What is Ignite?
Ignite is a Damaging Elemental Ailment that deals Fire Damage over time by default, and is typically inflicted by a Hit. The base Damage dealt by Ignite only accounts for the total Fire Damage in a Hit. Ignite does not stack by default and only the highest Damage Ignite on a target will deal its Damage. However, all inflicted Ignites will be present on a target and will remain for their full Duration, with the next highest Damage Ignite dealing its Damage once the current one expires.
Chance to Ignite
Any Fire Damage in a Hit has a chance to inflict Ignite. However, the chance to inflict Ignite is based on the total Fire Damage of a Hit relative to the target's Ailment Threshold. For the majority of monsters, their Ailment Threshold is equal to their maximum Life, with the exception of stronger monsters such as some Rares and all Unique bosses. Stronger monsters have their Ailment Threshold adjusted to allow players to inflict Ailments on them. By default, the Fire Damage in a Hit has 1% chance to inflict Ignite per 4% of the target's Ailment Threshold dealt by that Hit, calculated pre-mitigation.
Modifiers that grant an increased chance to Ignite do not apply a base chance, and instead directly scale the existing chance for a Hit to Ignite based on the Fire Damage relative to the target's Ailment Threshold. For example, if the Fire Damage in a Hit dealt Damage equal to 80% of the target's Ailment Threshold, it would have a 20% base chance to inflict Ignite. If the character had 50% increased chance to Ignite, that Hit would have 30% chance to inflict Ignite.
Some Skills or Unique items have special modifiers that interact with Ignite chance, such as
Perfect Strike which has the modifier 'Always Ignites'. This modifier bypasses the chance to Ignite calculation, but can still only Ignite with Fire Damage by default.
Magnitude of Ignite
The default Magnitude of Ignite is 20% of the Fire Damage of an Igniting Hit, dealt as Fire Damage per second. Modifiers to the Magnitude of Ignite, such as 'increased Magnitude of Ignite you inflict', are multipliers to the base Magnitude. For example with '25% increased Magnitude of Ignite you inflict', your Ignites will deal 25% of the Fire Damage of an Igniting Hit per second (20% * 1.25 = 25%). Note: modifiers to Magnitude do not affect Ignite Duration.
Calculating the Damage of Ignite
The Damage dealt by Ignite only accounts for the total Fire Damage in an Igniting Hit by default. The final Fire Damage dealt by a Hit prior to the target's mitigation will determine how much Damage is dealt per second. This is inclusive of any relevant Damage modifiers you have such as 'increased Fire Damage', or if the Hit was a Critical Hit, but does not include any modifiers that specifically apply to how much Damage the target takes from the Hit, such as Penetration.
Duration of Ignite
The default Duration of Ignite is 4 seconds and can be increased with modifiers to Ailment duration such as 'increased Ignite Duration on Enemies' or 'increased Duration of Damaging Ailments on Enemies'. Increasing the Duration of Ignite will improve the total Damage dealt across the full duration of the Ailment, but not the Damage dealt per second. Modifiers that make an Ailment deal Damage faster will cause the Ailment to deal its total Damage over a shorter period of time, essentially increasing the Damage dealt per second.