The hit dice are there in case you want to mix things up a bit. Maybe your players have memorized that Monster A has 10 hp but you want to make specific monsters more varied, then you know that you could roll 3d4+3 for that monsters hit points instead.
Short answer: Average HP (Amount of Hit dice rolled + Constitution Modifier)
Long answer: Based on the size category a creature gets a hit die assigned to it, in a way it's like the hit die used for your class, that is the "d4". Then figure out how many times you'll need to roll that die to fit it to the CR of the creature that is the "3" in 3d4. Then you take the Constitution modifier and multiply it by the number of times you rolled the d4, the "+3" part. This is the total amount of HP the creature has.
This gives you flexibility to make creatures more frail or hearty by randomly rolling, similar to damage rolls in a stat block.
More specifically, the number before the calculation is the average result of the roll. The average of a 3d4+3 is 10.5 – but we always roll the calculation down, so it'll be 10.
You'll find averages in lots of the calculations. Your DM might use the average damage for creature attacks too. Using the average flattens out the math and speeds up the game, as when there's multiple creatures they have to control, it can take a lot of time to make separate rolls for both attack and damage.
PCs don't get the luxury of average damage rolls, but you do get the option to take the average HP increase when levelling up.
ey guys so i just began playing d&d recently, on the creatures sheets it says for exemple 10 hp (3d4 + 3)
i thought they had 10 hp so what does the 3d4 + 3 mean?
faster to ask here then to search in the books thanks in advance guys
The hit dice are there in case you want to mix things up a bit. Maybe your players have memorized that Monster A has 10 hp but you want to make specific monsters more varied, then you know that you could roll 3d4+3 for that monsters hit points instead.
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ok makes sense thats pretty cool! thanks a lot
Short answer: Average HP (Amount of Hit dice rolled + Constitution Modifier)
Long answer: Based on the size category a creature gets a hit die assigned to it, in a way it's like the hit die used for your class, that is the "d4". Then figure out how many times you'll need to roll that die to fit it to the CR of the creature that is the "3" in 3d4. Then you take the Constitution modifier and multiply it by the number of times you rolled the d4, the "+3" part. This is the total amount of HP the creature has.
This gives you flexibility to make creatures more frail or hearty by randomly rolling, similar to damage rolls in a stat block.
More specifically, the number before the calculation is the average result of the roll. The average of a 3d4+3 is 10.5 – but we always roll the calculation down, so it'll be 10.
You'll find averages in lots of the calculations. Your DM might use the average damage for creature attacks too. Using the average flattens out the math and speeds up the game, as when there's multiple creatures they have to control, it can take a lot of time to make separate rolls for both attack and damage.
PCs don't get the luxury of average damage rolls, but you do get the option to take the average HP increase when levelling up.
Edit: Ninja'd. Thanks Thac0
It can also be handy if you hace multiple of the same monster, to roll their hp to give them more variance.
If players are hacing an easy time fighting monsters, you can raise their hp a bit to make them last longer.
Comparatively, if players are having a harder time killing a monster than you intended, you can lower its hp a bit to give them a break.
thanks a lot for all your answers this is really useful :)
Besides giving a range of possible HP values for the monster, the hit dice are also used for short rests.
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