An ability modifier is a number applied to a die roll when doing something related to that ability, such as attack rolls, saving throws and skills. The modifier is also used with some numbers that aren't die rolls. A positive modifier is called a bonus, and a negative modifier is called a penalty.
The modifier is equal to +1 for every 2 points above 10 in that ability score (or -1 for every 2 points below 10). For example, constitution 14 provides +2 hit points per level. The formula for this is (ability -10)/2, rounded down. (Note that -0.5 rounded down is -1.)
In addition, ability scores determine the number of bonus spells for spellcasters. The ability that spells relate to depends on the type of spellcaster: intelligence for wizards; wisdom for clerics, druids, paladins, and rangers; or charisma for sorcerers and bards. In addition to having a high ability score, a spellcaster must be of sufficient level in order to gain any bonus spells for a given level. Spellcasters must also have a 10 or higher in the appropriate ability score in order to cast any spells.
As a general rule, you get (ability modifier + 1 - spell level) / 4 (round up) bonus spells of any given spell level.
Modifier table
| Score | Modifier | Bonus spells per day | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 1st | 2nd | 3rd | 4th | 5th | 6th | 7th | 8th | 9th | ||
| 1 | -5 | Can't cast spells tied to this ability | |||||||||
| 2-3 | -4 | Can't cast spells tied to this ability | |||||||||
| 4-5 | -3 | Can't cast spells tied to this ability | |||||||||
| 6-7 | -2 | Can't cast spells tied to this ability | |||||||||
| 8-9 | -1 | Can't cast spells tied to this ability | |||||||||
| 10-11 | 0 | - | - | - | - | - | - | - | - | - | - |
| 12-13 | +1 | - | 1 | - | - | - | - | - | - | - | - |
| 14-15 | +2 | - | 1 | 1 | - | - | - | - | - | - | - |
| 16-17 | +3 | - | 1 | 1 | 1 | - | - | - | - | - | - |
| 18-19 | +4 | - | 1 | 1 | 1 | 1 | - | - | - | - | - |
| 20-21 | +5 | - | 2 | 1 | 1 | 1 | 1 | - | - | - | - |
| 22-23 | +6 | - | 2 | 2 | 1 | 1 | 1 | 1 | - | - | - |
| 24-25 | +7 | - | 2 | 2 | 2 | 1 | 1 | 1 | 1 | - | - |
| 26-27 | +8 | - | 2 | 2 | 2 | 2 | 1 | 1 | 1 | 1 | - |
| 28-29 | +9 | - | 3 | 2 | 2 | 2 | 2 | 1 | 1 | 1 | 1 |
| 30-31 | +10 | - | 3 | 3 | 2 | 2 | 2 | 2 | 1 | 1 | 1 |
| 32-33 | +11 | - | 3 | 3 | 3 | 2 | 2 | 2 | 2 | 1 | 1 |
| 34-35 | +12 | - | 3 | 3 | 3 | 3 | 2 | 2 | 2 | 2 | 1 |
| 36-37 | +13 | - | 4 | 3 | 3 | 3 | 3 | 2 | 2 | 2 | 2 |
| 38-39 | +14 | - | 4 | 4 | 3 | 3 | 3 | 3 | 2 | 2 | 2 |
| 40-41 | +15 | - | 4 | 4 | 4 | 3 | 3 | 3 | 3 | 2 | 2 |
| 42-43 | +16 | - | 4 | 4 | 4 | 4 | 3 | 3 | 3 | 3 | 2 |
| 44-45 | +17 | - | 5 | 4 | 4 | 4 | 4 | 3 | 3 | 3 | 3 |
| 46-47 | +18 | - | 5 | 5 | 4 | 4 | 4 | 4 | 3 | 3 | 3 |
| 48-49 | +19 | - | 5 | 5 | 5 | 4 | 4 | 4 | 4 | 3 | 3 |
| 50-51 | +20 | - | 5 | 5 | 5 | 5 | 4 | 4 | 4 | 4 | 3 |
| 52 | +21 | - | 6 | 5 | 5 | 5 | 5 | 4 | 4 | 4 | 4 |