AC calculations

Version 0.13: This article may not be up to date for the latest stable release of Crawl.

This page explains how the game calculates and uses AC. If you're not into math or don't want to be spoiled, stop reading right now.

Calculating AC from Armour

The formula for calculating the actual AC granted by wearing a piece of armour is as follows:

 (base AC) * (22 + Armour skill + Racial armour bonus/2) / 22

This means that every level in the Armour skill increases a piece of armour's AC by approximately 4.5% of its base AC. As for racial armour bonuses, they effectively act as bonus levels in the Armour skill. They are as follows:

• Dwarven armour grants all species a +4 bonus (roughly +9% to base AC). Deep dwarves receive an additional +4 for a total of +8 (roughly +18% to base AC).
• Elven armour grants high elves and deep elves a +2 bonus (roughly +4.5% base AC).
• Orcish armour grants Hill Orcs a +4 bonus (+9% base AC). If you're a follower of Beogh, you receive an additional bonus, increasing with Piety. This additional bonus maxes out at +9 when you reach 180 Piety, for a total of +13 (roughly +29% to base AC).

A hill orc follower of Beogh with max Piety and 27 Armour skill has the AC of all orcish armour pieces increased to roughly 252% of their base value.

Note that fractional AC from multiple pieces of armour does add! Fractions are not truncated until the contribution from all armour pieces are summed. For example, both a helmet and a pair of gloves have 1 base AC, so at an Armour skill of 12 they each provide 1.55 AC, which is rounded down to 1 AC when worn individually. When worn together, their contributions sum to 3.1 AC, which is rounded down to 3 AC.

Other Sources of AC

All other sources of AC are applied directly to a character's total AC and are not modified by Armour skill whatsoever. These include:

• Bonus AC from enchanted armour, rings of protection, or artefacts
• Magical AC boosts such as Ozocubu's Armour or Stoneskin
• AC from transmutations such as Necromutation, Statue Form and Dragon Form
• AC from mutations (e.g. scales) and intrinsics (e.g. a naga's increasingly tougher skin)