In D&D 3.5 how do you calculate the Encounter Level of an encounter when the encounter involves elements (usually enemies) of multiple CRs?
For example an encounter involves
- 4 Zombies (CR1/2)
- 2 Ghouls (CR1)
- 1 Ghast (CR3)
What would the CR be?
Rmorrisey's formula is correct, but to simplify: here's a rule of thumb: Doubles bump the EL by +2.
2 of any single monster is the same as the Base CR +2. Two CR1 monsters (the ghouls) is an EL3. (1+2=3)
The EL 3 ghouls and the CR 3 ghast are doubles, and thus an EL 5. (3+2=5)
4 CR1/2 zombies is not quite big enough to make an EL 5 (which would make this a 7, according to the double-bump principle), so it's a 6, more or less. Bigger than a 5, not quite a 7.
Here's a sideways trick you can use too: Class levels bump a full CR per level. NPC classes bump a 1/2 CR per level. As far as this formula is concerned, half CR's don't really count for much. Thus you can usually layer a single level of warrior or expert or acolyte (+1d8 hd and a +1 BAB) onto just about anything without really pushing the margin.
Edit: Here is how XP is done in D&D 3.5 (DMG pp. 36-38). It's actually much simpler.
For fractional CR creatures, use CR 1, and divide the result by 2.
Summoned creatures don't count.
Edit: Below is the way it's done in D&D 3.0.
The DMG pp. 101 says,
I don't have a nice formula like RMorrisey does, but I found this page which does the heavy lifting for you. It's not fool-proof, but I fed the bad guys you provided into the engine, and it came back with EL 6. You can also input the PCs and their levels to see an estimation of how their level and the bad guys compare. I used it extensively in the current campaign I'm running (and will DM the finale in our next session).
I think this link can be useful 'cause it calculates the EL and the XP/PC. I can say that for me the only problem with the d20 calculator is that it overestimates the difficulty for the party cause I don't think that the encounter of the ghouls&Co (EL6 for the calculator) is a very difficult challenge for a party of 4 4th lvl PC XD. However the GM can adjust the XP of any encounter how he/she thinks it's better.
Two encounters of CR X = 1 of CR X+2
4 zombies (cr0.5)= 2x CR2 = 1 CR4
but it's a soft CR6...
Any time you stage up multiple different beasties, they only match the calculation when they are used to proper effect.
It's often better to simply add the appropriate XP for each type of creature. In which case, use a CR 4, and two CR3's...