attacks are ranged and neither autoaim nor autotarget, so the enemy may move out of the way.
Still autohit attacks may miss someone. An enemy may have dodged or moved anyway, and some enemies may block the attack. And of course you always may yell 'LAAAAAG' as an excuse.

Attack Rating

Attack Rating for starting characters
Class	AR
complete Status Point Table
Amazon	95
Assassin	80
Barbarian	85
Druid	70
Necromancer	80
Paladin	85
Sorceress	75

im Charakterbildschirm angezeigte Trefferchance Attack Rating - AR - is needed to enhance the Chance to Hit for all your (non-autohit) attacks. It is displayed in the character screen, if you hover your mouse pointer over the Attack Rating figure, your AR against the last attacked monster will pop up. Sorry, that displayed info is not entirely accurate, since some skills have AR bonus which isn't counted in, others increase the figure, but due to some bugs won't change your actual AR. Furthermore skills to decrease enemy's defense don't figure in here.
Character's AR is composed out of a class-specific conversion of dexterity points (AR_base), straight AR boni from Items and percentage boni from items and skills.
A rule of thumb to quickly calculate first component is AR_base = dex * 5. You may notice a slight difference to the table especially in early levels. In higher levels the rule of thumb is approximately exact.

exact formula

AR_total = ((dex - 7) * 5 + AR_abs + K_char) * (1 + AR_% / 100%)

class-specific constant
Class	`K_char`
Amazon	5
Assassin	15
Barbarian	20
Druid	5
Necromancer	-10
Paladin	20
Sorceress	-15

AR_total: total Attack Rating dex: dexterity AR_abs: straight boni from items AR_%: percentage boni from items and skills K_char: class-specific constant

the 1.09 formula
AR_total = ((dex - 7) * 4 + AR_abs + K_char) * (AR_% / 100%)

Combat Shrine Icon A Combat Shrine will enhance AR_total by 200%, thus tripleing your total AR, It doesn't figure in as AR_%!
see also: Shrines

There are lots of different items granting various boni to AR. Some add a straight value, some a straight value based on character's level, some give a percentage bonus, and the prefix 'visionary' even grants a percentage bonus on character's level. As always I prefer to link to Atair's d2wissen for further reference.
Also you find items with race-specific boni to AR for fighting against demons or undead, this fits the above formula in AR_abs, just only versus that races and not versus everyone (this also isn't considered by the character screen as far as I know).

Barbarian's Taunt will decrease the taunted enemy's AR by a straight value. You may lower its AR to zero or even into negativa this way. An internal checking routine steps in and sets negative values to zero, the value is then added to defender's, eh, defense. The above formula is still mathematically valid.

'Monsters' and Attack Rating of ranged attacks

'Monster' also means summoned pets and most probably hirelings also.
Attack Rating of monsters is listed at monster sections of Arreat Summit or d2wissen Modify AS's values with MonLvl.txt before going further.

When a missile is launched by a monster the AR of that monster is passed to the missile as straight bonus. But base AR counts twice then since AR from firing unit will also figure into CtH check. This happens before potential percentage AR boni from items and/or skills like boss aurae etc.

AR_miss = (AR_abs + AR_abs + 5 * dex) AR_abs = AR_miss + AR_miss * (1 + AR_%skill / 100%)

AR_miss: straight Attack Rating of missile AR_abs: monster's Attack Rating from MonStats.txt dex: dexterity AR_%skill: percentage bonus via monster's skills

other Bugs

The percentage AR bonus granted by any of Amazon's ranged attack skills is displayed, but doesn't work ingame.
- brianc84's testing
- Balkoth's Testreihen
- fixing missile AR bug, by Nefarius
Paladin's Blessed Aim aura, also available via Act2 offensive hirelings from Normal or Hell, negates Amazon's percentage bonus granted by her Penetrate skill. As soon as the Zon enters a friendly BlA aura her own bonus is stripped away. This is caused by both skills using the same aurastate, you can't do anything about it in unmodified game.

Not a bug, but nice to know, a Paladin gains a hidden bonus of 5% AR for each hard point he has spent in Blessed Aim.
This will step in even when the aura itself is not activated.

Credits

Balkoth
brianc84
Donmom
Maxx Power
Myrdinn
Nefarius
Othin
RTB
SSoG

defense

defense of starting characters
classe	def
complete Status Point Table
Amazon	6
Assassin	5
Barbarian	5
Druid	5
Necromancer	6
Paladin	5
Sorceress	6

Defense Rating - DR - of a character determines the protection against physically delivered attacks. It is composed of a base DR figured from dexterity, defense boni from equipment and boni from skills.
The base DR rule of thumb is def_base = [dex / 4].

DR is subject to far more influences than AR, since it even can be lowered in various ways.
I won't discuss defense of Pets of any kind here. Basically it works similar to characters, but some pets are subject to varying figures for difficulties in MonStats.txt.

Boni to `DR_base`

straight defense values of your equipped gear, you may also read 'AC' for armor class
Some confusion here already. Often armor pieces have a percentage value listed. In this case the percentage value refers only to the actual piece of armor. If the item additionally gives a straight bonus the percentage bonus is applied firstly to the base defense of that armor piece, then the straight value is added.
Some gear which is in fact not really armor can add straight boni, too. As an example the unique amulet Cat's Eye or charms the character carries in inventory. There are items which grant a straight bonus based on character's level, too.
percentage defense boni on some weapons
Some weapons indeed enhance your defense. This is applied when all your armor defense is calculated already. Also the unique amulet Mahim Oak Kurio grants (apart from a straight bonus) a percentage bonus this way.
You may find some items which grant a straight bonus only in close combat (versus melee) or against ranged attacks (versus missiles).
see d2wissen for items
percentage defense boni granted by skills
- Assassin:
  - Cloak of Shadows (Sin only)
- Barbarian:
  - Iron Skin (passive, Barb only)
  - Shout (all allies in range when cried)
  - Concentrate (Barb only, just the attack)
- Druid:
  - Werebear (passive, Druid only)
- Paladin:
  - Holy Shield (Paladin only)
  - Defiance (all allies in range)
- Sorceress:
  - Frozen Armor
  - Shiver Armor
  - Chilling Armor
- the defensive Town Guard hireling from Act2 Normal or Hell:
  - Defiance (all allies in range)
- Bosses:
  - Stone Skin
Armor Shrine
see also: Shrines

penalties on `DR_base`

negative percentages caused by enemy's skills
- Assassin:
  - Cloak of Shadows
- Barbarian:
  - Battle Cry
- Paladin:
  - Conviction
negative straight values
- Amazon:
  - Inner Sight
- Barbarian:
  - Berzerk
- Dimoak's Hew substracts whopping eight points of your defense if you use it
(enemy's) attacks which ignore CtH check
- Amazon:
  - Guided Arrow
  - Lightning Bolt
  - Lightning Fury
  - plus all splashes caused by elemental arrows and triggered bolts from Charged and Lightning Strike
- Paladin (also some monsters):
  - Smite
- all Classes:
  - any native or crossclass spells which don't need physical deliverance via blowing, throwing, shooting
  - any physically delivered attack against a running character
negative percentage values caused by enemy's gear
Also known as item_fractionaltarget, or simply 'the Eth Rune'. This works only half of the listed value if enemy is a player, hireling or Boss. An internal check prevents enemy's defense becoming negative via this effect (this is the fix of the 1.09 'Eth Bug'). The common CtH formula stays intact.
- Eth Runes in weapons (while dual-wielding every single strike is checked, and just the weapon actually used will count)
- some other weapons and runewords, as listed on d2wissen
negative straight values caused by enemy's gear (-X defense per hit)
- weapons and runewords, as listed on d2wissen, the decreased value stays when enemy switches to another weapon; your allies won't benefit from you lowering enemy's defense this part of Info is possibly outdated or wrong, compare that Thread at the Basin
Ignore Target's Defense (ITD)
The name indicates otherwise, though this doesn't ignore the defense but it's set to zero. So the common CtH formula still is used. This property won't work against other players, hirelings and bosses.
- Jah Runes in weapons (while dual-wielding every single strike is checked, and just the weapon actually used will count)
- weapons and runewords, as listed on d2wissen

In Player versus Player all properties will work without penalty, unless it is explicitly written otherwise.

Formulas

base defense `def_base`

def_base = [dex / 4]

dex: dexterity [ ]: Floor Function

defense of a armor piece `def_item`

Grey and white items without the property 'superior' just use displayed value. All others:

def_item = [maxdef_itembase * Bonus_eth * (1 + item_%ED / 100%)] + def_absolut

maxdef_itembase: maximum possible defense of the base item Bonus_eth = 1,5: ethereal bonus, otherwise = 1 item_%ED: percentage bonus of that item def_absolut: straight bonus of that item [ ]: Floor Function

Note that straight boni def_absolut aren't increased by %ED from that item.
Socketing an ethereal piece of armor will cause a cumulative additional Bonus_eth.

overall defense `def_char` of a character before enemy penalties step in

def_char = (def_base + ∑def_item) * (1 + ∑def_%/100%) * def_Zerk + def_melee + def_missile

def_base: base value composed of dexterity points, see above ∑def_item: sum of all armor pieces, see above ∑def_%: sum of all percentage boni from skills, weapons, jewelry and shrine def_Zerk: 0 whilst Berserk, 1 with all other attacks def_melee: items granting straight bonus versus melee attacks def_missile: items granting straight bonus versus missile attacks

Berserk indeed does not reduce all your defense to zero, the boni dedicated especially versus melee and/or missile attacks are not affected by Zerk. def_Zerk is technically better known as Armor_overide_percent, a property only used by Zerk in unmodified game.

overall defense of a unit including enemy's penalties

Fit the penalties as negativa into last formula, straight to straight and percentage to percentage. As special case apply Eth Rune and items granting that same effect at the very end.

def_completely = def_char - def_char * ∑Eth_% / 100%

∑Eth_%: sum of enemy's weapon percentage penalties to opponent's defense

A quick summary, versus players, hirelings and bosses ∑Eth_% works only half the value displayed, skills and straight penalties work at full capacity, and ITD doesn't work at all.

In 1.09 the Eth Rune caused the enemy's Defense Rate to drop to -25%, which equalled zero, instead of decreasing his DR by 25%. This was fixed with 1.10.

If at any point during the calculation DR swaps into negative numbers it is internally set to zero, and the value is added to attacker's Attack Rating. The common formula still stays mathematically valid.

If an attack subdues DR it may still be blocked with a shield (or claws). An Amazon is able to dodge some attacks due to her Evsive skills, if even shield block fails.