Mechanics: Cooldown Reduction

Cooldown Reduction (CDR) is an odd mechanic. Most stats are less effective as they are stacked, or at least cause other stats to become better investments past certain points. CDR alone becomes more effective the more of it you have.

This can be unintuitive initially, especially because of how information regarding CDR is presented in game. We see the results of CDR in the numerical values of our cooldowns, which follow this equation:

Cooldown = Base Cooldown * ( 1 - CDR% )

To the mathematically inclined this is obviously a linear equation. Each additional % of CDR will reduce the cooldown by the same amount as the last. A 10 second cooldown is reduced by 0.1 seconds per % of CDR whether you're at 0% or 39% CDR. At face value CDR appears to have linear worth.

But the value of CDR isn't determined by how much we reduce the cooldown, but by how often we can use the ability. That equation looks like this:


# of Ability Uses = Time Frame / ( Base Cooldown * ( 1 - CDR% ) )

This equation isn't linear at all. Even though the cooldown decreases linearly, as a divisor its effect on the equation is anything but. Consider a graph of the equation using our hypothetical 10 second cooldown:



Looking at the graph it becomes self-evident why Riot capped CDR at 40%. Past that point the increase in value accelerates to game-breaking proportions.

Even though CDR's effectiveness at 40% isn't as dramatic as at 100%, it's still incredibly valuable. Many champions hit a threshold at or near 40% CDR where their crowd control or other abilities chain together in a way that isn't possible otherwise.

For example, Olaf's Axe Throw has an 8 second cooldown which is reduced by 4.5 seconds if you pick up the axe. That leaves 3.5 seconds of downtime between axes. However, with 40% CDR the cooldown is only 4.8 seconds, leaving a mere 0.3 seconds between axes. At that point Olaf can perpetually throw axes as fast as his animation allows, provided he instantly picks them back up. Many Olafs abuse CDR for super-fast dragon kills and for decimating targets in their immediate vicinity.

Crowd control also increases in effectiveness dramatically with CDR. At maximum rank Rammus' Taunt lasts for 3 seconds with an 12 second cooldown, leaving 9 seconds of freedom for an opponent. With max CDR the cooldown is only 7.2 seconds, cutting that freedom by more than half to 4.2 seconds. Some crowd control effects, such as Zilean's Time Warp, practically become permanent with CDR.

Not all champions can or should aim for maximum CDR, but as a stat CDR is perhaps one of the most undervalued.