Armour class

Version 0.30: This article may not be up to date for the latest stable release of Crawl.

When something injures you, your armour class (often abbreviated to AC) reduces the amount of damage you suffer. The higher the value, the better.

The most common way to improve your AC is by wearing armour. Wearing heavier body armour provides more AC, but comes at the cost of encumbrance - reducing EV, spell success rates, and stealth. Other types of armour, like helmets and boots, provide AC with no penalty.

Useful Info

AC reduces the damage of attacks by an absolute value between 0 and your current AC value. An AC of 10 will reduce damage by a random number between 0 and 10. This applies to damage that would traditionally ignore armour in other games, such as spell magic.

BEAM_ELEC damage (such as from Shock or Lightning Bolt) ignores half of the target's AC. BEAM_FRAG damage (such as from Sandblast or a tin of tremorstones) is affected three times as much by AC.

Guaranteed Damage Reduction

Main Article: GDR

When taking physical damage from a melee attack, guaranteed damage reduction will improve AC rolls. GDR% is equal to 16*(AC)^1/4. If GDR is applied, the minimum possible AC roll will equal (damage*GDR%) or (AC/2), whichever is lower. GDR only applies to melee attacks.

Calculating AC Value

First, the AC from worn armour is equal to:

  (base AC) * (Armour skill / 22 + 1) rounded down

Where "base AC" equals the sum of base AC from every worn armour (this excludes any enchantment). Rounding does not occur until all armour pieces have been calculated. Overall, Armour skill provides roughly +4.5% base AC per level.

Then, if you have the deformed or have Pseudopods, this value is halved, rounded down.

All other sources of AC are flat, additive bonuses to your AC score, which are not impacted by Armour skill:

• Value from armour enchantment, rings of protection, and artefact bonuses
• Ozocubu's Armour
• Protection brand
• Mutations
• Innate AC from transformations, like Dragon Form and Statue Form

List of Damage that ignores AC

• Torment and Holy Word.
• Smite.
• Damnation.
• Shared damage from Anguish, Injury Mirror, Injury Bond, Pain Bond, or spectral weapons (uses the original target's AC instead).
• Status effects: poison, sticky flame, flay.
• Hex effects: Pain / Necrotise, Mindburst, Anguish, Agony
• Weapon brand damage and monster attack flavours.
  • Note that weapon brand damage is often a percentage of AC-reduced damage in the first place.
  • Some monster attack flavours require a physical, AC-checking attack to deal damage to apply, though the attack flavour itself is not influenced by AC.
• Specific spells: Freeze, Vampiric Draining, Static Discharge, Ignite Poison, Dispel Undead / Dispel Undead Range, Lee's Rapid Deconstruction (directly on a monster).
  • The chilling blast breath attack of a white draconian.

History

• Prior to 0.29, Frozen Ramparts and Ozocubu's Refrigeration ignored AC.
• Prior to 0.26, Irradiate ignored AC.