These rules are loosely based on Chaosium’s Basic Roleplaying system, published in 1981 and revised across multiple editions. Games using a version of this system include Call of Cthulhu, RuneQuest, Stormbringer, Superworld, and more. The original creators include Steve Perrin, Steve Henderson, Warren James, Greg Stafford, Sandy Petersen, Ray Turney, and Lynn Willis.
Start with a percentage chance of success for a task under stressful, dangerous, contested, or unusual circumstances. (A non-challenging task should simply succeed.)
* 90% or more … master, visionary, epic-level
* 76% to 90% … expert, years of experience, high-level
* 51% to 75% … professional, experienced, competent, mid-level
* 26% to 50% … amateur, some talent or training, low-level
* 6% to 25% … novice, no training but some related skills
* 5% … slight chance of success by luck alone
* 0% … no ability, no chance of success
Roll percentile dice (d%) equal to or below the percentage value to succeed.
* If rolling half the % value or less, the outcome is a critical success.
* If rolling 100 (0 on both dice), the outcome is a critical failure.
Else, choose one die (or two or three six-sided dice) and follow the appropriate list below.
* A smaller die increases the chance of a critical success or failure.
* Roll within the range shown for success.
* If rolling the maximum possible number, the outcome is a critical success.
* If rolling the minimum possible number, the outcome is a critical failure.
* At very high skill levels, if a minimum possible number is rolled (usually a 1), re-roll the die and use the second result. If a second roll is also the minimum number, the outcome is a critical failure.
Regardless of dice rolled, if situation gives the character advantage, roll twice and use the better result.
If the character suffers disadvantage, roll twice and use the worse result.
If rolling percentile dice:
* 1 … critical success
* Equal or less than half % … major success
* Equal or less than % … minor success
* Above % number … failure
* 100 … critical failure
If rolling other dice:
* 1 … critical failure
* roll in range … minor success
* roll in range for half % skill … major success
* maximum possible roll … critical success
Outcomes
* Critical Success: Complete success with an exceptional additional benefit. Double attack damage and duration.
* Major Success: Complete success without complications. Full attack damage or duration.
* Minor Success: Partial success (half damage or duration), or complete success with a minor cost (resources, time, fatigue).
* Failure: The action does not succeed.
* Critical Failure: The action fails and causes a serious complication.
If two sides are directly competing, the size with the greater success (minor versus major versus critical) wins.
If using a d4, success if rolling…
* 76% to 100% … 2 to 4 (re-roll once if 1 is rolled)
* 51% to 75% … 2 to 4
* 26% to 50% … 3 to 4
* 1% to 25% … 4
If using a d6, success if rolling…
* 84% to 100% … 2 to 6 (re-roll once if 1 is rolled)
* 68% to 83% … 2 to 6
* 51% to 67% … 3 to 6
* 34% to 50% … 4 to 6
* 18% to 33% … 5 to 6
* 1% to 17% … 6
If using 2d6, success if rolling…
* 98% to 100% … 3 to 12 (re-roll once if 2 is rolled)
* 93% to 97% … 3 to 12
* 84% to 92% … 4 to 12
* 73% to 83% … 5 to 12
* 59% to 72% … 6 to 12
* 43% to 58% … 7 to 12
* 29% to 42% … 8 to 12
* 18% to 28% … 9 to 12
* 9% to 17% … 10 to 12
* 4% to 8% … 11 to 12
* 1% to 3% … 12
If using a 3d6, success if rolling…
* 99% to 100% … 4 to 18
* 96% to 98% … 5 to 18
* 92% to 95% … 6 to 18
* 85% to 91% … 7 to 18
* 75% to 84% … 8 to 18
* 63% to 74% … 9 to 18
* 51% to 62% … 10 to 18
* 38% to 50% … 11 to 18
* 27% to 37% … 12 to 18
* 17% to 26% … 13 to 18
* 10% to 16% … 14 to 18
* 6% to 9% … 15 to 18
* 3% to 5% … 16 to 18
* 1% to 2% … 17 to 18
If using a d8, success if rolling…
* 89% to 100% … 2 to 8 (re-roll once if 1 is rolled)
* 76% to 88% … 2 to 8
* 64% to 75% … 3 to 8
* 51% to 63% … 4 to 8
* 39% to 50% … 5 to 8
* 26% to 38% … 6 to 8
* 14% to 25% … 7 to 8
* 1% to 13% … 8
If using a d10, success if rolling…
* 90% to 100% … 2 to 10 (re-roll once if 1 is rolled)
* 80% to 89% … 2 to 10
* 70% to 79% … 3 to 10
* 60% to 69% … 4 to 10
* 50% to 59% … 5 to 10
* 40% to 49% … 6 to 10
* 30% to 39% … 7 to 10
* 20% to 29% … 8 to 10
* 10% to 19% … 9 to 10
* 1% to 9% … 10
If using a d12, success if rolling…
* 93% to 100% … 2 to 12 (re-roll once if 1 is rolled)
* 84% to 92% … 2 to 12
* 76% to 83% … 3 to 12
* 68% to 75% … 4 to 12
* 59% to 67% … 5 to 12
* 51% to 58% … 6 to 12
* 43% to 50% … 7 to 12
* 34% to 42% … 8 to 12
* 26% to 33% … 9 to 12
* 18% to 25% … 10 to 12
* 9% to 17% … 11 to 12
* 1% to 8% … 12
If using a d20, success if rolling…
* 96% to 100% … 2 to 20 (re-roll once if 1 is rolled)
* 91% to 95% … 2 to 20
* 86% to 90% … 3 to 20
* 81% to 85% … 4 to 20
* 76% to 80% … 5 to 20
* 71% to 75% … 6 to 20
* 66% to 70% … 7 to 20
* 61% to 65% … 8 to 20
* 56% to 60% … 9 to 20
* 51% to 55% … 10 to 20
* 46% to 50% … 11 to 20
* 41% to 45% … 12 to 20
* 36% to 40% … 13 to 20
* 31% to 35% … 14 to 20
* 26% to 30% … 15 to 20
* 21% to 25% … 16 to 20
* 16% to 20% … 17 to 20
* 11% to 15% … 18 to 20
* 6% to 10% … 19 to 20
* 1% to 5% … 20