[RPG] How to decide how many hit dice a custom creature has


I am planning to create some custom monsters for my games. I know how to determine a monster's average hit points, however I don't know how you determine the decomposition in hit dice.

For example if I create something similar to a Goblin I am able to tell it should have 7 hit points but I don't know how 2d6 is determined.

Best Answer

The DMG outlines the process for creating monsters as a DM.

There are two ways to decide how much HP your monster has:

Method 1: The CR Table

You can start with the monster's expected challenge rating and use the Monster Statistics by Challenge Rating table to determine an appropriate number of hit points. The table presents a range of hit points for each challenge rating. (DMG p.276)

In this method, you are going to do some math to go from the hit point range of your monster to the number of hit die your creature has.

In this case you use the table "Monster Statistics by Challenge Rating" (DMG p. 274) to look at the line with your expected challenge rating and see the range of hit points in that table.

Make sure at this point you know the size of your monster because that is what determines the size of the monster's hit die.

I am going to quote this AngryGM article because it explains this part of the process well:

For example, I might be designing a creature and need to roll between 80 and 100 hit points. If the creature is small and it has a Constitution modifier of +1, I can actually figure out exactly the dice code I need to get in that range. Follow the logic. Small creatures roll a d6 for hit points. So this creature is going to roll 1d6+1 some number of times to determine its hit points. What’s the average roll? Well, the average on 1d6 is 3.5 (half of 6 plus a half), so the average of 1d6+1 is 3.5+1 or 4.5. So, if I take, say, 85 and divide it by 4.5, I get 18.8. That means I need to roll 1d8+1 [ed: actually 1d6+1] about 18 times to get in the ballpark of 85. In this case, if I multiply 4.5 times 18, I get 81. Perfect. So, this creature has 81 (18d6+18) hit points.

After going through the above process you should now have the number of hit dice your creature possesses.

Method 2: Assigning Hit Dice

Alternatively, you can assign a number of Hit Dice to a monster, then calculate its average hit points. Don't worry if the hit points aren t matching up with the expected challenge rating for the monster. Other factors can affect a monster's challenge rating, as shown in later steps, and you can always adjust a monster's Hit Dice and hit points later on. (DMG p.276)

This one is very easy: you simply assign the monster's number of hit dice and make sure that the CR comes out the way you want by adjusting other features of the monster further down the line.