Determining XP for many low CR Creatures

[bhh39]

Is there a system for figuring out xp for many easy enemies other than just adding up the xp for many low CR encounters?

I ask because in tonight's session our party of 4 PC's at level 5 managed to defeat an army of 40 gnolls with a well placed fireball and a castings of glitterdust. All in all it was a rather easy fight, but doing the math for 40 CR1 opponents it looks like each of us took home 4000 xp each, the same amount we would have received from a CR11 encounter. This seems odd since the encounter was much easier than a CR11.

[keeper0]

XP and challenge calculations are always going to get a little strange at the margins. Situations like this are probably best handwaved to be rated based on the challenge the situation presented the party.

If somebody wants to get rules focused, point to the part of the rules where the tactical advantage can be used to modify the CR. If the 40 gnolls are in a position to be decimated by a fireball, their tactical situation was clearly a negative to the CR.

40 CR1 gnolls who know the party is coming and have a plan would be more of a challenge, though perhaps not CR11.

[freeAgent]

I've been gearing up to GM a campaign and this has been an area of interest for me. On the CR equivalencies table (see: http://paizo.com/PRD/gamemastering.html ), CR does not scale linearly with creature count. I've carried the scaling out to 64 creatures below. In order to get to CR+13, you need 96 creatures, and CR+14 requires 128 (though XP awards get ludicrous at this point given low CR creatures):

+1 CR/creature for creatures 2-4 (CR+4 for 4)
+.5 CR/creature for creatures 5-8 (CR+6 for 8)
+.25 CR/creature for 9-16 (CR+8 for 16)
+.125 CR/creature for 17-32 (CR+10 for 32)
+.0625 CR/creature for 33-64 (CR+12 for 64)

That would make 40 CR 1 creatures have a total CR of 10, not 11. Given that your party was able to take the gnolls out with a quick AoE spell, I'd give a -1 terrain penalty as well, making it a CR 9 encounter and 1,600 XP per PC. That would be strictly by the book and it's still a large load of XP, but not 4,000.

I think the encounter could have been designed better so a single fireball couldn't take out the entire group. The DM could have fudged and explained that the massive wall of burnt gnoll flesh acted as an insulator and allowed some lucky gnolls around the outer ring to survive the blast. That would have made the encounter more fun (IMO) and better justified the high XP award.