WCRPG is based on a d% type dice-rolling system. All crucial rolls in the game are made on two ten-sided dice, with one of them designated as "1d10x10" (a "tens-place" die). Valid results on a d% roll therefore range from zero to ninety-nine. Specifically, when a situation comes up wherein a character’s failure may affect the outcome of the game, a die roll is required against a certain failure threshold, known as a difficulty class (DC) (or hit difficulty (HD) in combat situations). These die rolls are known as Checks. The DC for all Checks equals the character's score in the Attribute, Skill, Specialization or Save being checked (usually a combination of one or more of these). Other attributes of the character (such as Traits) may modify the result of the roll. If the final result is lower than or equal to the DC, the action succeeds. If not, it fails. The amount by which a roll falls short of the DC is its degree of success; conversely, the amount by which a roll exceeds the DC is its degree of failure. These simple rules govern all die rolls necessary to play the game.
WCRPG commonly uses variants on the standard d% roll. The most common variants are as follows:
- xd10: This indicates a roll of x ten-sided dice, where x is a set number (for example, a roll calling for 3d10 needs three ten-sided dice). The player rolls the indicated number of dice and sums up the result. NOTE: There is a distinction between 2d10 and d%; 2d10 is an xd10 roll. Be careful not to confuse the two.
- xd5: This is similar to an xd10 roll, except that the ten-sided dice are treated as five-sided dice. To achieve this effect, take the result of an individual die, halve it, and round up (for example, a result of 7 becomes a result of 4). Zeroes count as 10 (a final result of 5) in this case.
- xd2: xd2 rolls are rare in the game, but sometimes occur. Roll the indicated number of ten-sided dice; treat all odd results as 1 and all even results as 2, and sum up the result as with an xd10 roll.
For rolls of d% or xd10, a result of 0 on a die counts as a zero (not 10) unless the situation specifically states otherwise. For example, the die results of a 3d10 roll are 2, 5, and 0. The result of the roll is 7, not 17.
Sometimes a player may roll exceptionally well (or exceptionally poorly) on the dice. Certain die rolls have what's known as critical potential. Critical potential awards or punishes die results above or below certain critical thresholds. Low results may indicate a critical success, denoting a particularly good outcome. In combat, a critical success is more commonly known as a critical hit. A character's critical success threshold equals zero plus one for every ten points in the given Skill Check DC; a roll of 00 is always a critical success. Conversely, very high rolls may result in critical failure (known as a critical miss in combat); critical failures often have very nasty effects. A character's critical failure threshold is 90 plus one for every ten points in the given Skill Check DC; a roll of 99 is always a critical failure. Only certain rolls have critical potential; some have outcomes for critical success only, some for critical failure only, and some for both critical success and critical failure. A roll that has critical potential will be noted in the rules, along with the effects of critical results. A critical result occurs if the player's roll falls within the bounds of a critical threshold regardless of whether or not the player would have otherwise succeeded against the Check's DC.