Step 1—Determine APL: Determine the average level of
your player characters—this is their Average Party Level
(APL for short). You should round this value to the nearest
whole number (this is one of the few exceptions to the round
down rule). Note that these encounter creation guidelines
assume a group of four or five PCs. If your group contains
six or more players, add one to their average level. If your
group contains three or fewer players, subtract one from
their average level. For example, if your group consists of
six players, two of which are 4th level and four of which
are 5th level, their APL is 6th (28 total levels, divided by six
players, rounding up, and adding one to the final result).
APL for six or more characters = Average Player Level (rounded up if not an integer) + 1
So if you have eight level 4 characters, the APL would be 5 by RAW.
Step 3—Build the Encounter: Determine the total
XP award for the encounter by looking it up by its CR
on Table 12–2. This gives you an “XP budget” for the
encounter. Every creature, trap, and hazard is worth an
amount of XP determined by its CR, as noted on Table
12–2. To build your encounter, simply add creatures,
traps, and hazards whose combined XP does not exceed
the total XP budget for your encounter. It’s easiest to add
the highest CR challenges to the encounter first, filling
out the remaining total with lesser challenges.
For example, let’s say you want your group of six
8th-level PCs to face a challenging encounter against
a group of gargoyles (each CR 4) and their stone giant
boss (CR 8). The PCs have an APL of 9, and table 12–1
tells you that a challenging encounter for your APL 9
group is a CR 10 encounter—worth 9,600 XP according
to Table 12–2. At CR 8, the stone giant is worth 4,800
XP, leaving you with another 4,800 points in your XP
budget for the gargoyles. Gargoyles are CR 4 each, and
thus worth 1,200 XP apiece, meaning that the encounter
can support four gargoyles in its XP budget. You could
further ref ine the encounter by including only three
gargoyles, leaving you with 1,200 XP to spend on a trio
of Small earth elemental servants (at CR 1, each is worth
400 XP) to further aid the stone giant.
Adding NPCs: Creatures whose Hit Dice are solely a
factor of their class levels and not a feature of their race,
such as all of the PC races detailed in Chapter 2, are
factored into combats a little differently than normal
monsters or monsters with class levels. A creature that
possesses class levels, but does not have any racial Hit
Dice, is factored in as a creature with a CR equal to its class
levels –1. A creature that only possesses non-player class
levels (such as a warrior or adept—see page 448) is factored
in as a creature with a CR equal to its class levels –2. If this
reduction would reduce a creature’s CR to below 1, its CR
drops one step on the following progression for each step
below 1 this reduction would make: 1/2, 1/3, 1/4, 1/6, 1/8.
High CR Encounters: The XP values for high-CR
encounters can seem quite daunting. Table 12–3 provides
some simple formulas to help you manage these large
numbers. When using a large number of identical creatures,
this chart can help simplify the math by combining them
into one CR, making it easier to find their total XP value.
For example, using this chart, four CR 8 creatures (worth
4,800 XP each) are equivalent to a CR 12 creature (worth
19,200 XP).
Six level 2 NPCs (CR 1, 400 XP each) would be equal to a CR 6 (2400 XP total) encounter.
Source: Chapter 12 (Gamemastering) of the Core Rule Book.