Every ability score has a modifier, and it’s the modifier — not the raw score — that you add to d20 rolls, attacks, damage, and saving throws. The formula: subtract 10 and divide by 2, rounding down. A 16 gives (16 − 10) ÷ 2 = +3; a 9 gives (9 − 10) ÷ 2 = −1 after rounding down.
Because you round down, scores improve in pairs: 14 and 15 both give +2, and 16 and 17 both give +3. That’s why even scores are efficient — an odd score doesn’t pay off until you raise it one more point.
Modifiers run from −5 (a score of 1) to +10 (a score of 30). At character creation the normal cap is 20; Ability Score Improvements, magic items, and epic play can push higher. This tool also builds scores with the standard array (15, 14, 13, 12, 10, 8) or 4d6 drop lowest.