Guaranteed damage reduction

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

Guaranteed damage reduction (GDR) gives players a layer of protection in melee and ranged combat. Normally, each attack that hits you is reduced by a random amount up to your AC. Against physical attacks, your minimum AC reduction is equal to GDR% of the attack, unless it is higher than 1/2 * AC. Therefore, GDR is a form of insurance against poor AC rolls.

What GDR Affects

GDR only works against physical damage, either in melee, with reaching, or with a ranged/throwing weapon.

It does not apply against any sort of spell or ability. Therefore, GDR does not apply to a Conjurations spell that does physical damage. Neither does it apply against "physical" abilities like Slug Dart or Harpoon Shot. It also doesn't work against elemental melee damage, such as an ice beast's additional cold damage.

Calculating GDR

Your GDR is calculated by the following formula: GDR% = AC^(1/4) * 16. Any means of obtaining AC counts towards your GDR.

GDR of Various AC Values

Base AC	GDR
2 (Robe)	19.0%
6 (Scale mail)	25.0%
10 (Plate armour)	28.5%
16 (+2 Robe, aux)	32%
32 (+10 Plate armour, aux)	38.1%
40 (+14 Crystal plate, aux)	40.2%
50	42.5%
100	50.6%

Auxiliary slots include a +2 Helmet, +2 Cloak, +2 Gloves, and +2 Boots. Percents on this table are rounded to the nearest tenth.

Damage Reduction

GDR is not a form of damage reduction independent from AC. If you could have excellent GDR and terrible AC, it would do you little good. It is effectively a minimum AC roll, or a guaranteed AC roll. It is capped at your GDR% of damage taken, or 1/2 of your AC, whichever is lower.

Let's say you take a 50 damage attack, but have 48% GDR. First, AC rolls. If AC roll is a 1, then GDR will raise the reduction by (50 * .48) = 24, meaning you actually lose 26 HP. But if your AC roll is 36, then GDR will not do anything, as you are above the minimum.

If you took said 50 damage attack with 16 AC and 32% GDR, then the AC/2 limit applies instead: you'll always reduce damage by at least (16 / 2) = 8 over the expected (50 * .32) = 16.

Strategy

GDR is tied to your AC; any form of extra AC will also increase GDR. This comes with the same costs and benefits; are you willing to sacrifice EV and spellcasting potential in order to become more bulky? Characters who want to avoid melee will still want to avoid melee as much as possible. However, GDR is a large reason why AC is more consistent in stopping damage.

For most characters, GDR will slightly reduce the maximum physical damage enemies can deal.

History

• Prior to 0.27, GDR was more complex. Each set of body armour had a defined GDR, calculated by (14*(Body Armour Base AC-2)^(1/2))%, no matter what your actual AC was. Certain transmutations also had a set GDR. Gargoyles increased base GDR, while Draconians (being unable to wear body armour) didn't have any. For more information, see this past revision.
• Prior to 0.8, GDR was equal to (Body Armour Base AC) * (13+Armour)/17.
• Prior to 0.5, GDR was higher.