Calculating Challenge Rating

[GoatToucher]

I have played far more than I have DMed 3.5. Can somebody please explain how to calculate challenge rating?

Thank you.

[Random832]

For NPCs with class levels, it's generally equal to their character level; otherwise it's given in the monster entry. Templates have a CR adjustment (e.g. Lich +2 CR)

[Darcand]

I think he was more interested in knowing how to combine CRs to get a total ECL. The answer of course is that there is no exact formula to calculate ECL.
A few guide lines though. Everytime you double the CR of the monsters involved you increase the the ECL by 2. Anything that makes standard fighting more difficult and only affects the PCs (i.e. fighting an oger in a room that is permanently Silenced) increases the ECL by 1.

[Douglas]

I think he was more interested in knowing how to combine CRs to get a total ECL. The answer of course is that there is no exact formula to calculate ECL.
A few guide lines though. Everytime you double the CR of the monsters involved you increase the the ECL by 2. Anything that makes standard fighting more difficult and only affects the PCs (i.e. fighting an oger in a room that is permanently Silenced) increases the ECL by 1.

ECL is Effective Character Level. You meant EL, for Encounter Level. Also, it's doubling the number of monsters that equates to +2 EL, not doubling their CR.

The official rules are:
For a single monster, EL = CR.
For 2 encounters both of EL X, the combination has EL X+2.
For 2 encounters of EL X and X-2, the combination has EL X+1.
For creatures with CR below 1, just add until you get to 1. Kobolds are CR 1/4, so 4 kobolds are collectively EL 1.

Example: 6 monsters, with CR 1, 2, 3, 4, 5, and 6. 1+3 = EL 4, giving 2, 4, 4, 5, 6. 2+4 = EL 5, giving 4, 5, 5, 6. 4+6 = EL 7, giving 5, 5, 7. 5+5 = EL 7, giving 7, 7. 7+7 = EL 9. This group of monsters has an encounter level of 9.

Or you could use this nifty online encounter calculator which does all that for you and looks up XP rewards too.

[Darcand]

an even simpler way to do it is just to add up the total CR of all monsters involved and try to keep that around 25% of the party's total level.

[Douglas]

Except that it really doesn't work that way. At all.

It might seem reasonably close at low levels. As your party goes up in levels, it will start to break down pretty quickly, especially for encounters with several monsters, and that approximation is utterly worthless at high levels. Four CR 5 monsters is barely even a speed bump to a level 20 party, yet your suggestion would rate it as a good encounter for them.

[grautry]

Just go here.

[RTGoodman]

Just go here.

Heh, exactly. I came here to say just that, but you beat me to it, good sir.

[AslanCross]

Challenge Rating is supposed to represent a monster that will be Challenging (ie, consume approximately 25% of a party's resources) in an encounter. The party should be able to fight four EL X encounters per day, where X = the party's level. This assumes everyone is of the same level and that there are four of them.

Encounter level is not simple addition, but the Encounter Calculators can do that for you.

Note however that monsters of the same CR are not necessarily equal. Even the Monster Manual admits it is not an exact science. For example, a 10th level fighter and a Juvenile Red Dragon are both CR 10. A 10th-level party won't break into a sweat with the fighter even if he might have good equipment. However, a CR 10 dragon is potentially a very difficult enemy, and I'd think both players and DM would only want to run a single dragon per day.

Sometimes, NPC classes may be BETTER than PC classes. A CR 1 Human Warrior 2 is potentially better than a CR 1 Human Fighter 1 due to better saves and BAB. The Fighter gets ahead with its extra feat, but say, Power Attack isn't really going to matter much at that level .

Ideally a party should be able to fight a monster of CR X+4, where X is their party level. These should be the exception and not the norm, or else the party will either get wiped out or gain XP too quickly. However, I've seen a 6th-level party take out an 11th-level human warblade. Humanoid opponents get shafted by the CR system, since while classes are supposed to boost their abilities much faster than advancing HD on monsters, they're usually medium (read: easily mobbed) and they only get one action vs the party's 4 or more actions.

[ericgrau]

Sometimes, NPC classes may be BETTER than PC classes. A CR 1 Human Warrior 2 is potentially better than a CR 1 Human Fighter 1 due to better saves and BAB. The Fighter gets ahead with its extra feat, but say, Power Attack isn't really going to matter much at that level .

I understand your point, but in this specific example you forgot NPCs get elite stats and better wealth:

Round 1, fight!
orc warrior 2 (falchion 2d4 18-20/x2, splint mail +6 AC):
str 17, con 12, dex 11, wis 8
fort/ref/will saves: 4/0/-1
weapon focus (falchion)
6 AB, 10.35 avg dmg incl. crits (9*115%), 11 HP, 16 AC
5.18 avg DPR, 1.54 rounds to drop fighter 1 to -1

VS orc fighter 1 (masterwork falchion, banded mail +6 AC):
str 19, con 14, dex 13, wis 10
fort/ref/will saves: 4/1/0
weapon focus (falchion), power attack (included in stats below)
6 AB, 14.95 avg dmg incl. crits (13*115%), 7 HP, 17 AC
8.22 avg DPR, 1.46 rounds to drop warrior 2 to -1

Win goes to fighter by a nose!
I know, if I really wanted to optimize purely for levels 1-2, they'd both get toughness. That would have heavily favored the lower HP fighter.

[AslanCross]

Ah yes, I did forget Elite Array.

[Mando Knight]

Just go here.

However, a lot of the time, you can get just as good an estimate here.

[Overlord Nicy]

Best bet: Know the party's ability, then eyeball it.

[GoatToucher]

I'm not worried about scaling, but about xp awards.

The calculator was helpful. Thanks a bunch.