Hit Dice, or Hit Die / HD, are mentioned in class kit descriptions as well as in some spells.
For nonplayer creatures, HD is roughly synonymous with character level, with higher HD often representing stronger and tougher creatures.
For playable characters, it’s one of the defining factors in their total Hit Points (HP). Each class has a set dice (d4, d6, d8, d10, or d12) which is rolled each time the class levels up to determine the total HP gained at that level. The character's Constitution will also affect their HP.
When starting a character at Level 1, the character will always roll the maximum starting HP. The Enhanced Editions have a gameplay option (Max HP On Level Up) which automatically adds the maximum roll possible at each level up.
Single Class HP[]
 The Dwarven Defender and Barbarian class kits gain 1d12 HP per level up to level 9, plus the Constitution modifier. From level 10 onwards, they will always gain 3 HP per level.
 All other Fighters, Rangers, and Paladins gain 1d10 HP per level up to level 9, plus the Constitution modifier. From level 10 onwards, they will always gain 3 HP per level.
 All Clerics, Druids, Monks, and Shamans gain 1d8 HP per level up to level 9, plus the Constitution modifier. From level 10 onwards, they will always gain 2 HP per level.
 All Thieves and Bards gain 1d6 HP per level up to level 10, plus the Constitution modifier. From level 11 onwards, they will always gain 2 HP per level.
 The Dragon Disciple class kit gains 1d6 HP per level up to level 10, plus the Constitution modifier. From level 11 onwards, they will always gain 1 HP per level.
 All other Mages and Sorcerers gain 1d4 HP per level up to level 10, plus the Constitution modifier. From level 11 onwards, they will always gain 1 HP per level.
Constitution modifiers to HP are capped at +2 per level for all classes except Warriortypes (including Barbarians), so a Constitution value of over 16 does not really benefit a Priest, Rogue or Wizard type.
MultiClass HP[]
When a MultiClass character levels up, the rolled Hit Dice is divided by 2 (for doubleclasses) or 3 (for tripleclasses). In the case of a decimal value, the result is always rounded down EXCEPT for values less than 1, in which case it is rounded up. For example, 1.666 rounds down to 1; 0.333 rounds up to 1.
For example, a level 1/1/1 Fighter / Mage / Cleric with 10 Constitution (no bonuses or penalties) has 7 HP (10 from Fighter+4 from Mage+8 from Cleric)/3=22/3=7.333=7 after rounding down).
 At 1,500 XP: the first class to level up next is the Cleric. The Hit Dice roll is a 1d8 divided by 3, which can result in 0.333, 0.666, 1, 1.333, 1.666, 2, 2.333, or 2.666. Thus, the character has a 62.5% chance of gaining 1 HP and a 37.5% chance of gaining 2 HP after the rounding is applied. (They will always gain 2 if the Max HP on Level Up option is enabled).
 At 2,000 XP: the next class to level up is the Fighter. The Hit Dice roll is a 1d10 divided by 3, which can result in 0.333, 0.666, 1, 1.333, 1.666, 2, 2.333, 2.666, 3, or 3.333. Thus, the character has a 50% chance of gaining 1 HP, a 30% chance of gaining 2 HP, and a 20% chance of gaining 3 HP after the rounding is applied. (They will always gain 3 if the Max HP on Level Up option is enabled).
 At 2,500 XP: the next class to level up is the Mage. The Hit Dice roll is a 1d4 divided by 3, which can result in 0.333, 0.666, 1, or 1.333. Thus, the character will always gain 1 HP after the rounding is applied.
 This continues until at 250,000 XP: The Fighter and Mage again level up together at which point the Hit Die roll is a 1d10+1d4 divided by 3, which can result in outcomes from 0.666 to 4.666, which after rounding is 1 to 4 HP.
 Note that, eventually, all classes transition to a fixed gain per level. These are also divided by 3 and rounded, which results in a gain of 1 HP for each class level up, unless two or more of the classes level up simultaneously.
When a character has a Constitution bonus (or penalty), this is also divided by the number of classes the character has. If the multiclass has a Fighter, the Warrior Constitution bonus is applied. However, unlike the normal Hit Dice roll, the Constitution bonus is calculated based on the total sum of class levels that the character has.
For example, a level 1/1/1 Fighter / Mage / Cleric with 18 Constitution (+4 HP/level bonus, since the character is part Fighter) starts with 7 HP from the normal Hit Dice roll. The additional HP from CON is calculated by (CON Bonus ÷ Number of Classes x Total Class Level). This calculates to (4 ÷ 3 x 3) = 4, for a total starting HP of 11.
 The first class to level up is the Cleric. The Hit Dice roll is calculated as above. The total class level is now (1 + 1 + 2) = 4. The additional HP from CON is calculated by (4 ÷ 3 x 4) = 5.333. This rounds down to 5, and the previous additional CON HP of 4 is subtracted so that the character gains 1 additional HP due to CON at this level.
 The next class to level up is the Fighter. The Hit Dice roll is calculated as above. The total class level is now (2 + 1 + 2) = 5. The additional HP from CON is calculated by (4 ÷ 3 x 5) = 6.666. This rounds down to 6, and the previous additional CON HP of 5 is subtracted so that the character gains 1 additional HP due to CON at this level.
 The next class to level up is the Mage. The Hit Dice roll is calculated as above. The total class level is now (2 + 2 + 2) = 6. The additional HP from CON is calculated by (4 ÷ 3 x 6) = 8. The previous additional CON HP of 6 is subtracted so that the character gains 2 additional HP due to CON at this level.
 Note that, eventually, all classes transition to a fixed gain per level and will not receive a CON bonus.
As another example, a level 7/7/7 Fighter / Mage / Thief with 10 Constitution (no bonuses or penalties) has 42 HP. If the character drinks a Potion of Fortitude, raising their CON to 18, the additional HP gain is calculated by (4 ÷ 3 x 21) = 28, resulting in a total HP of 70 for the duration of the potion.
Dualclass HP[]
DualClass characters will stop gaining HP once the dual begins until they reactive their first class, then they gain HP each level as the second class normally would.
For example, a Wizard Slayer with 15 Constitution will gain 1d10 + 1 HP per level up. If they DualClass to Mage at level 7, then they will gain no HP for Mage levels 17. When leveling up to Mage level 8, they will gain 1d4 + 1 HP, and once they reach level 11, they will start gaining a fixed + 1 HP per level as a Mage would.
Characters who dualclass into Fighter do only get additional bonus HP for constitution over 16 for those levels which they leveled as Fighters only. For example, a Thief who duals to Fighter at level 7 will only get max. +2 HP bonus per level from constitution for levels 17 but can get higher bonus HP (e.g. +4 per level with 18 constitution) for levels 8 and 9, after which constitution bonus ends anyway.
Mages keep their familiar from Find Familiar and its HP bonus, even while their mage class is inactive, if they have already summoned the familiar.
Quick reference table[]
Class group  Hit Dice per level  Max HD level  Fixed HP gain per level 

Warrior  
Fighter  d10  9  3 
Barbarian  d12  
Dwarven Defender  d12  
Paladin  d10  
Ranger  d10  
Priest  
Cleric  d8  9  2 
Druid  
Monk  
Shaman  
Wizard  
Mage  d4  10  1 
Sorcerer  d4  
Dragon Disciple  d6  
Rogue  
Bard  d6  10  2 
Thief  d6 
Average Hitpoints by Class[]
There is randomness in how many hitpoints are gained on levelup.
In practice, the game engine appears to take the best of 2 dice rolls (tested with a fighter d10 in EE 2.6.6.0).
Best of 2 dice  1  2  3  4  5  6  7  8  9  10  11  12 
1  1  2  3  4  5  6  7  8  9  10  11  12 
2  2  2  3  4  5  6  7  8  9  10  11  12 
3  3  3  3  4  5  6  7  8  9  10  11  12 
4  4  4  4  4  5  6  7  8  9  10  11  12 
5  5  5  5  5  5  6  7  8  9  10  11  12 
6  6  6  6  6  6  6  7  8  9  10  11  12 
7  7  7  7  7  7  7  7  8  9  10  11  12 
8  8  8  8  8  8  8  8  8  9  10  11  12 
9  9  9  9  9  9  9  9  9  9  10  11  12 
10  10  10  10  10  10  10  10  10  10  10  11  12 
11  11  11  11  11  11  11  11  11  11  11  11  12 
12  12  12  12  12  12  12  12  12  12  12  12  12 
The following table shows average hitpoints for each class.
Class  Dice  Average HP gain
Best of 2 Rolls^{[1]} to 1 d.p. 
Max HD Level  Average HPs
at Max HD, Constitution 714 
Average HPs
at Max HD Constitution High 
Later HP gain / Level 


d4  3.1

10 


1 

d6  4.5

10 




d8  5.8

9 


2 

d10  7.2

9 


3 

d12  8.5

9 


3 
The average HP gain on level up in a multiclass is a little less than expected because of rounding down. See MultiClass HP section above.
Class  Dice  SingleClass Average HP gain  2Way Multi Average HP gain  3Way Multi Average HP gain 


d4  3.1

1.4

1


d6  4.5

2.1

1.3


d8  5.8

2.7

1.6


d10  7.2

3.4

2.1


d12  8.5

Not Applicable  Not Applicable 
Average hitpoints for multiclasses are shown below.
MultiClass  Max HD Levels  k XP / Class  Average HPs
Max HD Level Con 714 
Dice  Calculation of Average HP  Average of Single Class HPsfor comparison 

Fighter/ Cleric  9/
9 
250  57.5  d10/
d8 
(10+8)/2 + 8(336/10^2) + 8(173/8^2)

60.85 
Cleric/ Ranger  9/
9 
300  57.5  d8/
d10 
(8+10)/2 + 8(173/8^2) + 8(336/10^2)

60.85 
Fighter/ Thief  9/
10 
250  53.4  d10/
d6 
(10+6)/2 + 8(336/10^2) + 9(74/6^2)

56.75 
Cleric/ Mage  9/
10 
250  40.6  d8/
d4 
(8 + 4)/2 + 8(173/8^2) + 9(23/ 4^2)

43.3 
Fighter/ Mage  9/
10 
250  46.7  d10/
d4 

49.65 
Fighter/ Druid  9/
11 
250  59.0  d10/ d8 

62.9 
Cleric/ Thief  9/
11 
225  47.9  d8/
d6 

51.4 
Thief/ Mage  11/
10 
250  36.6  d6/
d4 

40.2 
Fighter/
Mage/ Cleric 
9/
10/ 9 
250  45.9  d10/
d4/ d8 

51.3 
Fighter/ Mage/
Thief 
9/
10/ 10 
250  43.1  d10/
d4/ d6 

48.5 
Calculation when classes level together:
round[(d4+d6)/2]  1  2  3  4  5  6 

1  1  1  2  2  3  3 
2  1  2  2  3  3  4 
3  2  2  3  3  4  4 
4  2  3  3  4  4  5 
round[(d4+d6)/2]  1  2  3  4  5  
Probability  3/24  7/24  8/24  5/24  1/24  
round[(d4+d6)/2]  <=1  <=2  <=3  <=4  <=5  
Probability  3/24  10/24  18/24  23/24  1  
max of 2 results  <=1  <=2  <=3  <=4  <=5  
Probability  (3/24)^2  (10/24)^2  (18/24)^2  (23/24)^2  1^2  
X = max of 2 results  1  2  3  4  5  
Probability  (3/24)^2  (10/24)^2(3/24)^2  (18/24)^2(10/24)^2  (23/24)^2(18/24)^2  1^2(23/24)^2  
E(X)  
3.330 
Achievements[]
 Main article: Achievements
Juggernaut BG1[]
Reach 150 Hit Points during Baldur's Gate: Enhanced Edition or Baldur's Gate: Siege of Dragonspear.
Juggernaut BG2[]
Reach 150 Hit Points during Baldur's Gate II: Enhanced Edition.