The * Frilond * Campaign
Balancing Encounters

A few years back, Frank Mentzer developed an extremely useful system for tailoring encounters to meet party capabilities. A scaled-down version of this system appeared in the D&D Master Set, and Frank published a more expansive treatment in a Dragon article entitled "Planning It By the Numbers." We've adapted the original system for second edition AD&D, and it seems to work pretty well.

Frank's system looks at the Total Party Level (TPL), which is simply the sum of all the experience levels of all of the characters in the party—including NPCs like henchmen. By dividing the TPL by the total number of characters, you arrive at the Average Party Level (APL). With these two bits of information, you have a decent gauge of a party's relative strength. Mentzer's system compares monster strength to party strength, based on the relative relation between character level and monster hit dice.

Once the party's TPL and APL have been determined, the following procedure is used to set up an encounter.

  1. Select categories.
  2. Select monsters.
  3. Find the Modified HD of selected monsters.
  4. Find the total HD of monsters appearing.
  5. Calculate number appearing by dividing total monster HD by the Modified HD.
  6. (Optional) Multiply total monster HD appearing by 4.5; multiply this product by the number of monsters appearing to find the total monster hit point pool. Assign hit points to individual monsters by subtracting them from this pool.

Modified HD

Modified HD takes into account the special abilities of particular monsters. To find the modified HD of a monster, use its given experience point value and consult the following chart:

XP Modified HD
7 1/2
15 1
35 2
65 3
120 4
175 5
270 6
420 7
650 8
975 9
1400 10
2000 11
3000 12

For example, a gargoyle has 4+4 HD, and is worth 420 experience points. Its modified HD, then, is 7 HD.

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 party's goal
Total HD of Monsters Compared to TPL
Total HD Game Impact
Over 200% TPL Extreme danger; if PCs do not retreat or flee, they will probably die. You may need to drop hints to this effect.
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 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

Mentzer's system can also be used in reverse: a DM can estimate the toughness of an existing encounter by working backwards through the procedure. This is a good, quick way to test the difficulty of an encounter.