Guaranteed damage reduction

Wearing any body armour with AC greater than 2 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

In most cases, your GDR% = (14*(Body Armour Base AC-2)^(1/2))%. No other type of armour provides any benefits to GDR.

GDR of Various Armours

Armour	Base AC	GDR
Robe, animal skin	2	0%
Leather armour	3	14%
Troll leather armour	4	19%
Ring mail, steam dragon armour	5	24%
Scale mail, mottled dragon armour	6	28%
Swamp dragon armour	7	31%
Chain mail, fire dragon armour	8	34%
Ice dragon armour	9	37%
Plate armour, pearl dragon armour, storm dragon armour	10	39%
Gold dragon armour	12	44%
Crystal plate armour	14	48%

There are also two Transmutations spells which allow you to have GDR in spite of the fact that they effectively disable your body armour: Dragon Form (34%) and Statue Form (39%). Finally, gargoyle characters receive an innate boost to GDR. This boost is inversely proportional to the GDR provided by your body armour.

What GDR Affects

GDR only works against one thing: physical damage dealt by monsters attacking you in melee (including via a reaching brand). 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 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 have excellent GDR and terrible AC, it will do you little good. Regardless of your GDR%, the maximum amount of damage reduction it can provide is capped at 1/2 of your AC.

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

If an enemy's actual damage roll exceeds this guaranteed reduction, the game tests the attack against your AC. If the damage reduction from AC winds up being greater than your guaranteed damage reduction, that is used instead.

Strategy

GDR's usefulness depends heavily on how often you find yourself in melee combat, how high your AC is, and how much you're willing to sacrifice your evasion defense (in general, the worse an armour is for your evasion, the higher its GDR will be). Unless you intend to wade into close combat whenever possible, and you have no intention of training Dodging, you'll likely be better off avoiding super-heavy armour which maximizes GDR and focusing on optimizing your AC, EV, spells, and resistances to keep yourself alive. Characters that actively avoid melee, whether it be through fast movement speed or ranged attacks, will barely notice the effect of their GDR.