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.
Ability Scores and Modifiers
Each ability also has a modifier, derived from the score and used when making checks involving that ability. To determine an ability modifier, subtract 10 from the ability score and then divide the total by 2 (round down).
Because ability modifiers affect almost every attack roll, ability check, and saving throw, ability modifiers come up in play more often than their associated scores.
Some ability checks (notably untrained skills) use a different modifier calculation to increase the effect of ability score differences. To determine the ability modifier for these checks, subtract 10 from from the ability score. Do not divide by 2. See Ability Checks for additional discussion on when to use which modifier.