Spell Piercing Calculator

Your spell Caster level School Damage per hit, before resists

Hit type

Direct or channel DoT tick Binary

Spell Piercing Spell Piercing on this school The value from your character sheet, talents and gear included.

Target

The target's resistance to each school. No raid boss resistances have been measured yet, so enter the values you want to test.

Arcane Fire Frost Nature Shadow

Curse of the Elements Other resistance reduction Any other debuff that lowers the target's resistance on this school.

Result

Resistance after debuffs

After Spell Piercing

Average bonus

Expected damage per hit

What each hit can roll

Average damage change by Spell Piercing

The rest of your settings stay as they are. The dot marks your current piercing.

How Spell Piercing works

Spell Piercing doesn't add a flat amount of damage. Each hit rolls a step: +0%, +10%, +20%, and at high values +30%. In the combat log the bonus shows up as a negative resisted number.

Debuffs like Curse of the Elements lower the target's resistance, but never below 0. A debuff that would take it lower adds nothing.
Spell Piercing comes off what's left. Once it passes 0, the rest turns into bonus damage. Against a target with no resistance, all of it is bonus.
The average share is R / (|R| + 9 × your level), where R is the resistance after debuffs minus your piercing. A positive share is resisted damage, a negative one is bonus damage.
Up to an average of 10%, each hit lands on 0, 10 or 20% at 1 − 7.5a, 5a and 2.5a. Past 10%, every 10% step within 20 points of the average can come up, at 0.5 − 2.5 × its distance from the average.

The share depends on your level, not the target's, so the same piercing is worth less as you level. Holy has no resistance in Forever and takes no piercing.

Hit types

Direct or channel: rolls the full share. Channel ticks such as Mind Flay count here.
DoT tick: the sim rolls a tenth of the share. Logs show partial resists on ticks at about a tenth. No bonus from piercing has shown up on a tick yet.
Binary: spells with an effect besides damage, such as Frostbolt's slow. The average share is the chance to resist the whole spell, and one that lands still rolls its damage like a direct hit.

What the logs show

15 piercing at around level 22, almost 3,000 hits: +10% came up exactly twice as often as +20%, for an average bonus of 7.5%. Every school takes it, and crits roll the same as normal hits.
A Fireball log with more piercing shows the +30% step once the average passes 10%.
No bonus has shown up on DoT ticks in more than 1,000 ticks from a caster with 7 piercing.
A caster with 1 piercing got no bonus in 1,791 hits, where the formula expects a few percent.
The 9 × level constant is fitted to logs from levels 20 to 30. Above that the numbers are the model's prediction until someone logs it.