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
Leech mechanics in The Awakening
In the Awakening expansion the leech mechanics was changed with the goal to make it harder to leech against a single enemy. Earlier the leech recovered at a constant rate, this method does not take into account how many enemies that were hit by the skill. The leech mechanic was therefore changed to vary how fast the leech recovered depending on how many enemies were hit; with more targets hit by the skill the faster the character healed from the leech.
The total amount of life or mana leeched without Vaal Pact can be described as
 HP(t) = r * t
where r is the leech recovery rate and t is the time. The leech recovery rate, r, varies depending on how many enemies have been hit and how many times the enemies have been hit. ^{[1]}
Recovery rate
The leech recovery rate r is given by
 r = p * n * q * (1 + m)
where p is the character's maximum life (for life leech) or mana (for mana leech), n is the number of active leech instances, q is the Base Leech Rate and m is the sum of all the "X% increased (or reduced) Life/Mana Leeched per second" modifiers.
There is a upper limit of the leech recovery rate which is defined as
 r_{max} = p * (q_{max} + m_{max})
where q_{max} is the Base Maximum Leech Rate and m_{max} is the sum of all the "+X% of maximum Life/Mana per second to maximum Life/Mana Leech rate" modifiers.
If r > r_{max} those values will be substituted with r_{max}. The total amount of active leech instances allowed, n_{tot}, can be solved by
 r = r_{max}
 p * n_{tot} * q * (1 + m) = p * (q_{max} + m_{max})
solving for n_{tot} gives
 n_{tot} = (q_{max} + m_{max}) / (q * (1 + m))
Modifiers that alters the leech recovery rate increases the value of Life/Mana you're gaining per second for each leech effect. However, it does not affect the duration, t_{dur}, of the leech effects, meaning that more total life/mana will be gained from each leech instance as a result.^{[2]} This also implies that by increasing the Life/Mana Leeched per seconds gives a larger total of life leeched compared to Vaal Pact.
There are only a couple of ways for these factors to be altered, besides the passive tree:^{[3]}^{[2]}
 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.
 Vorici's master modifier also offers "X% increased Life Leeched per second".
The table below displays values of the base leech rates.^{[2]}
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} 
Leech duration
The leech recovery duration in seconds is given by
 t_{dur} = d * l / r_{0}
where d is the damage inflicted on the enemy, l is the total amount of leech and
 r_{0} = p * n * q
r_{0} is the recovery rate without any modifiers. If r_{0} > r_{0max} those values will be substituted with r_{max} and
 r_{0max} = p * q_{max}
To be able to sustain the max recovery rate, r_{max}, against a single target the total amount of instances allowed needs to be active at all times, n_{tot}. The duration with n_{tot} instances is
 t_{max} = d * l / r_{0max}
The attack time between each hit, t_{att}, needs to be
 t_{att} ≤ t_{max} / n_{tot}
or
 t_{att} ≤ d * l * q * (1 + m)^2 / (p * (q_{max} + m_{max})^2)
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 to enemy
 l = 1 % life leech
1 Enemy hit
 n = 1 leech instances/Enemies
 m = 0% increased Life Leeched per second
 m_{max} = 0% of maximum Life per second to maximum Life Leech rate
The recovery rate is:
 r = n * p * q * (1 + m) = 1 * 5000 * 0.02 * (1 + 0) = 100 HP/sec
Make sure that the recovery rate doesn't exceed the maximum recovery rate which is
 r_{max} = p * (q_{max} + m_{max}) = 5000 * (0.2 + 0) = 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_{dur} = d * l / r_{0} = 1000 * 0.01 / (1 * 5000 * 0.02) = 0.1 seconds
which gives the total life leeched
 HP(t_{dur}) = r * t_{dur} = 100 * 0.1 = 10 Life
5 enemies hit at once
 n = 5 leech instances/Enemies
 m = 0% increased Life Leeched per second
 m_{max} = 0% of maximum Life per second to maximum Life Leech rate
The recovery rate is:
 r = n * p * q * (1 + m) = 5 * 5000 * 0.02 * (1 + 0) = 500 HP/sec
Make sure that the recovery rate doesn't exceed the maximum recovery rate which is
 r_{max} = p * (q_{max} + m_{max}) = 5000 * (0.2 + 0) = 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_{dur} = d * l / r_{0} = 1000 * 0.01 / (5 * 5000 * 0.02) = 0.02 seconds
which gives the total life leeched
 HP(t_{dur}) = r * t_{dur} = 500 * 0.02 = 10 Life
11 enemies hit at once
 n = 11 leech instances/Enemies
 m = 0% increased Life Leeched per second
 m_{max} = 0% of maximum Life per second to maximum Life Leech rate
The recovery rate is:
 r = n * p * q * (1 + m) = 11 * 5000 * 0.02 * (1 + 0) = 1100 HP/sec
Make sure that the recovery rate doesn't exceed the maximum recovery rate which is
 r_{max} = p * (q_{max} + m_{max}) = 5000 * (0.2 + 0) = 1000 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_{dur} = d * l / r_{0max} = 1000 * 0.01 / (5000 * 0.2) = 0.01 seconds
which gives the total life leeched
 HP(t_{dur}) = r * t_{dur} = 1000 * 0.01 = 10 Life
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 being life leeched from by enemies through allocating the Bloodless notable passive skill.
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
 ↑ GGG_Mark (June 20, 2015). "Vitality Void: Leech Cluster Thread". Path of Exile subreddit. Retrieved July 20, 2015.
 ↑ ^{2.0} ^{2.1} ^{2.2} GGG_Neon (May 18, 2015). "Upcoming changes to Leech Thread". Path of Exile Forum. Retrieved July 20, 2015.
 ↑ Mark_GGG (March 4, 2014). "Leech: GGG clarify please!". Official Path of Exile Forums. Retrieved March 6, 2014.
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