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Calculating Monster Damage

by Mike on 20 June 2011

Since the Monster Manual 3, monster damage has changed drastically. Charts like that found in the most recent Dungeon Master’s Screen, the update to the Dungeon Master’s Guide, and my own Dungeon Master’s Cheat Sheet contain updated damage expressions to meet this change. When designing your own monsters, traps, and hazards, it may be important to know how to calculate these damage expressions yourself. This article will tell you how.

Base numbers

We begin with the core damage for any standard monster: level + 8. This is the average amount of damage that a standard creature does with a standard attack. A level 12 monster does an average of 20 points of damage on a basic attack.

We increase or decrease this average depending on the type of attack or the creature performing that attack. Here’s a few examples:

For limited attacks, increase the attack by 25% (multiply it by 1.25).

For brute attacks, increase it by 25% (multiply it by 1.25).

For attacks that target more than one creature, decrease damage by 25% (multiply it by .75).

Those are just a few basic modifications but generally you should consider increasing or decreasing damage by 25% based on the type of attack, how often your creature gets to use it, how many people it hits, or whether that creature is a heavy hitter (like a brute).

Let’s try it out. So we have a level 9 ogre pulverizer. This starts with a base average of 17 damage (level 9 + 8). He’s a brute, however, so we increase this by 25% to 21.25 (17 * 1.25). He has a limited attack called club sweep. It hits multiple targets (–25%) but it’s a limited attack (+25%) so we still have an average of 21.25 for this attack. He also has a Brutal Smash power that’s limited (+25%). That attack will have an average of 25.5 damage (17*1.5).

Dice averages

An average number like 25.5 isn’t exactly useful when we’re coming up with a damage expression, however. We need to figure in dice. To do this we need to know the average number on any given damage die. The Sly Flourish scientists are here, however, to give you these averages so you don’t have to hurt your head thinking it all up yourself:

d4 = 2.5 d6 = 3.5 d8 = 4.5 d10 = 5.5 d12 = 6.5

Since we now know what a die’s average is, we can subtract it from the average damage and come up with a damage expression. Generally speaking, half the damage should come from dice and half should come from the static modifier. Instead of having 1d6+22, it’s better to have 3d8+12.

To get multiple dice, add up two or three dice of any type to come up with an average. 3d8 comes out to an average of 13.5. For our ogre pulverizer we can transform his base attack (average 21.25) to 2d10+10 (11 average from the d10s + 10 static). His sweep will also stay at 2d10+10. His big smash (25.5 average) will increase to 3d10+9.

Higher dice increases swing

Now it’s time to talk about dice swings. The higher the number on the die, the bigger the swing is going to be. When you have a very precise monster, like a rogue or a were-rat, you want to stick to lower dice types. When you have a big massive creature swinging around a tree trunk, you want to use bigger dice.

When you’re rolling on the table, these big dice may roll low and produce glancing blows or they may roll high for a big surprising hit. Keep this in mind as you’re selecting dice types.

Minion damage

Last, we’ll cover minion damage. Chris Perkins, in his excellent Instant Monster article gives us the tools we need to calculate minion damage. In general, minion damage comes out to 4 + 1/2 the level of the monster. This isn’t perfect, but it’s good enough to work well at your table. If you want better precision, consult one of the charts mentioned in the beginning of this article.

Simple tools to help you DM

These simple tools should help you build out any monster you want. An easier way, however, is to stick to monsters found in the Monster Vault, the Monster Manual 3, and the Dark Sun Creature Catalog.