Monster hp vs. HD

[Vaahama]

Who or what dictate how many HP a monster have and how it as an impact on his CR?

I mean for exemple, an Ettin got 10D8+20 wich range from 30 to 100 hp so on wich consideration they decide to give it 65?

Typical average i assume?

Then my second question is if i wish to make a very nasty Ettin and give him full HP, wich is 100, does it afect his CR?

[Alexander Kilcoyne]

Go to the back of your bestiary, and look for the table that includes things like HP for CR, high attack for CR, average damage for CR, etc.

Generally if your monster falls mostly into a CR8 range with a couple of exceptions, its a CR8 monster.

Edit: As for the second part of your post, the HD's always work with average HP. In the ettin's case-

4.5*10= 45+20 from Con= 65

If you raised only his HP, I wouldn't say that would be enough to raise its CR if you gave it only one extra HP per HD. Even if the HP is a bit higher than normal for CR, it would need to be a significant HP change (think +50% or +100% to increase CR rating.

[AvalonXQ]

Monsters are listed with average HP. Each dX contributes (X+1)/2 HP to the monster's total. The ettin has 65 HP because 10d8+20=65.
Check the typical monster stats table for CR-appropriate HP. I think you'll find an extra 35 HP may make the monster harder than average for its CR, but if you don't change anything else it's probably not a large enough change to consider it CR+1.

[Tom Baumbach]

Vaahama wrote:

Then my second question is if i wish to make a very nasty Ettin and give him full HP, wich is 100, does it afect his CR?

Generally, no.

Some examples:

• If the party is larger than normal (6 players instead of 4), giving the monsters max hit points is a perfect way of evening out the CR. No CR adjustment needed.
  
• If the creature is at the low end of the "meaningful" range of CR challenges (such as a handful of CR 1 creatures vs. a level 5 party), giving the monsters max hit points is a useful way of making the encounter meaningful (meaning, to actually drain PC resources). No CR adjustment needed.
  
• If the creature's CR is spot on the party's average level, max hit points can make the fight a bigger challenge. If it's just one monster, probably no CR adjustment needed, but if a host of monsters add up to the CR, max hit points across the board probably would justify a +1 CR bump.
  

[IkeDoe]

From my experience using the full dice is about +1 to the CR of that creature, unless it gets so many hps from Constitution that the Hit Dice is almost irrelevant.

IMHO, Absolutely unofficial not based on RAW House Rule:
+50% of total average hps: +1/2 CR
+100% of total average hps (x2): +1 CR
+200% of total average hps (x3): +2 CR