THAC0 is an acronym for To Hit Armor Class 0 (Zero). It is a number that represents the chances that a physical attack hit its target. Lower numbers are better.
How it works Edit
Every time a creature makes a physical attack, the game simulates an attack roll, which is the roll of a 20-sided die. A roll of 20 is always a successful hit, a roll of 1 always a miss. Otherwise, the minimum roll required to hit successfully is obtained by subtracting the target's Armor Class from the attacker's THAC0.
Attack roll ≥ THAC0 - AC
For example, a character with a THAC0 of 17 trying to hit a target with an AC of 1 needs a roll of at least 17-1=16 to succeed. This is a chance of 5/20, or 25%. A character with a much lower THAC0 of 5 trying to hit the same target needs a roll of at least 5-1=4. This is a chance of 17/20, or 85%.
The game has an option that displays attack rolls on screen, in the logs. This option is located in the Options menu, Gameplay section, Feedback subsection. When this setting is enabled, the logs display the attack rolls in this format:
(Attacker's roll) + [(target's AC modifier) - (attacker's THAC0)] = result
With the result number plus target's effective AC, which is not shown, greater than or equal to zero will be a hit.
Ways to use it from a player's perpective Edit
As a player trying to assert which kind of AC I can reasonably hit, I could reword the equation to:
THAC0 ≤ AC + Attack roll
If I am only ready to accept missing about once out of seven attacks against AC -10 (~85% chance to hit), that means I need to aim for the following THAC0:
THAC0 = AC + (1 - Desired_Chance_To_Hit) x 20 + 1 -6 = -10 + (1 - 0.85) x 20 + 1
85% chance to hit AC -10 will be achieved if I can reach -6 THAC0.
Similarly, as a player trying to assert which kind of AC I need in order to reasonably resist against a given THAC0 value, I could reword the equation so that it describes to-miss rolls rather than to-hit. To miss, AC needs to be:
AC < THAC0 - Attack roll
If I am only ready to accept getting hit once out of five against opponents with THAC0 7 (80% chance to miss), that means I need to aim for the following AC:
AC = THAC0 - Desired_Chance_To_Get_Missed x 20 - 1 -10 = 7 - 0.8x20 - 1
80% chance to make a THAC0 of 7 miss is achieved if I can reach -10 AC.
There're four different THAC0 progression rate by class level:
- Warrior group (Fighter, Ranger, Paladin) and Monk have the best THAC0 progression, with a THAC0 that decreases at a rate of 1:1, to a max of base 0 at level 21.
- Priest group (Cleric, Druid, Shaman) except Monk have the second best rate of 2:3, to a max of base 6 at level 22.
- Rogue group (Thief, Bard) have a rate of 1:2, to a max of base 10 at level 21. In addition to the mathematical oddity of having a better THAC0 than a priest at level 3, rogues remain roughly on-par with priests (albeit slightly in the priest's favor) until level 9, and are thereafter surpassed by priest classes at an increasing pace.
- Wizard group (Mage, Sorcerer) have the worst THAC0 progression, at a rate of 1:3, to a max of 13 at level 22.
Regardless of a creature's THAC0, in Second Edition Dungeons & Dragons, a roll of 20 is always a critical (guaranteed) hit while a 1 is always a critical miss, both are 5% chances. See the Critical hit article for more details.
The previous progression rate gives the following table for THAC0 progression (note, that there are no further changes from level 22 on):
Improving THAC0 Edit
Beyond simply leveling, THAC0 can be improved in a number of ways:
- numerous magical weapons offer a "to hit" bonus
- increasing Weapon Proficiency
- depending on the weapon, improving either strength or dexterity
- improving luck
- improving chances of landing a critical hit
- the following potions improve THAC0:
- various items offer THAC0 bonuses (see table below)
- various spells, innate abilities and HLA's improve THAC0, either by boosting strength or effecting THAC0 directly
- for spells related to strength, see that article.
- for spells and abilities that directly boost THAC0, see the table later in this article
|Bracers of Archery||-2||Missile weapons only, not usable by Druid, Mage, Cleric, Kensai and Sorcerer|
|Circlet of Lost Souls||-2||Usable by Shaman|
|Gauntlets of Extraordinary Specialization||-1||Usable by Warriors, except Kensai and Wizard Slayer|
|Glimmering Bands||-2||Only usable by Rasaad yn Bashir|
|Helm of Balduran||-1||Not usable by Mage, Bard, Thief, Sorcerer, Kensai or Monk|
|Legacy of the Masters||-1||Not usable by Kensai or Wizard Slayer|
|Pale Green Ioun Stone||-1||
Original game: not usable by arcane magic users or thieves
|The Visage||-1||Not usable by good alignments, Bard, Mage, Mage / Thief, Thief, or Monk|
|Wondrous Gloves||-1||Usable by Bards|
|Xarrnous's Second Sword Arm||-1||Not usable by Kensai or Wizard Slayer|
Not included in the table above is the upgraded version of the scimitar Spectral Brand has a charge ability that will increase THAC0 by 10 for 3 rounds.
|Spell/Ability||Availability||Effect on THAC0|
|Bless||Priest||Bonus of 1|
|Boon of Lathander||Priest of Lathander||Bonus of 1|
|Chant||Priest||Bonus of 1|
|Enrage||Berserker||Bonus of 2|
|Holy Power||Priest||THAC0 becomes that of a Fighter of the same level as the caster|
|Offensive Spin||Blade HLA||Bonus of 2|
|Tenser's Transformation||Wizard (Alteration)||Base THAC0 becomes that of a fighter of the same level, with an additional bonus of 2|
BGWiki writing formats Edit
If a creature with a THAC0 of 6 wields a weapon with THAC0: +2 in its in-game description, the adjusted value will be 6-2=4, for this reason, weapons' THAC0: +n and all kinds of THAC0 or to-hit bonuses from any other sources will be written as -n bonus, where n is the positive integer, and vice versa.
- In Dungeons & Dragons 3rd Edition and later, the to-hit system has been reworked to always trend upward by using attack roll + attack bonus - enemy AC ≥ 0 to land hits, where better scores don't go into the negative numbers but rather continue growing in the positive integers.