Which number to use when using "hit dice" in ability calculations?

Question

I've got a debate in my group about what numbers to use when "hit dice" or "racial hit dice" are to be used in a calculation. We are using D&D 3.5.

For example, if calculating the damage of poison says to use half of a creature's "racial hit dice" and their hit dice is listed as "5d8+10" what number are we dealing with?

Let's say we break up the constituent parts of the hit dice listing as AdB+C=X where A is number of die, B is type of die, C is die modifier, and X is sum of A*B+C

The competing explanations within my group are as follows:

1. A is the hit dice number, so with the poison example 5 is the number to use, so 2.5 is the poison damage.

2. B is the hit dice number, so with the poison example 8 is the number to use, so 4 is the poison damage.

3. X is the hit dice number, so with the poison example 50 is the number to use, so 25 is the poison damage.

4. Actually do the rolling (Roll A number of B dice, add C to whatever that is) on a case by case basis, changing the amount of damage done each time.

Let's say all the creatures I use as an example use the same poison calculation as above.

The biggest point of contention is if you are just using A, which usually ties to the level of a creature a level 5 creature would always use 5 hit dice, so a level 5 dragon would do the same poison damage as a level 5 mosquito or something. However, if you are just using B then let's say a dragon has a d8 and a human has a d8, a level 20 dragon and level 1 human would do the same poison damage. Then of course, if you use X both level and type of dice are taken into consideration, so different creatures are doing proportional damage, but now the poison is doing way more damage than some people think it should.

Of course none of these could be the correct answer. Even reading the specific rules in the book, each of us are interpreting it differently. What say you, internet?

Answer 1

In the equation AdB+C=X...

• A is the number of Hit Dice
• B is the type (or size) of the Hit Dice (MM 5)
• C is "the number of bonus hit points[,] a function of the creature's [number of] Hit Dice and Constitution" (MM 298)
• X is hit points.

A, that is the number of HD, is what's meant in almost every discussion of Hit Dice. The game loves A and cares far less about B or C or X because A is--in theory--very, very roughly equal to character level, which is why number of HD is the controlling factor of so many monster special abilities. The monsters, essentially, level up in their virtual monster class when they gain more and more HD.

The Monster Manual scatters the information about the importance of the number of Hit Dice throughout its text, but the most important references occur in Chapter 4: Improving Monsters (290-4) and Chapter 5: Making Monsters (295-301). Start there if you're really interested.

By the way, a 5 HD dragon's poison and a 5 HD mosquito's poison could have exactly the same saving throw DC (if they share the same Constitution modifier--MM 313) because, although they're different types of creatures, if they've the same number of HD they're supposed to be roughly equally advanced as creatures, despite one being a freakin' dragon and the other a mosquito. The difference comes from the inequality of their types and the abilities inherent to their races and their types. The dragon type just straight-up comes with considerably more (including a larger size HD and usually a much higher Constitution) than the vermin type usually associated with the mosquito, much like the wizard class has considerably more to it than does the warrior class.