You calculate hit points in 5e based on your level, your class’s hit die, and your Constitution modifier. At 1st-level, a character’s hit points equal the maximum value of their hit die plus their Constitution modifier. For example, a Wizard (d6 hit die) with a +2 Constitution modifier would start at 1st-level with 8 hit points.
Every time you level up, you can either roll your class hit die or take the average value (rounded up) and add your Constitution modifier to determine how many hit points you gain. For example, a Wizard with a +2 Constitution modifier can either gain 1d6 + 2 hit points when they level up, or they can automatically take 6 hit points (1d6 average = 3.5, rounded up to 4, +2).
Starting Hit Points by Class in 5e
| Class | Starting Hit Points |
|---|---|
| Artificer | 8 + Con Mod |
| Barbarian | 12 + Con Mod |
| Bard | 8 + Con Mod |
| Cleric | 8 + Con Mod |
| Druid | 8 + Con Mod |
| Fighter | 10 + Con Mod |
| Monk | 8 + Con Mod |
| Paladin | 10 + Con Mod |
| Ranger | 10 + Con Mod |
| Rogue | 8 + Con Mod |
| Sorcerer | 6 + Con Mod |
| Warlock | 8 + Con Mod |
| Wizard | 6 + Con Mod |
Average Roll Results by Hit Die in 5e
| Hit Die | Average Result |
|---|---|
| d6 | 4 |
| d8 | 5 |
| d10 | 6 |
| d12 | 7 |
Player’s Handbook, page 12
Retroactive Hit Point Gains
If your character’s Constitution modifier increases, you retroactively gain +1 hit point for every character level you have, minus 1. For example, if you increase your Constitution modifier from +2 to +3 upon becoming 8th level, your character would gain 7 hit points from this (as well as use the new +3 Constitution modifier for determining hit points gained from this new level up).
This applies to all Constitution modifier changes, both positive and negative, permanent and temporary. For example, if a magic item increases a 10th-level player’s Constitution modifier by +2, that player gains 20 maximum hit points.
Alternatively, if a player’s Constitution modifier is reduced by an effect, their hit points are also reduced. For example, if a monster’s curse reduces a 5th-level player’s Constitution modifier by -1, that player loses 5 maximum hit points.
Player’s Handbook, page 177
Note that this also applies to the Tough feat, which grants +2 hit points per level, both after taking it and retroactively. For example, if an 8th-level player gains the Tough feat, they instantly gain 16 hit points (PHB 170).
