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Are there standard rules for calculating CR of groups? For example, in my Pathfinder bestiary, a single Stirge is listed as CR 1/2, and a group of 2d4 (average 5) of them is listed as an average CR 4.

Also, how do you scale CRs? I'm aware of the rule of thumb that says:

A party of 5 players at level 9 should be able to face 5 challenges of rating 9 in a day.

But how do you scale that to different levels? How many CR 4 challenges is suitable for a level 6 party?

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3 Answers

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The Challenge Rating of a monster is a very useful guide for judging the difficulty of an encounter, but it is not an exact science. Experience with your own players will tell you how tough an encounter they can handle, and what kinds of encounters they are best at.

When the entire party can gang up on a single monster (even one with multiple attacks) tactics on the battlefield can be less important than when they are outnumbered. However, the more monsters you have, the less certain the estimate of Encounter Level (EL) / "CR Equivalency" becomes. You should also be careful when advancing monsters (adding extra hit dice to increase their CR), as this too can lead to some nasty surprises.

I find this online calculator to be very useful for estimating EL, especially when I'm using monsters with a range of different CR's in a single encounter: http://www.d20srd.org/extras/d20encountercalculator/

As far as multiple encounters per day are concerned, this too depends on a number of things. A fighter or a warlock can likely go for longer than a wizard, but even they likely have limited use magic items (particularly healing potions) that they will run out of eventually. When they are near a settlement where they can go shopping and are likely to get an uninterrupted 8 hours of rest, it isn't a problem. However, when they're deep in the Underdark, surrounded by enemies, things can get a bit tough.

Rather than having multiple EL 4 encounters in a row, you should aim for variety. Table 3-2 Encounter Difficulty, in the 3.5 edition DMG, recommends the following:

10% easy (EL lower than party level)
20% "easy if handled properly" (whatever that means...)
50% challenging (EL equals Average Party Level)
15% very difficult (EL 1-4 higher)
5% overwhelming (EL 5+ higher)

In Pathfinder, the range of encounters is a little tighter:

Easy (APL –1)
Average (=APL)
Challenging (APL +1)
Hard (APL +2)
Epic (APL +3)
(APL = Average Party Level)

However, I think that the advice still holds: don't aim to make every enounter "average". Now and then, throw something "epic" at them to keep 'em from getting too cocky. You should drop some pretty heavy hints, though, that they need to run away to avoid a TPK (especially the first time you do it). In a level-based game like D&D, it's always good to have something to strive for.

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That calculator is very helpful. I know raw numbers aren't the complete answer, but having them to hand makes it much easier. –  Marcus Downing Nov 14 '10 at 0:14
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The way the D&D power curve is that encounters too far under your APL do not deplete your abilities at all. The "5 encounter rule" is based on an average of 20% depletion of spells/hit points/whatnot. So really an APL 6 party can fight a near infinite number of CR3 encounters (at least until boredom sets in).

You can use the normal "CR math" in terms of total amount to toss in, like 2 CR4s = a CR6 and use that to gauge off, but once you get more than APL+2 the answer is "one" and once you go below APL-2 the answer is "very many."

Oh, and for the standard CR math, basically double the number of creatures means CR+2. A creature with a sidekick of CR-1 means the encounter is CR+1.

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Be sure to remember that CR and EL are not the same thing. CR is a property of a creature. EL is a property of a group of creatures and the environment you face off against them in. Creatures on their home turf getting bonuses for that make for a higher EL, and creatures who are out of their element reduce the EL (usually just by +/- 1). For a single creature, CR = EL. For groups of creatures, EL can be calculated using the following rules of thumb:

  • Two things of the same CR have an EL of the CR+2. This can be repeated again, and sometimes a third time, before the players are too far ahead to be threatened by whatever is showing up in a huge group.
  • Fractional CR creatures are intended to be added together until you get a full CR 1, and then treat that "squad" as a single unit at that point. 3 dogs = 2 orcs = 4 kobolds = 1 gnoll = 2 goblins and 1 dog.

So yeah, two Stirge is EL 1, four of them is EL 3, and 5 of them is pushing into EL 4.

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I think you mean EL and there's no such thing in Pathfinder. –  mxyzplk Nov 12 '10 at 0:42
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Ah, yes, EL not ECL, my mistake. Write it wrong once and keep making the same mistake the whole way through, silly me. As to the idea that "EL doesn't exist in Pathfinder", well that's just absurd. Pathfinder is based on 3.5 with a million little edits. There aren't enough changes to the fundamental math to affect how the CR system works (that would break backwards compatibility), and so the same rules apply to all of 3.0/3.5/PF. –  Lokathor Nov 12 '10 at 6:06
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