# 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)