Guaranteed damage reduction
Wearing heavy body armour provides you with an amount of guaranteed damage reduction in melee combat. This is a percentage of an enemy's maximum possible damage output that you ignore from each of its hits. Because enemies rarely hit for their max damage, the GDR on some of the heaviest armours often reduces enemy damage to 0, and those hits that do get through are greatly reduced.
Your GDR% = (14*(Body Armour Base AC-2)^(1/2))% (see chart below for exact figures with each armour). However, the amount of damage you can ignore this way is capped at one half of your total AC. While equipment in other slots, enchantments, and the Armour skill will raise your AC and thus raise the cap, your GDR% is only determined by the body armour you wear. So long as enemy attacks aren't hitting the cap, however, you will see them deal 0 damage per hit GDR% of the time.
What It Works Against
GDR works against one thing: physical damage from monsters bashing 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 melee attacks that are non-physical, such as an ice beast's cold damage. However, most such attacks need to do damage to you in the first place to trigger their added effects, so high GDR will make them happen less often.
GDR of Various Armours
| Armour | Base AC | GDR |
|---|---|---|
| Robe, animal skin | 2 | 0% |
| Leather armour | 3 | 14% |
| Troll leather armour | 4 | 19.79% |
| Ring mail, steam dragon armour | 5 | 24.25% |
| Mottled dragon armour, scale mail | 6 | 28% |
| Chain mail, swamp dragon armour | 7 | 31.30% |
| Splint mail, fire dragon armour | 8 | 34.29% |
| Ice dragon armour | 9 | 37.04% |
| Plate mail, pearl dragon armour, storm dragon armour | 10 | 39.60% |
| Gold dragon armour | 12 | 44.27% |
| Crystal plate armour | 14 | 48.50% |
Strategy
GDR is a very desirable property to have in your armour. It is a major reason for casters to consider wearing leather armour over a robe: 14% is a significant amount of guaranteed damage reduction. On the other hand, the powerful archmagi and resistance brands are only found on robes, so the decision is not always clear-cut.
For fighter types, GDR is the main reason to go for heavier and heavier armours: If you aren't casting and are planning on training Armour, you should wear the first plate mail you find: the GDR is well worth the accuracy penalties. Later on, you might want to upgrade to a crystal plate mail or a gold dragon armour, though a branded, enchanted plate mail is a worthy ascension kit item as well.
Example
30 AC with a +6 chain mail is worse than 28 AC with a +0 plate mail.
The graph below lists the average damage taken after GDR calculations for a 28 AC plate vs a 30 AC chain. The plate has 40% damage reduction while the chain has 28% damage reduction. Note this omits the AC damage reduction check that can be greater than the GDR value.
This chart illustrates an important aspect of GDR: GDR will help reduce damage from small and numerous attacks. With monsters that have a low max damage, you will take less damage with the higher GDR plate mail. For example, for a monster that has a max damage of 15, you will take on average 4.2 damage with the chain mail and 3.0 damage with the plate mail. So while you have a higher AC with the chain mail, you will be taking 40% more damage, on average, than if you wore the plate mail.
The higher AC only starts to overcome the better GDR when the max damage surpasses 50, but by then the difference in reduction is negligible.
Spell-Based GDR
There are 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%).
