AC calculations

Version 0.14: 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.

Calculating AC from Armour

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

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

This means that every level in the Armour skill increases a piece of armour's AC by approximately 4.5% of its base AC.

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)