How this instrument works
In D&D 5e, a character's maximum hit points grow every level based on their class's hit die — d6, d8, d10 or d12 — plus their Constitution modifier. At 1st level you start with the hit die's maximum value plus your Con modifier. At every level after that, the rules let you either roll the hit die and add Con modifier, or take the fixed average value shown in the class table, which is the die's average result rounded up.
This instrument uses that fixed-average option, since it's deterministic and matches the number most tables use by default for NPCs and quick character building. A d8 class, for example, averages 4.5 per roll, which rounds up to 5; a d12 barbarian averages 6.5, rounding up to 7 — these rounded averages are added to Con modifier at every level past 1st.
Knowing your expected HP total matters for more than just tracking damage: it tells you how much punishment your character can absorb before dropping, helps a DM gauge whether an encounter's damage output is survivable, and lets you compare the survivability of different classes or Constitution builds before committing to one at character creation.
- Choose your class's hit die — d6, d8, d10 or d12 — from the Hit die selector.
- Enter your character's Constitution modifier (this can be negative for a low Constitution score).
- Enter your character's current level, from 1 to 20.
- Read Total hit points for the character's maximum HP using the fixed-average rule at every level past 1st.
- HP gained per level after 1st shows the flat amount (rounded-up die average plus Con modifier) added at each level beyond the first, useful for checking your math level by level.
Worked example — level 5 d8 class with +2 Constitution
Choose d8 as the hit die, enter a Constitution modifier of +2, and a level of 5. The average d8 roll is 4.5, which rounds up to 5, so HP gained per level after 1st = 5 + 2 = 7.
Level 1 gives the die's maximum plus Con modifier: 8 + 2 = 10. Levels 2 through 5 are four additional levels, each adding the 7-per-level average: 4 x 7 = 28. Total hit points = 10 + 28 = 38, exactly what a level 5 Cleric, Druid, Bard, Monk, Rogue, Artificer or Warlock with a +2 Con modifier would show on their character sheet using the fixed-average rule.
Questions
Should I roll for HP or use the fixed average?
Either is legal under the official rules — it's a table preference. Rolling can give you more HP than the average on a lucky night, but it can also give you less; the fixed average this instrument uses removes that variance entirely and is what most published D&D adventures assume for consistency when building NPCs and pre-generated characters.
Why is level 1 HP calculated differently from every level after it?
At 1st level, a D&D 5e character takes the maximum possible value of their hit die (not the average), plus their Constitution modifier — the game guarantees your starting HP isn't left to chance. Only levels 2 and beyond use the averaged (or rolled) hit die value, which is why this instrument adds the die's max once at level 1 and then the average for every level after.
What's my hit die if I multiclass?
Multiclass characters gain a hit die from each class they take a level in, and add up the HP from each class's die separately using that class's hit die size. This instrument calculates a single class's contribution; for a multiclass character, run each class's levels through it separately and sum the totals.
Can my Constitution modifier be negative?
Yes — a Constitution score of 8 or 9 gives a -1 modifier, and lower scores go further negative. The instrument accepts negative Constitution modifiers, though the rules guarantee a minimum of 1 HP gained per level even if the math would otherwise produce zero or less.
Why does a d12 class average 7 HP per level and not 6?
A d12's arithmetic average is 6.5 (the midpoint between 1 and 12), and the rules round that up to 7 before adding the Constitution modifier. Every hit die rounds its average up under this rule: d6 rounds 3.5 to 4, d8 rounds 4.5 to 5, d10 rounds 5.5 to 6, and d12 rounds 6.5 to 7.
Does a Constitution score increase apply retroactively to past levels?
Yes. If your Constitution modifier goes up (from an Ability Score Improvement or magic item, for example), the official rules say your hit point maximum increases by 1 for every level you've already gained, not just future ones. Re-run this calculator with the new modifier and your current level to see the updated total.