How to you calculate encounter CR if PC-s have allies?

[stenver]

So my PC-s, after a long day of fighting, are probably going to meet some fun guys

1 Lich Simulcarum CR 15
2 Custom skeleton mages(CR 1)
4 skeletons(CR 1/4)
1 "white walker"(CR 4)
Finally this guy(CR 10): https://i.imgur.com/4ykIJtM.jpg

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The 5 PC-s themselves are level 14.
However, they also have allies

1 Archmage - CR 12
1 Frost Giant king - CR 14

How do I calculate the difficulty and XP of the battle?

[Zman]

Firstly, subtract the XP value of the allies from the total XP value of the enemies. Then compare that total to the expected Encounter difficulty based off of the characters levels. The assumption is that the allies XP is roughly equivalent to the enemies XP values and you compare the party to what is left.

[stenver]

Thats so simple, thanks!

[Zman]

Thats so simple, thanks!

No problem, glad I could help! That should work for most cases like this, there are outliers ie you have a ton of allies, then you will need to shift things down one or two for the outnumbered multipliers, etc. Or alternatively, if the allies XP is greater than the total value of the enemies You'll need to alter it a bit, but for situation where there are a couple of NPCs of reasonable CR against a group of enemies it works well.