I’ve been playing D&D for 3 years, and recently I started to seriously look into it to become a better DM. I noticed that it would make sense for the HP to go up every time you level up, but how does the HP get determined? I know how to get the starting HP (hit die + CON modifier), just not the L2+ HP (if it does exist).
From the basic rules:
Each time you gain a level, you gain 1 additional Hit Die. Roll that Hit Die, add your Constitution modifier to the roll, and add the total (minimum of 1) to your hit point maximum. Alternatively, you can use the fixed value shown in your class entry, which is the average result of the die roll (rounded up).
The size of the hit die is determined by the class, as can be seen on the Classes Summary chart, and as noted in the class descriptions, which also note how hit points increase. For example, the barbarian class description states:
Hit Dice: 1d12 per barbarian level
Hit Points at 1st Level: 12 + your Constitution modifier
Hit Points at Higher Levels: 1d12 (or 7) + your Constitution modifier per barbarian level after 1st