Guaranteed damage reduction

Version 0.27: This article may not be up to date for the latest stable release of Crawl.

Wearing any armour provides you with an amount of guaranteed damage reduction (GDR) in melee combat. Normally, each attack that hits you is reduced by a random amount up to your AC. GDR is a form of insurance against poor AC rolls; if you have sufficient AC, you are guaranteed to reduce the damage taken by a percentage of the enemy's maximum attack damage equal to your GDR up to half of your AC. As an enemy's damage output is about as variable as your AC, a high GDR will allow you to completely negate an enemy's attacks much more often than normal, and those attacks that do get through will always be significantly diminished.

Calculating GDR

Your GDR% = AC^(1/4) * 16. Any means of gaining AC counts towards your GDR.

GDR of Various AC Values

Base AC	GDR
2 (Robe)	1%
6 (Scale mail)	25%
10 (Plate armour)	28%
16 (+2 Robe, aux)	32%
32 (+10 Plate armour, aux)	38%
40 (+14 Crystal plate, aux)	40%
50	43%

Aux = +2 Helmet, +2 Cloak, +2 Gloves, and +2 Boots

What GDR Affects

GDR only works against one thing: physical damage dealt by monsters attacking you in melee (including via reaching). It does not work against any sort of ranged attack, be it from a physical launcher or a Conjurations spell that deals physical damage. Neither does it work against elemental melee damage, such as an ice beast's additional cold damage; however, most such elemental attacks need to deal physical damage to you before their added effects trigger, so high GDR will make them happen less often.

Guaranteed Damage Reduction Cap

GDR is not a form of damage reduction independent from AC. If you could have excellent GDR and terrible AC, it will do you little good. The damage reduction is capped at your GDR% of the enemy's maximum damage or 1/2 of your AC, whichever is lower.

For example, let's say you have 48% GDR and 100 AC, and you're facing an opponent capable of dealing up to 50 damage in a single hit. This would give you a guaranteed damage reduction of 24 (48% of 50), taking full advantage of the 48% GDR. The same 48% GDR coupled with 20 AC would only give a guaranteed reduction of 10, a mere 20% of the enemy's max damage, because it is capped at 1/2 of your AC. If you are instead facing an opponent capable of dealing 200 damage, then even at 100 AC, you'd only be guaranteed 50 damage reduction (25%), while the 20 AC character would still be stuck with 10 (which is now only 5%). In each of these cases, the GDR is still helping; even in the case of 20 AC vs. 200 max damage, it still mitigates the worst possible rolls.

Guaranteed damage reduction is effectively a minimum AC roll. If your damage reduction is 24 (as in the first example above) and you roll a 1 on your AC against a given attack, your roll will be boosted up to 24 instead. But if you roll a 36 on your AC, you won't get any bonus from GDR because you're already above the minimum.

Strategy

GDR is as useful as AC; any form of AC will 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. For most characters, GDR will slightly reduce the maximum 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 a set GDR Gargoyles increased base GDR, while Draconians notably didn't have any. For more information, see this past revision.