D&D 5E DMG page 274.

[JEB]

No, you don't.
Divide the target HP by (average per die + Con Modifier)...

Simple algebraic solving for Y.

So, let's say you want 221-235 (CR11), and you have a medium critter (thus d8's), and Con 22 (for +6)...

Hd = Thp / (Apd + Conmod)
Hd = 235 / (4.5 + 6)
Hd = 235/10.5 = 22.38.
So, we give it 22 HD, and 22x10.5 = 220+11=231hp
​

Ah man, you got me all excited that there was an easy way, but I just tried it... with Con 22 and 22 HD, it's 22d8+132, and that approach does get an average result of 231 HP... but a maximum result of 308 HP, which should be CR 16, not 11.

That said, I think you're onto something, since that's precisely 5 CR off, and trying it with a few other combinations were similarly off...

 

[aramis erak]

Ah man, you got me all excited that there was an easy way, but I just tried it... with Con 22 and 22 HD, it's 22d8+132, and that approach does get an average result of 231 HP... but a maximum result of 308 HP, which should be CR 16, not 11.

That said, I think you're onto something, since that's precisely 5 CR off, and trying it with a few other combinations were similarly off...

DO NOT use Maximum HP. It's Average HP that's used on the table. See pg 276, last paragraph.

Your conceptual error, not my math, is the problem you're having.

 

[JEB]

DO NOT use Maximum HP. It's Average HP that's used on the table. See pg 276, last paragraph.

Your conceptual error, not my math, is the problem you're having.

Actually, it's average hit points if you're using the "Assign Hit Dice" approach, where you pick Hit Dice that seem fair and "don't worry if the hit points aren't matching up with the expected challenge rating for the monster".

If you're using the table on page 274 (an approach referred to, obscurely, as "Use the Table"), it describes that as presenting "a range of hit points for each challenge rating." Meaning an expected maximum and minimum for that CR, and not an average amount (though such can easily be calculated by averaging the two ends of the range).

 

[ZombieRoboNinja]

Why would hit dice matter on a monster?

 

[aramis erak]

Why would hit dice matter on a monster?

In case you charm it to accompany you, or let it get away for a bit... it can then have the healing rules applied.

 

[SaxonDouglass]

Ah man, you got me all excited that there was an easy way, but I just tried it... with Con 22 and 22 HD, it's 22d8+132, and that approach does get an average result of 231 HP... but a maximum result of 308 HP, which should be CR 16, not 11.

That said, I think you're onto something, since that's precisely 5 CR off, and trying it with a few other combinations were similarly off...

I am still waiting for the release of the DMG in my region, but I wanted to chime in on calculating hit dice. I don't think the hit point ranges are to be taken as the rolled minimum and maximum HP for the monster, but instead as the range of average HP values.

Take the range above for a Challenge 11 monster (221-235) and assume we want d8 hit dice. We know that the maximum HP is from rolling maximum hit dice (all 8s in this case) and the minimum is from rolling all 1s. So the difference between maximum and minimum hit points gives us the desired range of our roll:
(235-221)/(8-1) = 2
Therefore the creature has 2 hit dice of d8s. Then the constant term is:
221-2 = 219
Which if we split into hit dice and Con mod gives us a Con mod of +110 (Con score 230). So we give the monster 2d8+219 hit points.
This is probably undesirable!

Hence the approach outlined by aramis erak should probably be used in preference, which simply gives you the desired average hit points.

Note that for a given Challenge rating the calculated hit dice depends on both the die type chosen (presumably corresponding to creature type) and the Con mod (which can be freely adjusted). I would use the minimum HP as my target average when building spellcasting or lurking monsters, and the maximum HP for brutes. In between the two would be soldier and ranged monster types. Then adjust the Con mod to give a desirable number of hit dice (a quick glance at the MM suggests between 1 to 2 hit dice per Challenge level is typical).

 

[Grazzt]

In case you charm it to accompany you, or let it get away for a bit... it can then have the healing rules applied.

Monsters heal when the DM says they heal; no healing rules needed.

 

[the Jester]

Monsters heal when the DM says they heal; no healing rules needed.

Well... yes... but....

I had a thing in my game where two pcs, empowered with water breathing by a nixie, confronted a merrow... and ALMOST beat him down before he fled. They, too, were battered and pretty badly in need of healing.

So I confronted them with the choice- take a short rest and spend HD to heal, but in so doing so, let the bad guy do the same; or not.

The choice wouldn't have been half as interesting without me being able to point at the HD rules and say, "You realize that this is how it works, right?"

 

[aramis erak]

Monsters heal when the DM says they heal; no healing rules needed.

Taken to extremes, that approach is the same as saying "Nothing needs rules because the DM handles it all"...

 

[Authweight]

I am still waiting for the release of the DMG in my region, but I wanted to chime in on calculating hit dice. I don't think the hit point ranges are to be taken as the rolled minimum and maximum HP for the monster, but instead as the range of average HP values.

Take the range above for a Challenge 11 monster (221-235) and assume we want d8 hit dice. We know that the maximum HP is from rolling maximum hit dice (all 8s in this case) and the minimum is from rolling all 1s. So the difference between maximum and minimum hit points gives us the desired range of our roll:
(235-221)/(8-1) = 2
Therefore the creature has 2 hit dice of d8s. Then the constant term is:
221-2 = 219
Which if we split into hit dice and Con mod gives us a Con mod of +110 (Con score 230). So we give the monster 2d8+219 hit points.
This is probably undesirable!

Hence the approach outlined by aramis erak should probably be used in preference, which simply gives you the desired average hit points.

Note that for a given Challenge rating the calculated hit dice depends on both the die type chosen (presumably corresponding to creature type) and the Con mod (which can be freely adjusted). I would use the minimum HP as my target average when building spellcasting or lurking monsters, and the maximum HP for brutes. In between the two would be soldier and ranged monster types. Then adjust the Con mod to give a desirable number of hit dice (a quick glance at the MM suggests between 1 to 2 hit dice per Challenge level is typical).

One slightly bizarre aspect of this approach is it means that higher con creatures will have fewer HD than similar challenge lower con creatures. If we're letting monsters use HD like PCs it means that the highest con monsters will have the worst healing abilities.