Ability score modifier
For a comprehensive summary of the mechanics behind all rolls and modifiers, see Die Rolls.
An Ability Score Modifier is a Die Roll modifier based on an Ability Score of a creature. It can affect every type of roll. Depending on the Ability Score in question, it can be shortened to Strength Modifier, Dexterity Modifier, Constitution Modifier, and so on.
When it's applied
Attack Roll
During Attack Rolls with Weapons, the attacking creature's Strength or Dexterity will be used to determine the Modifier. See the linked Attack Roll and Weapon pages for details.
For a spell that uses an Attack Roll, the Ability Score to be used depends on the Class of the creature casting the spell. For example, a Wizard uses Intelligence, a Cleric uses Wisdom, and a Sorcerer uses Charisma. See the Attack Roll page for details.
Damage Roll
For Damage Rolls made with Weapons, the exact same rules apply as for the Attack Roll.
For spells, the Ability Score Modifier is normally not used, though there are exceptions. For example, the class feature Agonising Blast allows a Warlock to add their Charisma Modifier to the damage of Eldritch Blast; and the spell Shillelagh imbues a weapon with magic, such that it benefits from the wielder's Wisdom Modifier.
Saving Throw
Each Saving Throw is tied to a certain Ability Score, which determines its type, such as Strength Saving Throw, Dexterity Saving Throw, and so on, which determines the Ability Score Modifier that will be used.
Ability Check
The Ability Check is similar to the Saving Throw in that it's categorized based on the Ability Score used. (The clue is in the name.)
A special type of Ability Check is the Skill Check, where a specific skill like Athletics, Perception, Sleight of Hand, etc. is used, but each Skill maps one to one to a corresponding Ability Score, so the Ability Score Modifier still applies. For example, Athletics is based on Strength, whereas Sleight of Hand is based on Dexterity.
Value
Following is the table for the Ability Score Modifier value based on how high the corresponding Ability Score of a creature is:
| Ability score | Modifier value | Ability score | Modifier value | |
|---|---|---|---|---|
| 1 | -5 | 16-17 | +3 | |
| 2-3 | -4 | 18-19 | +4 | |
| 4-5 | -3 | 20-21 | +5 | |
| 6-7 | -2 | 22-23 | +6 | |
| 8-9 | -1 | 24-25 | +7 | |
| 10-11 | +0 | 26-27 | +8 | |
| 12-13 | +1 | 28-29 | +9 | |
| 14-15 | +2 | 30 | +10 |