AC calculations
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 elves (high elf, deep elf, or sludge elf) 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)
AC Damage Reduction and Guaranteed Damage Reduction
Not all attacks have their damage reduced by AC. The vast majority of physical attacks (both melee and ranged) do, as do most spells or abilities (except Hellfire, and White Draconian's chilling blast). If a melee attack has some kind of brand, the portion of damage corresponding to the brand isn't reduced by AC.
Whenever a character is hit by a melee attack, the attack's main portion of damage is always at least reduced by an amount referred to as the guaranteed damage reduction, or GDR for short. The formula for the GDR is as follows:
GDR percentage = 14*sqrt((base AC of body armour)-2)
Guaranteed Damage Reduction = min((Max damage)*(GDR percentage)/100, AC/2)
You can look up the GDR percentage of any body armour in this table.
If you aren't wearing any body armour, your GDR percentage is zero. But there are two exceptions: Your GDR percentage is 34% in Dragon Form, and 39% in Statue Form.
The AC reduction is always at least the GDR, but can go up to the character's full AC value. For example, a character in crystal plate armour (48% GDR), with a total AC of 30, would reduce the damage of an attack whose maximum damage is 50 by 15 to 30 damage (not 24 to 30):
AC/2 = 15
GDR percentage*Max damage = 0.48*50 = 24
GDR = min(24, 15) = 15
Among melee attacks, Constriction is a special case. GDR is completely ignored, and the damage reduction is a random number between 0 and AC/2.
Ranged attacks and conjurations aren't affected by the GDR either. The damage reduction can be any number between 0 and AC. There are 4 exceptions to this rule:
- Hellfire: No damage reduction.
- White Draconian's chilling blast: No damage reduction.
- Freeze: No damage reduction.
- Beams of electricity (Bolt of lightning, Chain Lightning, ...): The reduction is between 0 and AC/2.
- Lee's Rapid Deconstruction: The AC is rolled three times (random number between 0 and AC), and each roll reduces damage.
