Pathfinder Skill Check Probability Calculator
Calculate the probability of succeeding on a Pathfinder skill check (1d20 + modifier vs. DC). Includes critical success and critical failure chances per Pathfinder 2e rules.
Sum of ability modifier + proficiency bonus + item bonus + status bonus + circumstance bonus, etc.
The target number set by the GM or ability description.
Results will appear here.
Formulas
Core roll: d20 + Total Modifier ≥ DC → Success
Pathfinder 1e / D&D 3.5 (simple pass/fail):
- P(Success) = (number of d20 faces where roll + modifier ≥ DC) / 20 × 100%
- Natural 1 always fails on skill checks; Natural 20 does not auto-succeed on skill checks in PF1e.
- Minimum roll needed = DC − modifier (clamped to 1–20 range)
- Success faces = max(0, 20 − max(1, DC − modifier) + 1)
Pathfinder 2e (four degrees of success):
- For each face f (1–20): total = f + modifier
- Base degree: total ≥ DC + 10 → Critical Success; total ≥ DC → Success; total ≥ DC − 9 → Failure; else Critical Failure
- Natural 20 upgrades degree by one step (max Critical Success)
- Natural 1 downgrades degree by one step (min Critical Failure)
- P(outcome) = count of faces producing that outcome / 20 × 100%
Assumptions & References
- A standard fair d20 is used; each face (1–20) has equal probability of 5%.
- PF2e degrees of success follow the Pathfinder 2e Core Rulebook, p. 445–446 (natural 1/20 degree shift rules).
- PF1e skill checks do not have automatic success on a natural 20 (unlike attack rolls); a natural 1 always fails.
- Total modifier includes: ability modifier, proficiency bonus (PF2e: untrained/trained/expert/master/legendary + level), item bonuses, status bonuses, circumstance bonuses, and any penalties.
- Bonuses of the same type do not stack in PF2e; only the highest applies.
- This calculator does not account for Hero Points, Fortune/Misfortune effects, or Aid Another bonuses.
- DC values: Common DCs in PF2e range from 14 (level 1) to 50 (level 25+); see the PF2e GM Core DC-by-level table.