Tenacity

 Tenacity (or Crowd Control Reduction) is a stat that reduces the duration of all incoming  crowd control effects except  suppression,  nearsight,  stasis,  displacements, and  drowsy. The stat is the percentage by which the duration is reduced. This reduction is limited, the duration can never drop below 0.5 seconds. The duration of the crowd control is calculated at the time of application and does not change if the tenacity changes.

Increasing tenacity

Tenacity can be increased with champion abilities, items, runes, and summoner spells, stacking both  multiplicatively and  additively depending on the combination of the effects.

Items

• Mercury's Treads
• Sterak's Gage

Elixirs

• Elixir of Iron

Champion abilities

• Dr. Mundo's Burning Agony
• Garen's Courage

Runes

• Legend: Tenacity
• Unflinching

Summoner Spells

• Cleanse

Game Modes

•  Ultra Rapid Fire

Stacking

These are the combinations that stack additively:

• Elixir of Iron with Unflinching's 10%/20% bonus
• Mercury's Treads with Sterak's Gage
• Burning Agony / Courage with  Brittle and Unflinching's 15% bonus

All the other combinations stack multiplicatively.

Reducing Tenacity

Tenacity can be reduced to negative values, causing crowd control to last longer.

Champion Abilities

• Ornn's Bellows Breath and Call of the Forge God apply  Brittle, a unique effect that also increases  Knockup duration.

Trivia

• Until patch V1.0.0.118, Tenacity was an unnamed effect unique to Mercury's Treads. That patch promoted the effect to a champion statistic and added three new items: Cloak and Dagger (later removed), Eleisa's Miracle (later removed) and Moonflair Spellblade. Tenacity from multiple sources did not stack, similar to Enhanced Movement. In Season Three there was an item overhaul, named item effects were introduced, and Tenacity was reverted to an item effect. In V3.01, all sources of crowd control reduction were changed to say "Tenacity" and Tenacity was reintroduced as a champion statistic.
• Until Season Three, Tenacity's in-game icon was the same as the icon used for Cho'Gath's Vorpal Spikes and Baron Nashor's passive. Between the introduction of season 3 changes and V3.01, it shared the same icon as the Tenacious mastery. As of V3.01, it no longer has an icon in the buff bar.

Last updated: November 20, 2019, patch V9.23

• The biggest amount of Tenacity you can obtain is 98.652% with Garen.
  • Courage + Unflinching (+75% tenacity)
  • Elixir of Iron + Unflinching (+45% tenacity)
  • Legend: Tenacity (+30% tenacity)
  • Mercury's Treads + Sterak's Gage (+60% tenacity)
  • Cleanse (+65% tenacity)

1 - ((1 - ( 0.6 + 0.15)) × (1 - ( 0.25 + 0.2)) × (1 - 0.3) × (1 - ( 0.3 + 0.3)) × (1 - 0.65) = 

1 - (0.25 × 0.55 × 0.7 × 0.4 × 0.35) = 98.652%

• With this amount of tenacity, Nasus' Wither (longest crowd control in the game) would only last 0.067 seconds. The minimum crowd control duration prevents this.
  • In comparison, Nasus' Wither would last 6.5 seconds if the target has no tenacity and is  Brittled.
• Note that this amount of tenacity can only be achieved briefly with Courage wearing off almost immediately, and with Unflinching and Sterak's Gage which grant a temporary buff.
• With the Ultra Rapid Fire buff enabled, the maximum tenacity achievable is 98.989%.

1 - (0.25 × 0.55 × 0.7 × 0.4 × 0.35 × 0.75) = 98.989%

Champion statistics

Offensive	Ability power ·  Attack damage ·  Attack speed ·  Critical strike chance ·  Critical strike damage ·  Armor penetration ·  Magic penetration ·  Life steal ·  Spell vamp ·  True damage
Defensive	Heal and shield power ·  Health ·  Health regeneration ·  Armor ·  Magic resistance ·  Tenacity ·  Slow resist
Utility	Cooldown reduction ·  Energy ·  Energy regeneration ·  Mana ·  Mana regeneration
Other	Experience ·  Gold generation ·  Movement speed ·  Range