Ability Scores And Modifiers

Each of a creature's abilities has a score, a number that defines the magnitude of that ability. An ability score is not just a measure of innate capabilities, but also encompasses a creature's training and competence in activities related to that ability. A score of 10 or 11 is the normal human average, but adventurers and many monsters are a cut above average in most abilities. A score of 18 is the highest that a person usually reaches. Adventurers can have scores as high as 20, and monsters and divine beings can have scores as high as 30.
Each ability also has a modifier, derived from the score and ranging from -5 (for an ability score of 1) to +10 (for a score of 30). The Ability Scores and Modifiers table notes the ability modifiers for the range of possible ability scores, from 1 to 30.
Ability Scores and Modifiers Table Score Modifier 1 -5 2-3 -4 4-5 -3 6-7 -2 8-9 -1 10-11 +0 12-13 +1 14-15 +2 16-17 +3 18-19 +4 20-21 +5 22-23 +6 24-25 +7 26-27 +8 28-29 +9 30 +10