You have one hit die per character level, and its size is set by your class (table below). They do two jobs: they set your hit points as you level (the HP calculator does that part), and they are a pool of healing you can spend on a short rest. Because they only heal at the end of a short rest, they can’t save a character who drops to 0 hit points mid-fight; that is when death saving throws start.
Spending hit dice on a short rest
At the end of a short rest (at least 1 hour), you can spend any number of your unspent hit dice. For each one, roll it and add your Constitution modifier; you regain that many hit points. You can roll one at a time and decide whether to spend another after each roll (SRD 5.1 "Resting"; SRD 5.2 Rules Glossary, "Short Rest").
- •2024 rules: each die restores at least 1 hit point, even with a negative Constitution modifier.
- •2014 rules: there is no stated minimum, so a low roll with a negative Constitution modifier can restore 0.
On average a die restores (die + 1) ÷ 2 + Con: 3.5 for a d6, 4.5 for a d8, 5.5 for a d10 and 6.5 for a d12, before Constitution.
Getting them back on a long rest
This is the big 2014/2024 difference:
- •2014 (SRD 5.1 "Long Rest"): you regain spent hit dice up to half your total, rounded down, minimum one. With eight hit dice you get back four.
- •2024 (SRD 5.2 "Long Rest"): you regain all spent hit dice.
Under 2014 rules, spending every die on day one means starting day two with only half of them, which is why many players spend them carefully.
Multiclass hit dice
Dice of the same size go in one pool; different sizes are tracked separately. A Paladin 5 / fighter 5 has ten d10s, while a paladin 5 / Cleric 5 has five d10s and five d8s (both SRDs, "Multiclassing"). You choose which pool to spend from. The multiclass calculator checks the requirements for a new class.
Worked example
A level 5 fighter with Constitution 14 (+2) took a beating and has 18 of 44 hit points. Spending two d10s restores 2 × (5.5 + 2) = 15 on average (between 6 and 24), bringing her to about 33. A long rest gives both dice back in either edition, because the 2014 cap is half of five rounded down, which is 2. The editions split once she has spent all five: 2014 returns 2 and 2024 returns all 5.