Hi folks,
I'm pretty new to GMing, and I have been having the hardest time figuring out how to calculate the CR of an encounter with multiple creatures. I swear I saw something in one of the books on how to figure it out, but I haven't been able to find it again. I would appreciate any insight or advice on this.
Thanks!
As a general rule of thumb, when dealing with creatures that are individually CR 1 and higher, two creatures of a given CR are equivalent to a single creature that is two CRs higher. So, two CR 1 monsters = one CR 3 monster; two CR 2 monsters = one CR 4 monster; etc.
When combining creatures of multiple CRs in a single encounter, you need to compare the total XP value of all the creatures combined to the CR chart. Whichever number you are closest to, rounding normally, is the CR range that encounter falls within.
For example, say I want to create an encounter using three dire bats led by a werebat. The werebat is CR 3 (worth 800 XP), and each dire bat is CR 2 (worth 600 XP). 800 + (3 x 600) = 2,600 XP.
Examining the "XP Rewards by CR" table (not sure where this is, just working off monster CRs here), I see that a CR 6 monster is worth 2,400 XP, whereas a CR 7 monster is worth 3,200 XP. Our little cadre of bats is closer to the CR 6 benchmark than the CR 7 one, so we'd consider this group of creatures a CR 6 encounter.
Did that help? =]