Preliminary calculations	Procedure	-	Examples	Reversing the Process
1st Edition AD&D	-	Dragon #101	-	Dragon magazine

(Author’s note: The following was included
in the original manuscript for the
D&D® Master Set. It was, however,
thought by the editors to be too heavily
mathematic for easy use, and was replaced
by an alternate system. But I still like this
one, and use it in my own campaign —
though modified for AD&D® game use.)
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It is possible to generate encounters by
applying a bit of math, using the experience
of the PCs as the major variable factor. Try
the following system; but be aware that it
may not apply to some campaigns. It can be
easily modified, applying the same general
principles to data found more accurate to
your own style of play.

By applying this or a similar system, you
can predict the game impact of a wandering
or placed encounter before it occurs. This
system is thus recommended when you
cannot accurately estimate the monster
power needed to challenge the characters.
Give yourself enough time to use it; try to
find out what PCs will be played at least an
hour, if not longer, before the game starts.

This system is also useful when, for entertainment
reasons, you want to finish the
game session with a rousing encounter,
designed (quickly and just before it is used)
specially for a given group of characters. If
you can work the math quickly enough, try
it during a break, or while the players are
busy role-playing without needing you as a
judge. But don't delay the game while
calculating!
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Preliminary calculations
Before you start, find the Total Party
Levels (TPL) by adding all the levels of all
the characters being played. Divide this
total by the number of characters to find the
Average Party Level (APL).

Multi-classed AD&D game characters use
their highest single level plus one-half of the
level attained in any other class(es); thus, a
fighter/magic-user/thief of levels 2/2/2 has a
figure of 4 for purposes of this calculation.
Dual-classed characters use the total of all
levels they have attained. Drop any fractions
that remain after the TPL arithmetic
is completed.

The difficulty of an individual monster, in
comparison to a given level of PC, can be
expressed in terms of the APL, as shown on
Table 1.

When the TPL is compared to the total
hit dice of monsters appearing, the overall
impact of the encounter can be estimated,
as shown in Table 2.

Table 1: Monster HD compared to APL

HD of 1 monster	Comparison to PC abilities
Over 200% APL	Extremely dangerous opponent
150-200% APL	Tough opponent
110-150% APL	Major opponent
90-110% APL	Average opponent
50-90% APL	Minor opponent
30-50% APL	Easy opponent
20-30% APL	Very easy opponent
Up to 20% APL	Nuisance opponent; omit or modify unless related to greater party goal.

Table 2: Total HD of monsters compared
to TL

Total HD	Game impact
Over 200% TPL	Extreme danger. If the PCs do not retreat or flee, they will probably die; you may need to drop hints to this efect.
150-200% TPL	Tough encounter; may require many party resources to ensure success, and may cause deaths.
110-150% TPL	Major encounter; can become one of the game session's major activities.
90-110% TPL	Average placed encounter, Tough wandering encounter.
50-90% TPL	Minor placed encounter,  Major wandering encounter.
30-50% TPL	Easy placed encounter,  Average wandering encounter.
20-30% TPL	Very easy placed encounter, Minor wandering encounter.
Up to 20% TPL	Too easy for placed encounter, Easy wandering  encounter.

Procedure
Keep track of decimals in all of these
steps, without rounding. An electronic
calculator is very helpful.
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1. Select categories: Decide what impact
you want the encounter to have, in terms of
monster strength (Table 1) and overall
encounter impact (Table 2). Be sure you
have the APL and TPL handy, as well.

2. Select the monsters: Multiply the APL
by each of the percentages given in the
Table 1 category you have chosen. Select a
monster whose HD are within that
range, preferably near the average. (This
will often be based on those available in the
adventure setting.) Make a note of its name
and hit dice, including the number of asterisks
(special abilities).

3. Find the Power Factor (PF): Find the
number of asterisks by the monster's HD,
and use the following chart to find the
PF (to be used in step 5).
 

Asterisks	Power Factor
0-1	1
2-3	2
4-5	3
6-7	4
8 or more	5

4. Find total HD appearing: Multiply
the TPL by each of the percentages given in
the Table 2 category you have chosen. Keep
both the smaller and larger figures. You
may wish to note two or more ranges and
their effects, to provide alternatives.

5. Calculate number appearing: Divide
the total hit dice (from step 4) by the hit
dice of a single monster, then divide that
result by the Power Factor. (Thus, if the PF
is 1, it has no real effect.) The number(s) to
the left of the decimal point indicates the
number of monsters appearing.

6. (Optional) Convert decimals to HP:
Find the average hit points of one
monster by multiplying its hit dice (without
plusses) by 4.5 (average for 1d8), then add
any plusses to the total. Multiply the entire
result of step 5 by this number of hit points.
Round the result to the nearest whole number;
it is the total number of hit points to be
divided among all the monsters appearing.
Divide it however you wish, as long as each
monster has no fewer than the minimum
and no more than the maximum hit points.

Example: A giant rockfish has 5 + 5 HD
(5d8 + 5 hp); its average hit point total is
4.5 times 5 (= 22.5) + 5, or 27.5 hp. If your
calculations show 3.45 rockfish appearing,
they have a total of 94.8 (rounded to 95) hp;
you divide this as you like among the 3
rockfish appearing (as shown by the wholenumber
part of the 3.45), so that each has
10-45 hp.

(Designer's note: This is the system I use,
but a few HP here and there don't
mean much, and this last step is a bit timeconsuming.
Estimate, round off, or substitute
if you wish.)
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Examples
A. Low Level Party; size = 5, TPL 26,
APL 5.2

Situation #1: Wandering encounter
1. Impact desired: Average opponent,
average encounter.

2. HD of one monster: 90-1 10% APL, or
90% of 5.2 to 110% of 5.2, or 4.7 to
5.7. Average = 5.2; monster chosen:
cockatrice (HD 5**).

3. Power Factor: 2 (asterisks).

4. Total monster HD: 30-50% TPL, or
30% of 26 to 50% of 26, or 7.8 to 13.

5. Number appearing: 7.8/5 to 13/5, or
1.56 to 2.6, each divided by 2 (PF) =
0.78 to 1.3. One cockatrice wanders
by.

6. Convert decimals: Average hp = 22.5,
multiplied by 0.78 and 1.3. The cockatrice
has 18-29 hp.
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B. Mid-level party; size = 6, TPL 122,
APL 20.3

Situation #1: Wandering encounter

1. Impact desired: Average opponent,
Easy encounter.

2. HD of one monster: 90-110% APL, or
18.27 to 22.33. Average = 20.3; monster
chosen: tyrannosaurus rex (HD
20).

3. Power Factor: 1 (may be ignored).

4. Total monster HD: Up to 20% TPL,
or up to 24.4.

5. Number appearing: Up to 24.4/20, or
up to 1.22. One dinosaur wanders by.

6. Convert decimals: Average hp = 90;
the tyrannosaurus rex has up to 110 hp
(minimum 20).

Situation #2: Placed encounter
1. Impact desired: Major opponent,
extremely dangerous encounter.

2. HD of one monster: 110- 150% APL,
or 22.33 to 30.45. Average = 26.39;
monster chosen: NPC party (APL 26).

3. Power Factor: Assume 3.

4. Total monster HD: Over 200% TPL,
or over 244.

5. Number appearing: Over 244/26, or
over 9.38; divided by 3 (PF) = over
3.13. Three NPCs are placed.

6. Convert decimals: Average hp =
special. Each member of the NPC
party has about 104% of average hp.
Recommended makeup: 1 fighter, 1
magic-user, 1 cleric; all level 25-27,
equipped similar to PC party but with
about 28% more power (NPC APL
divided by PC APL = 1.28).

C. High-level party; size = 4, TPL 132,
APL 34

Situation #1: Wandering encounter

1. Impact desired: Easy opponent, Minor
encounter.

2. HD of one monster: 30-50% APL, or
10.2 to 17, average = 13.6. Monster
chosen: cloud giant (HD 13*).

3. Power Factor: 1 (may be ignored).

4. Total monster HD: 20-30% TPL, or
26.4 to 39.6.

5. Number appearing: 26.4/13 to 39.6/
13, or 2.03 to 3.04; 2-3 giants wander
by.

6. Convert decimals: Average hp = 58.5;
the 2-3 cloud giants have a total of
119-178 hp (each with 13-104 hp).

Situation #2: Placed encounter
1. Impact desired: Minor opponent,
Average encounter.

2. HD of one monster: 50-90% APL, or
17 to 30.6. Average = 23.8; monster
chosen: Huge red dragon (HD
20 * * * * ).

3. Power Factor: 4.

4. Total monster HD: 90-110% TPL, or
118.8 to 145.2.

5. Number appearing: 5.94 to 7.26,
divided by 4 (PF) = 1.48 to 1.81; one
dragon appears.

6. Convert decimals: Average hp = 90;
the one huge red dragon has from 133
to 160 (maximum) hit points.
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Reversing the process
By finding the total HD of a group of
monsters who are about to be encountered
by a party (according to your encounter
key), you can estimate the impact in advance.
If you feel that the result would be
something other than desirable, feel free to
modify the number appearing or their hit
points -- and the treasure, as well.

There are many good reasons for modifying
existing details (often called ?winging
it?). If the hour is late, you might wish to
avoid a long, involved encounter. If the
party is exceptionally damaged, you might
want to avoid killing characters. If the party
has been remarkably unlucky in treasurefinding
during the adventure, you might
want to add more, with a corresponding
increase in danger. Or you might simply
have underestimated or overestimated the
PCs? abilities. By using an effective impactprediction
system, you can avoid guesswork
in making the changes. You may have trouble
with the system at first, but stick with it.
The more you use is, the faster it?ll become,
until you can estimate impacts quickly and
accurately without even a calculator. Don?t
believe it? Try it!

One final note. The concept of variable
game details may shock some DMs --
especially those who wipe out whole parties
while saying "That's what it says right here.
. . . Gee, isn't that a shame?" But most
DMs fake rolls occasionally, announcing the
results they prefer -- which could be to the
party's advantage, or the reverse. The
deadliest games I've ever seen are ones in
which everyone (including the DM, in
melees) rolls the dice out in the open, for
better or worse. I make up results regularly,
to keep the game fun -- and isn't that why
we're all playing?

You can do lots more with APL and TPL.
Think about it, and write to me with your
ideas!