Cooldown and Resource Cost Reduction Explained

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Introduction

Cooldown Reduction (CDR) and Resource Cost Reduction (RCR) are utility stats in Diablo III. The former lets you use your skills more often while the latter makes them consume less resource.

You can find your current CDR and RCR values on the UI, CDR under offense section in details sheet and RCR in the resource section further below. Although some items give skill or element specific CDR or RCR and that is not displayed in any way.

This guide will cover all different sources of CDR and RCR and other similar effects, rules for their stacking, as well as different item and skill interactions.

Cooldown Reduction

CDR comes in two varieties: percentage based and flat Cooldown Reduction. The former is the most widespread one and it comes from a lot of different sources. Percentage CDR reduces the cooldown of your skills by the specified percentage:

$$\textrm{Cooldown} = (1 - \textrm{CDR} ) \cdot \textrm{Base Cooldown}$$

This CDR is the one that's displayed in your details sheet. On top of that, there's also CDR that only affects certain skills. Regardless if it's general or skill specific CDR, all separate sources of it stack multiplicatively with each other.

$$\textrm{CDR} = 1 - (1 - \textrm{CDR}_1) \cdot (1 - \textrm{CDR}_2)$$

This means that there's no diminishing returns to CDR and that each separate effect does exactly what it says on the tin: "Reduces cooldown of all skills by X%". For example two separate sources of 50% CDR will stack together giving us 75%. Which means that when we add the second source of CDR, the cooldown goes from 50% of the base value down to 25% i.e. becomes 2 times shorter.

Flat Cooldown Reduction is a lot more rare and usually only affects specific skills. Instead of reducing the cooldown by a portion of base value it just straight up subtracts a certain number of seconds from it. Flat CDR is calculated before percentage-based one:

$$\textrm{Cooldown} = (1 - \textrm{CDR} ) \cdot (\textrm{Base Cooldown} - \textrm{Flat CDR})$$

One exception to this rule is Witch Doctor passive Tribal Rites: it changes the base cooldown on the skill and is calculated before flat CDR.

There's a cap to CDR's effectiveness: neither percentage nor flat CDR can reduce the cooldown of a skill below 0.5s, meaning that above formula can be more accurately written as:

$$\textrm{Cooldown} = max \bigl ( 0.5; (1 - \textrm{CDR} ) \cdot (\textrm{Base Cooldown} - \textrm{Flat CDR}) \bigr )$$

One consequence of this cap is that when skill's cooldown is supposed to be reduced to 0 (e.g. In-Geom with 10s or less base cooldown skills or items such as Aether Walker and Frydehr's Wrath), it instead gets unreducible 0.5s cooldown.

CDR affects only your active skills; cooldowns on your passives (such as Spirit Vessel) and on your Potion always stay the same. For skills with charges, CDR affects charge cooldown instead. When you cast a skill, it goes on cooldown calculated with your CDR value at that very moment. This means that if your CDR changes after the skill is already on cooldown, that cooldown will not change. In other words, CDR is snapshot at the moment of cast. Some skills don't trigger their cooldown when they are cast but only after a certain delay (Serenity, Smoke Screen, Lut Socks, Cam's Rebuttal). In that case CDR is snapshot at the moment the cooldown is triggered.

Sources of Cooldown Reduction

General CDR:

• Paragon Offense Tab: 10%
• Rolls on Weapons: 10%
• Rolls on Gloves, Shoulders, Offhands, Jewelry: 8%
• Roll on Vigilante Belt: 8%
• Diamond in Helmet: 12.5%
  (25% with Leoric's Crown)
• Gogok of Swiftness: 15%
• Captain Crimson's Trimmings (2) Bonus: 20%
• Born's Command (3) Bonus: 10%
• Evocation: 20%
• Beacon of Ytar: 20%
• Fervor: 15%
• Decrepify Borrowed Time: 20%
• Empowered Shrine: 50%
• Channeling Pylon: 75%
• Red Soul Shard: 75%

The maximum possible general CDR is 83.77% (97.97% with Shrines), attainable only on Monk (Beacon of Ytar and Dual Wielding).

Skill Specific CDR:

There's only one source of general flat CDR currently in the game, In-Geom. For skill specific flat Cooldown Reduction there are a few options:

• Boon of Bul-Kathos
• Golemskin Breeches
• Last Breath
• The Oculus

Instantaneous Cooldown Reduction

Apart from described above persistent Cooldown Reduction effects, there's also instantaneous CDR which affects skills that are currently on cooldown. Only one such effect is percentage based - Blood is Power, all the others reduce ongoing cooldowns by flat amount. All these effects are calculated after all the persistent CDR, meaning that all flat instantaneous CDR effects become more powerful as we stack more persistent CDR.

These effects have no cap and if the skill's remaining cooldown is less than the magnitude of the effect at the time it triggers, then it's immediately set to 0 and the skill can be used again. Just like persistent Cooldown Reduction, instantaneous CDR effects can affect either all skills (or any random skill) or only certain specific skills:

In-Geom and Channeling Pylon give both instantaneous and persistent Cooldown Reduction. The former at the moment of activation and the latter from the buff. Leech Blood Flask is the only CDR-like effect in the game that affects your Potion.

Resource Cost Reduction

Resource Cost Reduction is a stat similar to Cooldown Reduction, using a similar formula as seen below.

$$\textrm{Cost} = (1 - \textrm{RCR} ) \cdot \textrm{Base Cost}$$

You can find it in same amounts on the same pieces of gear:

• Paragon Utility tab: 10%
• Rolls on Weapons: 10%
• Rolls on Gloves, Shoulders, Offhands, Jewelry: 8%
• Topaz in Helmet: 12.5%
  (25% with Leoric's Crown)

On top of that there's a handful of unique effects providing Resource Cost Reduction:

* Yang's Recurve has a unique RCR roll that provides between 40% and 50% RCR. On top of that it can also roll a normal 6-10% RCR same as any other weapon. These two rolls stack multiplicatively using normal rules and are displayed together as one roll, sometimes resulting in RCR with decimals in its value.

All these sources of general Resource Cost Reduction are displayed in your details sheet. Again, similar to Cooldown Reduction, there's also Skill Specific Resource Cost Reduction which are not displayed anywhere:

And one element-specific RCR source - Cindercoat for the Fire element.

All the different sources of Resource Cost Reduction stack multiplicatively:

$$\textrm{RCR} = 1 - (1 - \textrm{RCR}_1) \cdot (1 - \textrm{RCR}_2)$$

Flat Resource Cost Reduction also exists but it's limited to two wizard skills Storm Armor Power of the Storm and Diamond Skin Prism. Just like CDR flat RCR is calculated before the percentage-based:

$$\textrm{Cost} = (1 - \textrm{RCR} ) \cdot (\textrm{Base Cost} - \textrm{Flat RCR})$$

Finally, there's a cap to the effectiveness of Resource Cost Reduction. Indeed you cannot reduce the cost of any skill below 1 resource with RCR.

$$\textrm{Cost} = max \bigl ( 1; (1 - \textrm{RCR} ) \cdot (\textrm{Base Cost} - \textrm{Flat RCR}) \bigr )$$

Resource Cost Reduction only affects resource costs of the skills; neither charge costs or Necromancer's health costs are reduced by it. For Demon Hunters, RCR reduces both Hatred and Discipline costs, although there's Perfectionist passive which only affects the latter.

Resource Cost Reduction only reduces base costs; skills that read "consume all remaining resource" (e.g. Ancient Spear Boulder Toss or Meteor Star Pact) will still use all your resource even if you have 100% RCR. Although reducing the base cost of these skills will let more of your resource go towards the "consume all resource" effect, making it slightly stronger.

Apart from Resource Cost Reduction, there's also a bunch of effects that outright remove the resource cost (or all costs) of a certain skill or even all skills. For example Channeling Pylon, Kekegi's Unbreakable Spirit, Land of the Dead Invigoration. These effects are not displayed in your details sheet and don't count as RCR. One such effect that isn't always at 100% magnitude (although it can be) is Deadly Reach Strike From Beyond - reducing the cost of next Spirit Spender by 8% per enemy hit (stacks additively with itself and multiplicatively with normal Resource Cost Reduction).

Channeling Pylon in particular is an interesting effect because apart from removing your resource costs, it also removes charge cost of the skills that use charges like Dashing Strike. This lets you spam those skills freely.

Resource Generation

Resource generation always goes hand in hand with Resource Cost Reduction. After all, you can only spend as much resource as you generate. If you want to cast more spells there's two ways to go about it: by making you spells cost less (via RCR) or simply by generation more resource.

Most resource generating effects are pretty straightforward: they say "Gain X resource" or "Gain X resource per second" and that's exactly what they do. One stat here that deserves a special mention is increased resource generation - a very rare stat only found on a handful of items and skills:

• Hexing Pants of Mr. Yan
• Animosity
• The Guardian's Path (with 2h weapon)
• Inspire (for Monk, Barbarian and Necromancer)

These buffs stack additively with each other and affect all sources of resource generation, be it generator skills, passive regeneration or legendary effects.

Despite similar wording, Witch Doctor's Vision Quest only affects passive mana regeneration and stacks multiplicatively with Hexing Pants.

Captain Crimson's Trimmings

Captain Crimson's Trimmings (3) Bonus gives you damage boost equal to your CDR and damage reduction equal to your RCR. Effectively that makes RCR into a very strong defensive stat, while CDR becomes a decently strong offensive stat with very severe diminishing returns. This bonus only works with percentage-based general CDR and RCR, none of the flat and skill-specific RCR/CDR have any effect. Neither do any "remove all costs" (and Strike from Beyond) effects. You always get exactly what's written in your details sheet.

Each 8% RCR roll becomes an 8.7% boost to your toughness, which usually makes it the best defensive stat you can have on an item, only outshined by Vitality and Life % in situations where life pool vs mitigation concerns do not apply.

CDR as a damage stat is a lot weaker in comparison. For builds that don't stack any CDR and sit at about 40% in total, an additional 8% CDR roll translates into a 3.5% damage increase. For the builds that stack a lot of CDR (60%+), this bonus is even smaller - 2% or less.

Summary

Overall Resource Cost Reduction and Cooldown Reduction in Diablo III are not terribly complicated and behave fairly intuitively. To sum up all the important information:

• Flat Cooldown Reduction stacks additively, percentage multiplicatively.
• Flat Cooldown Reduction is calculated first before percentage.
• Cooldowns cannot be reduced below 0.5 seconds.
• Resource Costs cannot be reduced below 1 resource.

Credits

Written by Northwar.