How Does Cooldown Reduction Stack in Diablo 3?

Cooldown Reduction is a key component to a number of builds in Diablo 3: Reaper of Souls, be it for maintaining constant Akarat’s Champion for Crusaders, or for ensuring you get the most out of your Raekor charge Barbarian. The trouble is, if you’ve ever tried to add up how much cooldown reduction you have, you may have noticed that your calculated number doesn’t match up to what your stat sheet says. Why is that?

Before we answer that, let’s go over what, exactly, cooldown reduction is. Some skills have cooldowns – a certain number of seconds that you have to wait before you can use the skill again. Cooldown reduction is a property that reduces that waiting time by a given percentage.

For example, you may have a weapon with 10% cooldown reduction. If you are using a skill with a 30 second cooldown, then you will only have to wait 27 seconds before using it again – 3 seconds, or 10%, fewer.

Now, what happens if you have a weapon with 10% cooldown redcution, and a ring with 5% cooldown reduction? Your total cooldown reduction should be 15%, right?

Not exactly. Cooldown reduction bonuses are not additive – they are multiplicative. What this means is that you cannot simply add them together to obtain your total – you need to apply a special formula, shown in the image, below. The total will actually come out to less than if you had simply added them together.

What this formula tells us is that our 10% and 5% cooldown reduction combine to give us 14.5% cooldown reduction. That’s only half a percent lower than if we had simply added the numbers; no big deal, right? Well, it becomes a big deal as you add more and more sources of cooldown reduction (CDR). For instance, if we add a second ring that gives us 5% CDR, then the total is 18.8%. That’s 1.2% less than the added total (10+5+5=20). As you continue to add sources of CDR, this discrepancy becomes larger and larger.

So if you’re following a build that tells you to aim for a certain amount of total CDR, or you’ve decided yourself to aim for a certain total CDR in order to achieve a desired effect, keep in mind that the sum of your individual CDR sources will actually have to be higher than your target. But don’t worry – you don’t have to run these calculations yourself to figure out what your actual target should be. Here’s a handy website that runs the math for you, factoring in the total CDR you can get for each gear slot, for each class, with various gear and skill bonuses.

For instance, if we visit that website, select the crusader, and hit “Enter Max,” we can see the maximum theoretical CDR that a crusader can achieve. Across item slots, you can have 10% CDR on your weapon, 8% CDR on your shield, gloves, shoulders, amulet, and two rings, and 12.5% with a diamond in your helm. If you have the legendary helm, Leoric’s Crown, rolled with perfect stats, then the diamond bonus is instead 25%. If you have the Born’s Command set items equipped, you gain an additional 10%, and if you combine that with the Captain Crimson’s Trimmings set, that’s an additional 10%. The Vigilante Belt legendary item can add another 8%, and by spending 50 paragon points in CDR, you can get another 10%. Lastly, factor in another 15% you can get with the legendary Gem, Gogok of Swiftness, and our sum total CDR is a whopping 136%! However, when plugged into the formula, we’re actually brought down to only 76.67% – we lose almost half of our CDR.

Don’t get me wrong – I don’t mean to say that you shouldn’t stack CDR. I just want to impress how difficult it is to get your total CDR number high up there, and how much you have to sacrifice to get there. But for some builds, it is completely worth it. Apart from the Crusader, which currently needs 100% uptime on Akarat’s Champion through cooldown reduction, there are a number of zDPS builds (zero damage per second builds, aka support builds) that don’t need to deal damage and benefit tremendously from ridiculously high CDR.

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CJ Miozzi is a Senior Editor at The Escapist and is also known as Rhykker on Youtube. You can follow his livestreams on Twitch and Tweet to him @Rhykker.