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Damage Amplification or Damage Amp is a stat that multiplies the damage caused by the player to affected enemies and NPCs.

Video explaining Damage Amplification

Damage Reduction

Damage Reduction is a negative form of Damage Amplification, reducing the damage output of enemies or its user. Mechanically, standard Damage Reductions are treated as negative Additive Amps (see Calculation section).

Calculation

Contrary to what the percentages might suggest, multiple sources of damage amplification do not all stack in the same way. There are three distinct categories of Damage Amplification, each interacting with the damage formula differently:

• Additive Amps: These add together with each other (inculding additive Damage Reductions) into a single pool.
• Multiplicative Amps: These multiply the total damage one by one.
• Instance Amps: These are nearly identical to multiplicative amps, main exception being they create a new instance of damage.

The Damage Formula

Based on testing, the generalized formula for how these amps apply to weapon damage is as follows:

$\text{Damage}=[(\text{Base Bullet Damage}\times \text{Weapon Multiplier})\times (1+\text{Total Additive Amps})+\text{Flat Bonus}]\times {\text{Multiplicative Amp}}_1\times {\text{Multiplicative Amp}}_2\dots \times {\text{Instance Amp Damage}}_1\times {\text{Instance Amp Damage}}_2$

Example:

1. A hero has a base bullet damage of 100 and 0 flat weapon damage for simplicity.
2. The hero is affected by Inhibitor (an Additive Damage Reduction of -30%). Their damage drops to 70 (100 * 0.70 = 70).
3. Ivy casts Air Drop on the hero (an Additive Amp of +20%). The additive pool is now -30% + 20% = -10%. The hero deals 90 damage (100 * 0.90 = 90).
4. McGinnis inflicts the target with Spectral Wall (a Multiplicative Amp of +20%). This is applied after the additive pool. The hero now deals 97.75 damage (90 * 1.20 = 108).

Damage Reduction Stacking

Valve's May 22 patch notes state that Damage Reduction debuffs are intended to stack diminishingly (i.e., multiplicatively, in the same manner as Resistance).

However, live testing confirms that standard Damage Reduction debuffs have not been fully migrated to this new system. Currently, Damage Reductions (such as Inhibitor or Fire Scarabs) simply act as negative Additive Amps. This means they add directly together with other additive amps (positive or negative) rather than multiplying against each other.

Bugs, Quirks, and Exceptions

• Additive Amps do not apply to many flat values, such as flat weapon damage bonuses, base damage for Melee abilities (Such as the bonus +60 damage from Calico's Leaping Slash T3 upgrade), Capacitor active, Backstabber, etc.
• Headhunter Interaction: Multiplicative non-instance amps are the only amps that work correctly with Headhunter damage.
• Return Fire / Reflection: Because Multiplicative Amps act as defensive stats on the target, if an attacker fires at a target with Return Fire while the attacker has an M-amp active against them, the attacker will take increased reflected damage.
• Barrier Interaction: Billy's bullet amp from Blasted is unique as it is a post-mitigation true damage amp. Because of this, it does not work at all against barriers.
• Silencer / Infernus Afterburn T2: Silencer and Infernus's Afterburn Tier 2 Spirit Damage Reduction are exceptions to the standard Damage Reduction rule. They stack additively with themselves, but multiplicatively with other additive amps.
• Celeste: Celeste's self Spirit damage amp works like a multiplicative amp but is affected by additive amp bugs.

Sources of Damage Amplification

Additive Amplifiers

List of heroes

Hero	Ability	Tier	Stat change
Calico	Ava	3	+18% Ramping Damage Amp
Ivy	Air Drop	0	+20% Damage Amp on Ivy and carried ally
Shiv	Killing Blow	0	+8% Damage Amp
		2	+16% Damage Amp

Multiplicative Amplifiers

List of heroes

Hero	Ability	Tier	Stat change
Infernus	Napalm	0	+16% Damage Amp
		3	+17% Damage Amp
McGinnis	Spectral Wall	1	+20% Damage Amp
Mo & Krill	Scorn	3	+15% Damage Amp per Stack
Bebop	Hook	0	Damage Amp against hooked targets
All Heroes	Parry	0	+25% Damage Amp (Parried target takes 25% increased damage from all sources)

Instance Amplifiers

List of heroes and items

Hero / Item	Ability	Tier	Stat change
Drifter	Bloodscent	0	+15% Damage Amp on Isolated Heroes
		3	+11% Damage Amp on Isolated Heroes
Lady Geist	Malice	0	+7% Damage Amp per Stack
		3	+8% Damage Amp per Stack
Paradox	Pulse Grenade	0	+4% Damage Amp per Stack
Pocket	Barrage	0	+6% Damage Amp per Stack
		3	+4% Damage Amp per Stack
Sinclair	Rabbit Hex	0	+15% Damage Amp
		3	+7% Damage Amp
Escalating Exposure	-	-	+4.5% Damage Amp per Stack

Sources of Damage Reduction (Negative Additive Amps)

Weapon and Spirit Reduction

List of heroes

Hero	Ability	Tier	Stat change
Mirage	Fire Scarabs	0	-20% Damage Reduction
		3	-15% Damage Reduction

This is a dynamic list "Damage Reduction". Edit contents.

Name	Cost	Category	Stat change
Haunting Shot	Legendary	Weapon	-40% Damage Reduction
Cheat Death	6,400	Vitality	-60% Damage Reduction
Inhibitor	6,400	Vitality	-30% Damage Reduction
Golden Goose Egg	800	Spirit	-10% Damage Reduction
Cursed Relic	6,400	Spirit	-25% Damage Reduction

• Cheat Death, Golden Goose Egg and Cursed Relic reduce damage dealt by its user.

Spirit Reduction (need explanation here)

List of heroes and items

Hero / Item	Ability	Tier	Stat change
Infernus	Afterburn	2	-35% Spirit Damage Reduction (Note: Additive with itself, but multiplicative with other additive amps.)
Silencer	-	-	Spirit Damage Reduction (Note: Additive with itself, but multiplicative with other additive amps.)