Effect stacking refers to how effects change when applied in succession and before the first application had expired. Players encounter effect stacking often due to boons and conditions, which both have effects that become more potent as stacks increase. Effects that have charges, such as Arcane Shield aren't generally considered stacking if the effect has a guaranteed duration and/or a guaranteed effect. Effects that stack duration are more numerous than those that stack intensity.
A stack can refer to either the application or the intensity of an effect. It is correct to say "Regeneration has been stacked on me; the first application was 5 seconds, and the second was 10 seconds, giving me 15 seconds of Regeneration total". It is also correct to say "I have 10 stacks of Might for 30 seconds, giving me 300 Power at level 80".
Effects in Guild Wars 2 may stack in duration or intensity. An effect that stacks in duration will generally have the same effect regardless of the effect's source (Regeneration being the most notable exception). An effect that stacks in intensity will strengthen with more stacks, and weaken as stacks expire.
- Duration Example: The Swiftness boon can increase in duration, but more applications will not increase movement speed beyond its set 33%.
- Intensity Example: When applying the Burning condition, each application of burning will take effect and expire independently of other applications. Damage increases as the number of stacks increases.
- Duration-stacking effects
Applications of duration-stacking effects apply sequentially and only one application is effective at a given time. Duration-stacking effects have a maximum number of stacks: 3, 5 or 9 (depending on the effect, see list below). Attempts to further stack the effect will be nullified, yet longer duration stacks are able to overwrite shorter duration stacks that are currently inactive. However the currently active stack cannot be overwritten with a longer stack and will stay active for its remaining duration. After that the stack with the greatest duration will be used up first. Regeneration is an exception here, as it is the only stacking effect that will cause the active stack to temporarily pause if a new stack with greater Healing Power is applied. Effects that extend the duration will extend the currently active stack.
- Examples of duration-stacking effects
- Alacrity stack up to 9 times: 9 stacks of a 2s duration each are applied to a target, resulting into a total of 18s (2+2+2+2+2+2+2+2+2). Immediately 3 new stack of 4s are applied, thus 3 of the 2s stacks will be replaced, resulting into a new total of 24s (2+4+4+4+2+2+2+2+2). Therefore even tho additional 12s tried to be applied, the stack limit of 9 caused 3 stacks of 2s to be replaced, resulting in only additional 6s of Alacrity.
- Quickness stacks up to 5 times: 5 stacks of a 2s duration each are applied to a target, resulting into a total of 10s (2+2+2+2+2). Immediately 5 new stacks of 4s are applied, yet replacing only 4 out of the 5 stacks, as the active one cannot be overwritten, resulting into a total of 18s (2+4+4+4+4). Afterwards Signet of Inspiration (extend all boons by 3s) is being cast, which will cause only the active stack to be extended by 3s, resulting into a new total of 21s (5+4+4+4+4).
- 10 seconds of Regeneration are applied to a target from a level 80 player with 1500 Healing Power. 5 seconds later, 10 seconds of Regeneration are applied to the same target from a level 80 player with 2000 Healing Power. The target will experience 317.5 health per second for the first 5 seconds, then 380 health per second for the next 10 seconds (the first stack was "paused" in order to let the stack with higher Healing Power take priority), then 317.5 health per second for the next 5 seconds.
- Intensity-stacking effects
Applications of intensity-stacking effects apply simultaneously and all applications are effective until they expire. Effects that stack in intensity can stack up to 1500 times, except Might, Stability and Vulnerability which are limited to a stack size of 25. The sources with the greatest intensity and number of stacks will take priority in calculating damage. Removing effects, usually with condition and boon removal, ignores the intensity of the effect. Effects that extend the duration will extend all stack.
- Example of an intensity-stacking effect
- 1 stack of 10 second Bleeding is applied to a target. 1 second later, a 6-second stack of bleeding is applied to the same target by a second source. For 6 seconds, the target will take damage over time from both stacks of bleeding. After that, the second bleed effect will expire, and for 3 seconds the target will continue to suffer the bleeding from the first effect, which then expires.
Appearance in the user interface
The duration of an effect is indicated by a border around the effect icon that gradually disappears as the duration decreases (the icon itself vanishes when the effect ceases). The number of applications of the effect is indicated by a number in the bottom right of the effect icon. The duration shown for an intensity-stacking effect is from the source with the greatest duration.
List of duration-stacking effects
Swiftness does not stack intensity with other effects that increase movement speed. Boons are limited to a maximum of 5 stacks, with the exception of Aegis, Alacrity, Fury and Swiftness.
- Aegis (max 9 stacks)
- Alacrity (max 9 stacks)
- Fury (max 9 stacks)
- Protection (max 5 stacks)
- Regeneration (max 5 stacks)
- Resistance (max 5 stacks)
- Retaliation (max 5 stacks)
- Swiftness (max 9 stacks)
- Quickness (max 5 stacks)
- Vigor (max 5 stacks)
- Blinded (max 9 stacks)
- Chilled (max 5 stacks)
- Crippled (max 5 stacks)
- Fear (max 5 stacks)
- Immobile (max 3 stacks)
- Slow (max 9 stacks)
- Weakness (max 5 stacks)
List of intensity-stacking effects
Each stack applies its own intensity for its listed duration, independently from the others, except if the effect is removed as a whole.
- Bleeding (max 1500 stacks)
- Burning (max 1500 stacks)
- Confusion (max 1500 stacks)
- Poisoned (max 1500 stacks)
- Torment (max 1500 stacks)
- Vulnerability (max 25 stacks)
- Stacking sigil effects
- The effects gained from some sigils which increase a character's attribute with every kill, such as the Sigil of Bloodlust, stack in intensity.
- Damaging effects (retaliation, bleeding, poison, burning, torment, and confusion) and regeneration of a specific source will update in intensity if the source's stats change by various means (e.g. might, food, etc.).
- Currently no information on how stacks are dropped when extras occur.
- Absolutely no rounding happens for duration totals (tooltips only round for display) but it is unknown what happens at certain split seconds for tick based effects.
- Some skills like Well of Corruption can transform boons into conditions or vice versa.
- When a stack of a condition exceeds 100, the number will be replaced with an exclamation point (!).