Dealing Elemental Damage

Elemental damage never rolls to hit and never meets defense: a spell that reaches its target does its number, and the only things between the number and the monster's life are the multipliers on your side and the resistance on theirs. There are three multipliers and they do not overlap.

The three multipliers

1. The skill's own damage at its level, with +skills, synergies and the skill's per-level brackets (the skill page's table).
2. +X% to Fire Skill Damage (Fire Skill Damage and its kin, from items) and the mastery (Fire Mastery, Lightning Mastery: skills.txt passive_fire_mastery / passive_ltng_mastery): both are "% skill damage", added together into one multiplier on the skill's number.
3. Pierce — "−X% to Enemy Fire Resistance" on items (Enemy Fire Resistance Reduction), Cold Mastery's passive_cold_pierce, Conviction, Lower Resist: these do not change your number, they lower the monster's resistance before it is applied.

damage dealt = skill damage × (1 + skill damage % + mastery %) × (100 − (monster resist − pierce)) ÷ 100

Pierce is worth more the closer the monster is to immune: taking a monster from 75% to 50% resistance doubles the damage that gets through, while +25% skill damage on the same monster adds a quarter.

Resistance, immunity and the floor

A monster at 100% or more is immune and takes nothing until the immunity is broken (Resistances and Immunities): against an immune, Conviction and Lower Resist count for a fifth of their value and item pierce for nothing, and a sunder charm sets the resistance to 95% first. Once the monster is under 100, every reduction counts in full, down to the −100% floor, where it takes double damage.

Pierce that goes past the floor is wasted: a monster already at −100% cold resistance gains nothing from more Cold Mastery. Past that, only skill damage and masteries add anything.

Poison

Poison is a rate over time: the skill's damage is spread over its length, and poison resistance cuts the rate while Poison Length Reduced on the target cuts the time. See Poison Damage.

On the receiving end

What reaches a player goes through the defender's chain in Taking Damage: Energy Shield first, then resistance, then absorb, which heals what it takes. Against a player the number is multiplied by 17% before any of that (Player versus Player).