HD is better than hp

TLDR: instead of tracking hp, make a box or circle for each HD a creature has and tick them off as the hits come in, rounding up or down to "5" per box.

Let's start with this - hit points (hp) is just a more granular way of representing hit dice (HD). In most old school games, creature HD come in d8s. An average roll on a d8 is 4.5. So 1 HD = 4.5 hp. 

Weapon damage varies from d4 to d10, usually. The ranges, averages look like this:

• d4: 1-4, 2.5
• d6: 1-6, 3.5
• d8: 1-8, 4.5
• d10: 1-10, 5.5

But just as aside, let's remember that initially all weapons did d6 damage, rather than variable dice damage, in D&D (1974, 3LB). In which case the average hit was around 4 hp, which is also the average point of the above range of averages. 

One might even say that the average hit (1:4) and the average HD (1:4.5) are about the same. That's not coincidence. Hit points are just Gygax's fancy way of amping up the meaning of hits and hit dice – of making them more granular. (Or is that less granular? I never get that right. What I mean to say is: more finely grained.)

Running encounters with hp is the norm that most of us have used countless times. If a creature has 7 HD (32 hp), we start with 32 and whittle it away as the hits come in. Let's say the damage done to a creature by a party over the course of a few rounds goes like this: 2, 9, 4, 6, 7, 1, 10! That's a total of 39, with the final shot getting in 7 points of "overkill" damage. That's all fine and good. Basic addition/subtraction math is pretty easy. 

But why all this granularity? Is it for some notion of "realism" or to make things less predictable? Does it actually make the game more fun? Consider the following way of tracking things as the GM.

Same monster, same fight. The monster's HD are marked as bubbles like:

Monster: HD 7 OOOOOOO

As the damage comes in the bubbles get marked off as follows:

• 2 damage: no bubble is marked, but maybe a dot or tick mark is used to show a weak hit. 
• 9: two bubbles gone! (10 tracked damage in bubble form /11 actual damage by weapons.)
• 4: another bubble (totals now 15 tracked / 15 actual)
• 6: one more (20/21)
• 7: one more (25/28)
• 1: none? one? This is the GM's call if their gut says they are running behind actual damage. Let's say none for argument's sake. (25/29)
• 10!: two (35/39) - the monster is defeated!

Basically, this combat would have gone the same number of turns whether we were tracking HD bubbles or actual hp. Having used HD bubbles rather than hp a number of times, I believe this example rings rings true – the difference is negligible. Maybe sometimes it takes one more or one less hit than it would have if we were doing the math. 

What is the upside? I find it a lot more satisfying to tick off a bubble than to subtract one number from another and write down a new number. The tracking of wounds takes up a little less headspace than tracking hp does, and even the note-taking is slightly faster. Anything that makes me more fluid at the table is a big deal. An odd, but useful side effect is that I know exactly how much page space tracking wounds is going to take in my notes. With math I have to save margins or some other white space for recording hp and hits.

Here's a two-fight scenario I ran last Sunday from my journal. I could find you a prettier example, but I'm going to be "real" here and show you the most recent one. There's a lot going on here. Bubbles for spell-like abilities as well as HD, lots of chicken-scratches and shorthand concepts that only make sense to me, etc. But the point is I replaced at least 7 pages of an actual printed material with this two-page journal spread, and it made running the session far easier for me. 

The other argument I might add for using HD bubbles is that they give me a little room to pace the fight better without fudging, but that argument gets a little nuanced and may not hold up. I say it isn't like dice fudging, but it is a bit like squinting to get the overall effect rather than trying to see detail. 

Any way, this is my current method. Your mileage may vary, but I can't see the downside of tracking HD bubbles instead of hp numbers. 

I suppose this also begs the question, do I do the reverse? Ask players to track their HP as Levels/Wounds and deliver hits from creatures in increments (i.e. a hit of 14 damage means I say "mark off three wounds")? No, I don't do that. Players have a lot less to track and their hp means infinitely more to them than monster hp does to me. The same goes for their weapon damage. I let them roll damage and take damage in hp. In my notes the damage they deal gets converted on the fly to a relative amount of HD bubbles.