How to calculate Adjusted XP for heterogenous monster groups?

Question

To recap my understanding...

• The point of Adjusted XP is provide a metric with which to judge the difficulty of a combat, to give your party well-balanced encounters and well-balanced adventuring days overall.
• The encounters for the day "should" sum to the Daily XP Threshold; Notably, multiple separate encounters don't increase the Adjusted XP for each Encounter - that's accounted for in the Daily Threshold.
• For a single monster encounter "Adjusted XP" = "Monster XP".
• Obviously (due to action economy) fighting multiple monsters simultaneously is disproportionately hard: Fighting 2 Werewolves is more than twice as hard as fighting 1 Werewolf.
• The Encounter Multipliers table accounts for this:

Monsters	Multiplier
1	x 1
2	x 1.5
3-6	x 2
7-10	x 2.5
11-14	x 3
15+	x 4

• So 2 Werewolves are 3 times as hard as 1 Werewolf.

End of Recap

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This concept works great when your encounter is "n copies of the same monster".

Question: How do I calculate the Adjusted XP for a mixed monster encounter?

It's trivially obvious that just applying a single Multiplier to the whole lot doesn't work.

Consider an extreme case of fighting a Adult Red Dragon plus 2 Giant Rats. By RAW that encounter is now 2 x (18,000 + 25 + 25) = 36,100 Adjusted XP! But it's completely obvious that this encounter isn't even slightly comparable to "fight an Adult Red Dragon ... and then after you've killed it, its mate appears", which would also be a total of 36,000 Adjusted XP.

On the other hand, in a less lop-sided encounter, say ... a Tree Blight flanked by a handful of Needle and Twig Blights ... there is a material impact to having to deal with them all at once, so you couldn't just use "apply the multiplier to groups of the same CR, and then add the results together.

So ... are the any published, or common, ways of handling Adjusted XP for heterogenous monster groups?

Answer 1

Don't count very weak monsters.

This is actually addressed in the Encounter Building rules in the DMG (p.82) where you got that multiplier chart. Step 4 ends with a note:

When making this calculation, don't count any monsters whose challenge rating is significantly below the average challenge rating of the other monsters in the group unless you think the weak monsters significantly contribute to the difficulty of the encounter.

If you're using a group of monsters that are close to the same level -- say, a CR 5 leader and a squad of CR 2 goons -- you do indeed count them all up and use the multiplier, and it'll work fine.

In your extreme example, a couple of giant rats aren't anywhere near the threat of the dragon, so you can just ignore the rats entirely. It is, at that point, functionally just a dragon fight. (Don't get overly worried about how to take a mathematical mean here -- we could posit 20 rats and a dragon or something and get into a big discussion about whether Legendary creatures count as more than one and whether average actually works in this case, but the point is just to ignore really weak creatures unless you have some kind of crazy synergy effect going on.)

There is also another caveat that might come up, relevant to a point in your summary. You noted that multiple separate encounters don't increase the Adjusted XP for each Encounter, and that's true in general, but if you have it arranged so the party can't rest between the fights and the total XP is at least a third of the daily XP total, the fights will be tougher than they look. In the rules that's referred to as a "multi-part encounter" but it applies equally if the party is prevented from resting for other reasons, like a plot-relevant time limit or something.