Hit Dice on the Fly

I don't use any monster manual very much. Generally I invent stats for things on the fly, based on some measure of common sense. Hit points are an important part of this, so I've had to develop a consistent system. Hit Dice for a given monster is equivalent to its mass plus its combat experience.

Here is my system for mass-based HP (using a d8 for each hit die):

For every 5' of length along the spine's axis, I give 1 HD unit

For every 2' of width along the shoulders' axis, I give 1 HD unit

For every 1' thick from chest to back, I give 1 HD unit

Multiply all these together, and you have total Hit Dice. For example: a wolf is 5' long, 3' along the shoulders, and 2' thick from chest to back. This gives 1 unit x 1.5 units x 2 units = 3 HD.

Add to this hit dice for combat experience as follows:

Predator (animal intelligence): +1 HD

Fights among its own kind: +1 HD

Every 10 years of life: +1 HD

Humanoid intelligence: +1 HD

Supernatural entity: +1 HD

Every actual class level: +1 HD as class

The wolf is a predator and fights among its own kind, giving it 2 HD of combat experience. Thus its HD is written as 3+2 HD (average of 22.5 hp), and it has attacks and saves as a 2nd level fighter.

Another example: a troll is 10' tall, 4' wide, and 2.5' thick. This gives 2 x 2 x 2.5 = 10 HD for mass. It fights its own kind, has humanoid intelligence, is supernatural, and let's say it is 20 years old, for 5 HD of combat experience. It thus has 10+5 HD (average of 67.5), and attacks as a 5th level fighter.

Note that you will start having serious trouble with particularly large creatures. C'est la vie. If purple worms are 100' x 10' x 10', they will have 20 x 5 x 10 = 1000 HD, or 4500 hp on average. That's a loooonng combat. If you wish, you can determine HD based on the smallest length at which the creature is still "itself" (that is, worms and snakes have a long, repetitive bodies; only calculate HD for a fraction of this body). Using this system, let's say that each "segment" of the purple worm is 20' long, for 4 x 5 x 10 = 200 HD, or 900 hp. Still a monster, but I'm sure your PCs can at least outsmart the bastard!

This system is mostly based on Alexis' HD = m + e, a series of posts that can be found at the following links:

http://tao-dnd.blogspot.com/2008/08/new-concepts-in-hit-points.html

http://tao-dnd.blogspot.com/2008/08/unified-theory-hd-m-x.html

http://tao-dnd.blogspot.com/2009/02/mass-experience.html

http://tao-dnd.blogspot.com/2009/02/you-cant-kill-whale.html

http://tao-dnd.blogspot.com/2009/02/big.html