Leech
Life leech restores over time an amount of life relative to damage inflicted on an enemy. Likewise, mana leech restores mana in the same way.
Contents
PreAwakening Mechanics
The total amount of life or mana leeched is a percentage of the damage inflicted and is restored over time. This is different from gaining life or mana on hit, which restores a certain amount instantly.
Recovery rate
The leech recovery rate r is given by
 r = p * q
where p is the character's maximum life (for life leech) or mana (for mana leech), and q is the recovery rate factor including any modifiers.
Life leech has a fixed recovery rate equal to 20% of the character's maximum life per second.^{[1]} Similarly, mana leech has a fixed recovery rate equal to 20% of the character's maximum mana per second.^{[2]}
The leech recovery rate factor, q, is therefore 0.2 for life and mana. There are only a couple of ways for this factor to be altered:^{[1]}
 The of Smothering modifier for maps slows life and mana leech recovery rate by 50%.
 For life leech by a skill supported by the Life Leech gem, the recovery rate is increased by 1% for each point of quality.
 Vorici's master modifier also offers increased life leech rate.
Example
If the character has 1,000 maximum life and leeches life with the default recovery rate factor of 0.2, that life leech will always restore 200 life per second.
 p = 1000
 q = 0.2
 r = 1000 * 0.2 = 200
Recovery duration
The leech recovery duration in seconds t is given by
 t = l / r
where l is the total leech amount, and r is the recovery rate.
Example
If the character has 1,000 maximum life and lands a single hit that would leech a total of 600 life with the default recovery rate factor of 0.2, it would take 3.0 seconds for that amount of life to be restored.
 3.0 = 600 / (1000 * 0.2)
Example with successive single hits
Assume the character has 1,000 maximum life and lands a single hit that would leech a total of 300 life with the default recovery rate factor, then after 0.5 seconds the character lands a single hit that would leech a total of 600 life.
Using the formula for recovery duration, the first leech effect would last 1.5 seconds, and the second would last 3.0 seconds. The second leech effect starts 0.5 seconds after the first, so the total duration that any leech recovery would take place is 3.5 seconds. Both of these leech effects have the same recovery rate of 200 life restored per second, which means neither leech effect takes precedence over the other. Because only one leech effect can restore life at any given moment, a total of 700 life is restored over 3.5 seconds.
 3.5 * 200 = 700
Example with an area of effect hit
Assume the character has 1,000 maximum life and lands an area of effect hit that would leech a total of 600 life with the default recovery rate factor, and that four enemies are hit simultaneously.
Using the formula for recovery duration, each leech effect would last 3.0 seconds. All four leech effects are applied at the same time, so the total recovery duration is 3.0 seconds. Therefore, 600 life is restored over 3.0 seconds.
 3.0 * 200 = 600
This demonstrates that the total amount of leech recovery is not affected by the number of enemies hit.
Example with differing recovery rates
Assume the character has 1,000 maximum life and lands a single hit that would leech a total of 800 life with the default recovery rate factor. After 1.0 second the character uses a skill supported by a quality Life Leech gem and lands a single hit that would leech a total of 400 life with a recovery rate factor of 0.24. Using the formula for recovery duration, the first leech effect would last 4.0 seconds, and the second would last around 1.7 seconds. The second leech effect starts 1.0 second after the first, and ends before the first ends. The total duration that any leech recovery would take place is 4.0 seconds, however the second leech effect has a higher recovery rate and therefore takes precedence during its duration over the first leech effect. As a result, 200 life is restored over 1.0 second, followed by 408 life over 1.7 seconds, and finally 260 life over 1.3 seconds for a grand total of 868 life over 4.0 seconds.
 (1.0 * 200) + (1.7 * 240) + (1.3 * 200) = 200 + 408 + 260 = 868
Leech mechanics in The Awakening
The total amount of life or mana leeched is a percentage of the damage inflicted and is restored over time.
 d * l = r * t
where d is the damage inflicted on the enemy, l is the total amount of leech, r is the leech recovery rate and t is the time. The life leech recovery rate varies depending on how many enemies have been hit and how many times the enemies have been hit. ^{[3]}
Recovery rate
The leech recovery rate r is given by
 r = n * p * q
where n is the number of active leech instances, p is the character's maximum life (for life leech) or mana (for mana leech) and q is the Base Leech Rate including modifiers. There is a upper limit of the leech recovery rate which is defined as
 r_{max} = p * q_{max}
where q_{max} is the Base Maximum Leech Rate including modifiers. If r > r_{max} those values will be substituted with r_{max}. The upper limit can be reached by hitting at least 1/q_{max} enemies at once or hitting 1 enemy quick enough that so many leech instances are active at the same time.
The table below displays values of the base leech rates.^{[4]}
Symbol  Value  Description  Unit 

q  2 % 2 % 
Base Life Leech Rate Base Mana Leech Rate 
s^{1} s^{1 } 
q_{max}  20 % 20 % 
Base Maximum Life Leech Rate Base Maximum Mana Leech Rate 
s^{1} s^{1} 
Recovery rate modifiers
Modifiers that alters the leech recovery rate are called "X% increased (or reduced) Life/Mana Leeched per second", this increases the value of Life/Mana you're gaining per second for each leech effect. However, it does not affect the duration, t, of the leech effects, meaning that more total life/mana will be gained from each leech instance as a result.^{[4]}
There are only a couple of ways for these factors to be altered:^{[1]}^{[4]}
 The of Smothering modifier for maps slows life and mana leech recovery rate by 50%.
 For life leech by a skill supported by the Life Leech gem.
 From the passive tree.
 Vorici's master modifier also offers "X% increased Life per second".
Note that the only way to increase the Base Life Leech Rate, q, is to do so with the affix "X% increased Life Leeched per second". You can't stack it with Life Leech and passives for instance.
Leech duration
The leech recovery duration in seconds t is given by
 t = d * l / r
Examples
When using skills such as Dual Strike linked with Melee Splash several enemies are hit at once, the leech rate will vary considerably depending on how many enemies are hit. The character used in the example has the following stats:
 p = 5000 maximum life
 d = 1000 damage
 l = 2 % life leech
The total amount of life leeched the character will get with those stats are:
 d * l = 1000 * 0.02 = 20 HP
1 Enemy hit
 n = 1 leech instances/Enemies
The recovery rate is:
 r = n * p * q = 1 * 5000 * 0.02 = 100 HP/sec
Make sure that the recovery rate doesn't exceed the maximum recovery rate which is
 r_{max} = p * q_{max} = 5000 * 0.2 = 1000 HP/sec
The limit has not been exceeded, meaning that no substitution is necessary.
The time it takes to recover the total amount of life leeched is
 t = d * l / r = 1000 * 0.02 / 100 = 0.2 seconds
5 enemies hit at once
 n = 5 leech instances/Enemies
The recovery rate is:
 r = n * p * q = 5 * 5000 * 0.02 = 500 HP/sec
The limit has not been exceeded with this many enemies hit at once, meaning that no substitution is necessary.
The time it takes to recover the total amount of life leeched is
 t = d * l / r = 1000 * 0.02 / 500 = 0.04 seconds
11 enemies hit at once
 n = 11 leech instances/Enemies
The recovery rate is:
 r = n * p * q = 11 * 5000 * 0.02 = 1100 HP/sec
The limit has been exceeded, therefore the recovery rate needs to be changed to r = 1000 HP/sec.
The time it takes to recover the total amount of life leeched is
 t = d * l / r = 1000 * 0.02 / 1000 = 0.02 seconds
Sources of leech
Sources of leech include support gems, passive skills, and modifiers on equipment.


These keystone passive are related, but don't add any life leech as a percentage of physical damage.
The Vaal Pact Keystone will allow life leech to apply immediately at the cost of losing life regeneration. Ghost Reaver applies life leech to energy shield instead of life.
Name  Bonuses 

Vaal Pact 

Ghost Reaver 

Immunity to leech
A player can become immune to leech by allocating specific passive skills.
Name  Bonuses 

Mental Fortitude 

Bloodless 

Immunity can also be achieved by equipping certain unique items.
Name  Modifiers 

Sin Trek  +(20 to 30) to Dexterity +(20 to 30) to Intelligence (80 to 100)% increased Evasion Rating +(70 to 100) to maximum Energy Shield 30% increased Movement Speed Enemies Cannot Leech Life From You 
Mindspiral  (10 to 15)% increased Cold Damage (10 to 15)% increased Lightning Damage +(100 to 150) to maximum Mana Enemies Cannot Leech Mana From You (5 to 10)% of Damage taken Gained as Mana when Hit Cannot Leech Mana 
Deidbell  +(20 to 30) to Strength +(20 to 30) to Dexterity +(200 to 300) to Armour 20% increased Melee Damage Cannot Leech when on Low Life 
References
 ↑ ^{1.0} ^{1.1} ^{1.2} Mark_GGG (March 4, 2014). "Leech: GGG clarify please!". Official Path of Exile Forums. Retrieved March 6, 2014.
 ↑ Malice (November 13, 2011). "Mechanics Thread". Path of Exile Forum. Retrieved March 8, 2014.
 ↑ GGG_Mark (June 20, 2015). "Vitality Void: Leech Cluster Thread". Path of Exile subreddit. Retrieved July 20, 2015.
 ↑ ^{4.0} ^{4.1} ^{4.2} GGG_Neon (May 18, 2015). "Upcoming changes to Leech Thread". Path of Exile Forum. Retrieved July 20, 2015.