Challenge Ratings and Encounter Level
(originally posted at the RPG Stack Exchange)

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:
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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