Effect stacking refers to the how an effect increments upon application. An effect may stack in duration or in intensity, where the intensity is indicated by the number in the bottom right of the effect icon. Players encounter effect stacking often due to boons and conditions, which both have effects that stack in duration and intensity. Effects that have charges, such as conjure weapon effects, aren't generally considered stacking if the effect only decrements.
Effects that stack in duration typically follow a simple queue where multiple applications are cumulative. For effects that depend on other values, such as Retaliation, the effect with greatest intensity takes priority. Most effects have a stacking limit, where the duration of the effect can't increase beyond a certain point. This limit depends on each source, as multiple sources can stack the effect for longer than a single source, such as the player alone. For a majority of effects players encounter, the effects will simply stack in duration.
The less common version, effects stacking in intensity increment in both duration and degree. The degree is indicated in the number of stacks in the bottom right corner of the effect icon. The duration indicated in an intensity-stacking effect is the source with the greatest duration. Effects that stack in intensity are limited to 25 stacks rather than duration. The sources with the greatest intensity and number of stacks will take priority in calculating damage. Removing effects, usually with condition and boon removal, will ignore the intensity of the effect.
 List of intensity-stacking effects
Each stack applies its own intensity for its listed duration, independently from the others, except if the condition is removed as a whole.
- 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.
10 seconds of bleeding are applied to a target. 1 second later, another 6 seconds of bleeding are 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.
 List of duration-stacking effects
The effect from the source with the greatest intensity will be used for calculations for the duration of the effect. If there are two unique sources of Burning, the effect with the greater condition damage value will be used until that source ends, at which point, the intensity from the second source is used for the remaining duration. Similarly, swiftness does not stack intensity with other effects that increase movement speed. Boons are limited to a maximum of 9 stacks, with the exceptions of Retaliation and Stability. The duration of each stack is irrelevant.
- Retaliation (max 5 stacks)
- Stability (max 3 stacks)
- 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, then 317.5 health per second for the next 5 seconds.
- A Guardian stacks swiftness by repeatedly casting Symbol of Swiftness. Once 9 stacks of swiftness have been applied, no further sources of swiftness can be given until at least one stack has expired. Symbol of Swiftness applies 4 seconds of swiftness per tick; 9x4 = 36 seconds of swiftness. Until one of those stacks is gone, at 32 seconds, other sources of swiftness, such as Save Yourselves will not apply any additional duration.
- Damaging effects (retribution, bleeding, poison, burning, and confusion) of a specific source will update in intensity if the source's stats change by various means (e.g might).
- However, same does not apply to Regeneration. Using Banner of Tactics will not update the next healing ticks from regeneration if banner was applied after the regeneration was.
- 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.