How are monster levels calculated?

[Leonaru]

The sample troll given in M&M describes the troll as a level 7 monster, suggesting that the bonus hit points grant it another level in addition to its six HD. But how many bonu hit points are needed for one level? Three, so the rounded-down average hit pints per hit die (using a d6)?

[Finarvyn]

My interpretation of "6+3" hit dice is "more than six, so seven." In other words, it's not hit points so much as hit dice.

[Stormcrow]

Gary explained this in the D&D FAQ in The Strategic Review: "For purposes of experience determination the level, of the monster is equivalent to its hit dice, and additional abilities add to the level in this case. A gorgon is certainly worth about 10 level factors, a balrog not less than 12, the largest red dragon not less than 16 or 17, and so on. The referee’s judgement must be used to determine such matters, but with the foregoing examples it should prove to be no difficulty."

The Sleep spell indirectly suggests that the bonus hit point cutoff you're looking for is +1. "A Sleep spell affects from 2-16 1st level types (hit dice of up to 1 + 1), from 2-12 2nd level types (hit dice of up to 2 +1), from 1-6, 3rd level types, and but 1 4th level type (up to 4 +1 hit dice)." If this is correct, then +2 or more on hit dice should be considered an "additional ability." I wouldn't follow this blindly, however.

Monster	Hit Dice	Special Abilities	Given Level
Troll	6 + 3	Regeneration	7
Gorgon	8	Turn to Stone	10
Balrog	10	Magic Resistance +1 Sword Immolation	12+
Largest Red Dragon	11	Breathe Fire Resists attacks of fire, air, water 13% chance of spell-use Flight Cause troops to check morale	16+

[Deleted]

Nice job, Fin and Stormcrow. Well explained.

[Leonaru]

Alright. So it's kind of the S&W challenge level.

[Deleted]

This seems to be a preference thing. Barker, in his Empire of the Petal Throne, ignores all pluses. So a 6+3 is a 6, both for XP and to-hit purposes. Complete Warlock uses the +2 or better == next level rule.

[Stormcrow]

The '81 and '83 Basic Sets also defined monster level as equal to hit dice, but monster levels weren't actually used for anything except determining where in a dungeon a monster would go. But these rules also put asterisks after monsters' hit dice; each asterisk indicated a special ability that garnered bonus XP. Given that the monster attack table goes in rows of HD of 1+ to 2, 2+ to 3, 3+ to 4, and so on, any bonus hit points always mean "attack as one level higher." So you can easily calculate monster level in these rules by simply starting with the base hit dice, adding 1 if there are any bonus hit points, and adding 1 for each asterisk after the hit dice. These extra additions weren't official—the rules only said "if a monster has several special powers, the DM may consider it one level greater than its hit dice."

[Deleted]

I assume this is the quote you were reading in MEN & MAGIC, but on page eighteen it clearly says "...defeating a troll (which is a 7th level monster, as it has over 6 hit dice)..." and that looks pretty clear to me. The "by the book" interpretation would be to adjust +1 and higher up to the next dice level.

[tetramorph]

Delta has blogged a series of posts on this topic:

deltasdnd.blogspot.com/2016/03/monte-carlo-measures-of-monster-levels_14.html