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What is giving me trouble is the phrase

+ your Constitution modifier per [class] level after 1st.

Does this simply mean I add my CON modifier once each time I level up, or add my CON modifier a number of times equal to my class level?

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up vote 11 down vote accepted

Each time you gain a level you will add your constitution modifier and one roll of your hit die to your previous max HP. If a barbarian with 18 constitution levels up he takes his previous max plus a roll of a d12 plus 4 for his constitution modifier.

On a related note if your CON modifier increases later you gain hit points retroactively for all previous levels.

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I have to agree that the wording is a bit convoluted or rather short in the class definitions. However, it is clarified in PHB p. 177:

If your constitution modifier changes, your hit point maximum changes as well, as though you had the new modifier from the 1st level. [...]

(And where some people may get quite wrongly confused, the example they give is positive and not negative... but unfortunately, it does go both ways.)

The math is pretty straight forward and can be expressed as follow:

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There are many reason why your constitution will go up or down and thus your max. HP (and current HP too!) will vary if your constitution varies. It is up to your DM to have you decrease your current HP or not, but it would make more sense for you to lose the same amount in your current HP, although the math can be a bit tricky (i.e. if you current HP represents what you had at level 3, then you should lose your constitution modifier × 3, and not your constitution modified × current level.) For sure, your current hit points cannot be any higher than your maximum.

In my formula, I put a minimum of 1 HP per level which does not appear in the PHB, at least not for HP. But there are various places where that max() function is used. For example, in PHB p. 84, the Paladin Spellcasting ability says that the Paladin gets a number of spells equal to:

[...] Charisma modifier + half your Paladin level, rounded down (minimum of one spell) [...]

There is also the Enlarge/Reduce spell in PHB p. 237 which says (very end):

[...] (this can't reduce the damage below 1).

Also, without the minimum, you could end up dead just and only because you lose your constitution (i.e. a really bad constitution of 1 would give you a -5 modifier, so at level 20, you would lose 100 HP from your rolls... and if your maximum was close to 100 [sorcerers and wizards have 1d6 or an average of 4 × level... would 80 HP - 100 HP = -20 HP], you would die right there.)

As a side note, that applies to all effects that make use of a modifier. So you could lose spells you've learn before because your Intelligence decreases. Similarly, and quite important, your Dexterity could decrease and thus does your AC. That is much more common than losing constitution or intelligence (also, you may want to read Feeblemind, PHB p. 239 in link with abilities going to 1, some magical treasures also have such wonderful effects...)

As pointed out by Miniman, some creatures (Vampire, Wright) and spells (Harm) may reduce your maximum number of hit points. In that case, this reduction has nothing to do with your constitution nor your level, however, it can further complicate your HP math.

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So you are saying that if I level up I may get 0 or even -1 HP for gaining a level? – Alexis Wilke Feb 18 '15 at 5:49
Sorry, it appeared that you were talking about what happens when your Con decreases. That said, with a sufficiently low Con modifier there's no reason you couldn't gain 0 or less max hp on level up. – Miniman Feb 18 '15 at 6:02
Alexis, this seems to be responding about con changes, but the asker only wanted to know if they add their con modifier onto their new level's HP gain once, or once per class level they have: i.e. nothing to do with con changes. That would make this not an answer. However, if you still want to preserve this writing, you could create a self-answered question for this content. – doppelgreener Feb 18 '15 at 6:35
Guys, I don't get you, the formula I present in big in my answer represents a direct answer to the question: how should the constitution modifier be added? Then I included points about where you can find the information and why I would use the max() for the minimum of 1 HP/level. Not only that, if the person asking the question does not understand the basics, they are not likely to understand that if the constitution changes, so are the HP which my answer points out. Both are the exact same thing. Constitution is not a static parameter and thus HP is not either... – Alexis Wilke Feb 18 '15 at 6:59
@Alexis Because CON changing isn't something the asker's confused about or considering. I've reread your answer and it seems it isn't as not-an-answer as I thought, but it's still off target and wandering into issues that aren't even present here. They're asking, assuming a particular hit die and CON modifier: "This thing says to add my constitution modifier per class level. When I hit level 3, do they mean I add 1d8 + 4 health, or do they mean I add 1d8 + (4 + 4 + 4) health?" – doppelgreener Feb 18 '15 at 7:20

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