Whenever you attempt an action that has some chance of failure, you roll a twenty-sided die (d20). To determine if your character succeeds at a task you do this:
If the result equals or exceeds the target number, your character succeeds. If the result is lower than the target number, you fail.
Dice rolls are described with expressions such as “3d4+5,” which means “roll three four-sided dice and add 5 to the total” (resulting in a number between 6 and 15). The first number tells you how many dice to roll (adding the results together). The number immediately after the “d” tells you the type of die to use. Any number after that indicates a quantity that is added or subtracted from the result.
Percentile dice work a little differently. You generate a number between 1 and 100 by rolling two different ten-sided dice. One (designated before you roll) is the tens digit. The other is the ones digit. Two 0s represent 100.
A modifier is any bonus or penalty applying to a die roll. A positive modifier is a bonus, and a negative modifier is a penalty.
In most cases, modifiers to a given check or roll stack (combine for a cumulative effect) if they come from different sources and have different types (or no type at all), but do not stack if they have the same type or come from the same source (such as the same spell cast twice in succession). If the modifiers to a particular roll do not stack, only the best bonus and worst penalty applies. Dodge bonuses and circumstance bonuses however, do stack with one another unless otherwise specified.
Modifiers can be applied to other modifiers. For example, a creature can have a +2 natural armor bonus to Armor Class and a +2 enhancement bonus to natural armor, resulting in an overall +4 bonus to Armor Class.
In general, if you wind up with a fraction, round down, even if the fraction is one-half or larger.
Exception: Certain rolls, such as damage and hit points, have a minimum of 1.
Sometimes a rule makes you multiply a number or a die roll. As long as you’re applying a single multiplier, multiply the number normally. When two or more multipliers apply to any abstract value (such as a modifier or a die roll), however, combine them into a single multiple, with each extra multiple adding 1 less than its value to the first multiple. Thus, a double (×2) and a double (×2) applied to the same number results in a triple (×3, because 2 + 1 = 3).
When applying multipliers to real-world values (such as weight or distance), normal rules of math apply instead. A creature whose size doubles (thus multiplying its weight by 8) and then is turned to stone (which would multiply its weight by a factor of roughly 3) now weighs about 24 times normal, not 10 times normal. Similarly, a blinded creature attempting to negotiate difficult terrain would count each square as 4 squares (doubling the cost twice, for a total multiplier of ×4), rather than as 3 squares (adding 100% twice).
Each ability has a score and a modifier. After your ability score is determined (and you have applied any racial bonuses, magical modifiers, and other adjustments), your ability modifier is equal to:
(Ability score - 10) / 2
In other words, for every 2 points by which your ability score exceeds 10, your ability modifier increases by 1.
| Ability score | Ability 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 |
The modifier is the number you apply to the die roll when your character tries to do something related to that ability. You also use the modifier with some numbers that aren’t die rolls. A positive modifier is called a bonus, and a negative modifier is called a penalty.
Each ability partially describes your character and affects some of his or her actions.
Strength measures your character’s muscle and physical power. This ability is especially important for fighters, barbarians, paladins, rangers, monks, and other warrior types because it determines accuracy and damage with melee attacks. Strength also limits the amount of equipment your character can carry.
You apply your character’s Strength modifier to:
Dexterity measures hand-eye coordination, agility, reflexes, and balance. This ability is very important for rogues, but it’s also high on the list for characters who typically wear light or medium armor (rangers and barbarians) or no armor at all (monks and arcanists), and for anyone who wants to be a skilled archer.
You apply your character’s Dexterity modifier to:
Constitution represents your character’s health and stamina. A Constitution bonus increases a character’s hit points, so the ability is important for all classes - especially classes like arcanists, rogues, and psions who get very few hit points naturally.
You apply your character’s Constitution modifier to:
If a character’s Constitution score changes enough to alter his or her Constitution modifier, the character’s hit points also increase or decrease accordingly.
Intelligence determines your character's memory, reasoning, and ability to learn. This ability is important for wizards because it affects how many spells they can cast, how hard their spells are to resist, and how powerful their spells can be. It’s also important for any character who wants to have a wide assortment of skills, such as a factotum or a rogue.
You apply your character’s Intelligence modifier to:
A wizard gains bonus spells based on her Intelligence score. The minimum Intelligence score needed to cast a wizard spell is 10 + the spell’s level.
An animal has an Intelligence score of 1 or 2. A creature of humanlike intelligence has a score of at least 3.
Wisdom describes a character’s willpower, common sense, perception, and intuition. While Intelligence represents one’s ability to analyze information, Wisdom represents being in tune with and aware of one’s surroundings. Wisdom is the most important ability for clerics and druids, and it is also important for paladins, rangers, and psychic warriors. If you want your character to have acute senses, put a high score in Wisdom.
You apply your character’s Wisdom modifier to:
Clerics, druids, paladins, and rangers get bonus spells based on their Wisdom scores. The minimum Wisdom score needed to cast a cleric, druid, paladin, or ranger spell is 10 + the spell’s level.
Charisma measures a character’s force of personality, persuasiveness, personal magnetism, and ability to lead. This ability represents actual strength of personality, not merely how one is perceived by others in a social setting. Charisma is most important for paladins, sorcerers, marshals, and bards. It is also important for clerics, since it affects their ability to turn undead.
You apply your character’s Charisma modifier to:
Sorcerers and bards get bonus spells based on their Charisma scores. The minimum Charisma score needed to cast a sorcerer or bard spell is 10 + the spell’s level.
When an ability score changes, all attributes associated with that score change accordingly.
An exception to this is skill points. A character who gains an increase to Intelligence does not retroactively receive skill points for previous class levels.
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