File:GDR Example.png

Guaranteed Damage Reduction (GDR) calculations for a 28 AC Plate vs a 28 AC Chain. Average damage done based on maximum possible damage. The plate has 40% damage reduction while the chain has 28% damage reduction. Average damage was calculated by taking a uniform distribution of the hit dice 1d(max possible damage) and applying damage reduction. Note this is without using the AC damage reduction that can be greater than GDR.

Note that at low hits, 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, in this case.