How to calculate CR

[Titanium Fox]

Players in my PbP, go away. Seriously, don't read this. >>

I'm trying to actually give balanced fights, and it's the one thing I suck at as a DM. I can put together all of the fluff, create NPCs, weave stories, all of that. However, when it comes to figuring out CR, or even what CR the party should be going up against, I'm usually at a loss. Either I hold back too much or go WAY overboard and dropping the party down to 1 member left standing. It makes it interesting, and it's good to know how to do that, yes, but I'd like to actually be able to do something in the middle.

So, for example, in my current PbP I've got a party of 4, all at level 10. Swift Hunter/Warblade, Warblade, Druid, Fighter. For the purposes of this fight, they'll also have a little bit of back up (namely three extra rangers, all around level 5 - 6).

I've figured out the enemies I want to throw at them. But seriously, how do I figure out what level each should be to put a challenge to the party? (Tentative levels are in parentheses) I have a pair of Swift Hunters (9), a Wizard (8), a Cleric (8) and a Crusader (10) to pit them against. Is that anywhere around what I should be shooting for? And like I said, how did you figure that out? I'd kill to know how to figure this out myself in the future.

[The Glyphstone]

Well, officially there is a formula, and you can automate it with this Encounter Calculator:

http://www.penpaperpixel.org/tools/d20encountercalculator.htm

For your example, that's a CR14 encounter against an average PL of 10.5, so Very Difficult.

Unofficially, CR is guesswork at the best of times. If CR is unreliable, sometimes you have to go into the grit and manually compare your party's damage output/survivability to various monster stat blocks before you choose them for an encounter.

[Lord Loss]

The fact that each party is good at different things and worse at others, combined with the fact that WotC can't judge a creature's CR for their life (MMII, I'm looking at you), combined with the fact that their formula doesn't work (10 CR 1 creatures will not challenge your average ECL 10 Party, no matter what the EL Calculator says) turns CR evaluation into a game of "guess and check", except with more TPKs.

EDIT: Ninja'd

[Cicciograna]

Vorpal Tribble made up a pretty nifty system to calculate the correct CR of any creature. You can find it here:
http://www.giantitp.com/forums/showthread.php?t=192329

[137beth]

Wait, is this for figuring out the CR, or the encounter level? For the encounter level, I actually don't think the official formula is all that bad, it just can't be abused (in fact, the DMG SPECIFICALLY says not to use monsters 8 levels below the players, regardless of what the formula says the EL is.)

As for the CR of an individual monster, this is substantially harder. Vorpal Tribble's method is fairly good, though it requires you to set appropriate save DCs, saving throws, attack rolls, and damage. You can start with vorpal tribble's method, then adjust your monster based on the guidelines in Savage Species for estimating CR (these guidelines are good for tweaking the CR if you already know the approximate range. WotC does not provide a good method for finding said approximate range, so we for that we must turn to vorpal tribble).

Once you've done all that, playtest your monster.

[Urpriest]

If you want to know the official way to calculate these things, the guide in my sig is updated past the point that should be relevant for the encounter your describe. Go to the Improving Monsters section.

[Psyren]

Well, first you look at the monster or monsters, then you look at the capabilities of the party. Then you close your eyes, spin in a circle until you fall down, and count the spots floating in your vision. This is called "the WotC method."

Or you can follow Ciccio's link, but that's not nearly as fun.

[Flickerdart]

Remember, though, that a level-appropriate encounter is supposed to be, essentially, a curb-stomp - players are only expected to expend 20% of their daily resources. A CR+2 or 3 can be a pretty serious challenge that the party can still be expected to beat at better than even odds, and just might force them to try and thing tactically instead of spamming the same old routines.

[Hunter Killer]

I tend to eye-ball it, but I'm reeeeeeeally familiar with 3.5 and tend to be able to roughly estimate the damage output of my PC's. Since we've been playing together for nearly a decade, I can usually guesstimate their tactics too.

However, in the off chance that an encounter doesn't go my way, I have backup plans.

If I notice that it is heading to a TPK, I start to pull the punches a little... Not be fudging dice or anything, but by not using abilities or spells as well as I can, having the monsters make mistakes on tactical maneuvers, or various other 'non-cheatery' ways of giving the PC's a little bit of advantage.

Note that this only happens if it's stepping into TPK territory. I do not start doing this in the event of one character death, especially if I knew it would be a difficult encounter.

If I notice that the encounter is too easy... Well, I'll just roll a % for "Reinforcements". It's a bit of a homebrew rule but basically for 1d4 rounds I roll to see if it lands on 15% or lower. If it does, I roll on the random monster charts in the DMG (Starting on Page 96). Rerolls on CR's that are way outside of the PC's range and/or monsters that don't make sense for the world region.

Just like with my TPK strategies, I only do this when it makes sense and when things are just looking way, way too easy. Mostly, it's done on things that are supposed to be keystone campaigns fights that I just way underestimated the players on (Again, it ain't happening often).

Neither of these are perfect systems, and I guess they're less fair than planning an encounter down to the most minute detail, but it keeps the challenge where it should be an I'm still allowing the dice to fall where they may. And, most importantly, I don't do these with every encounter.