::D&D Ability score tell us how proficient a character is in a particular ability. They are derived by subtracting 10 from the ability score, then dividing the result by 2, rounding down if necessary.
The formula is: Modifier = (Ability Score – 10) / 2.
Example Scenario 1: Character A has a Strength score of 16, therefore an Ability Modifier of +3.
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Modifier = (16 – 10) / 2
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Modifier = (6) / 2
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Modifier = +3
Example Scenario 2: Character B has a Dexterity score of 9, therefore an Ability Modifier of -1.
These modifiers are then used in gameplay for various checks, such as attack rolls, saving throws, and skill checks, adding or subtracting from the dice roll based on the modifier.
For example, let’s say character A is a Fighter class as they would have invented most of their Ability points into Strength and character B is a Sorcerer with a very low Dexterity score due to Clerics investing their Ability points into Wisdom instead.
In a typical D&D dungeon fight the following would play out:
Attack Rolls (when attacking)
When Fighter attempts to hit an opponent with a sword, they would add +3 to their roll:
For example: the Fighter locks eyes with a malicious goblin. He swings his sword with a fierce strike. Rolling a 12 on his d20, his total attack roll computes to 12 +3(Strength Modifier) +2(Proficiency Bonus) = 17.
As the Goblin has an Armor Class (AC) of 15, it would successfully cleave through the Goblin’s armor and land a damaging hit.
Armor Class (when being attacked)
If the Sorcerer is trying to dodge an attack, their lower Dexterity modifier would actually reduce their Armor Class (AC) below the base of 10:
For example: after the Fighter’s turn, the Sorcerer faces a ghastly ghoul that snuck up behind her from a crevice in the cave walls. Due to her class, she isn’t proficient in any armor types, and thus cannot wear any armor other than cloth (which has a 0 AC contribution), combined with her low Dexterity score, she is very susceptible to being hit in this situation.
The ghoul lunges at her, rolling a 15 on its attack. We then calculate her AC from base 10 + (−1 Dexterity Modifier) + (0 Armor Bonus) = 9, which is far lower than the 15 roll from the ghoul, therefore it lands a painful blow to her.