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 duration depending on the boon. Attempts to further stack boons past the maxed duration will be wasted. The source of the boons do not matter, as most boons have a static buff effect. Regeneration is an exception here, as whoever has the highest Healing Power will take priority. It is currently unknown how Regeneration is applied or overwritten with multiple sources.
- Examples of duration-stacking effects
- Alacrity has a max duration of 30 seconds: 2 stacks of a 5s duration each are applied to a target, resulting into a total of 10s (5+5). Immediately 25 seconds of Alacrity are applied to a target, and that total duration of 35 seconds is over the 30 second cap, resulting in 5 seconds of Alacrity being wasted.
- Slow has a max duration of 10 seconds: A 12 second slow is applied to a target. As Slow is capped at 10 seconds, the Slow is instantly set to ten seconds.
- 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. Damaging conditions can stack up to 1500 times. The boons Might and Stability as well as the condition Vulnerability are limited to a stack size of 25. For the cap of 25, longer duration stacks are able to overwrite shorter duration stacks while at the limit. 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 and will remove all stacks. 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 most other effects that increase movement speed. Boons are limited to a maximum of 30 seconds (swiftness 60 seconds), with the exception of Aegis and Regeneration. Conditions have a maximum of 10 seconds, with the exception of Fear.
- Aegis (max 9 stacks)
- Alacrity (max 30 seconds)
- Fury (max 30 seconds)
- Protection (max 30 seconds)
- Regeneration (max 5 stacks)
- Resistance (max 30 seconds)
- Resolution (max 30 seconds)
- Swiftness (max 60 seconds)
- Quickness (max 30 seconds)
- Vigor (max 30 seconds)
- Fear (max 5 stacks)
- Blinded (max 10 seconds)
- Chilled (max 10 seconds)
- Crippled (max 10 seconds)
- Immobile (max 10 seconds)
- Slow (max 10 seconds)
- Weakness (max 10 seconds)
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 (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 (!).
- The 2015-06-23 game update increased the maximum stacks for damaging conditions from 25 to 1500.
- The 2021-05-11 game update removed the stack limit on most duration stacking effects and set their maximum duration to 30 seconds for boons and 10 seconds for conditions.
- Swiftness was later increased to 60 seconds.