Pathfinder Skill Check Probability Calculator
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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.
Total Skill Modifier
Sum of ability modifier + proficiency bonus + item bonus + status bonus + circumstance bonus, etc.
Difficulty Class (DC)
The target number set by the GM or ability description.
Ruleset
Pathfinder 2e (critical success/failure on ±10) Pathfinder 1e / D&D 3.5 (simple pass/fail)
Calculate Results will appear here.
function patCalc() { var modEl = document.getElementById('pat-modifier'); var dcEl = document.getElementById('pat-dc'); var edEl = document.getElementById('pat-edition'); var resEl = document.getElementById('pat-result');
var mod = parseFloat(modEl.value); var dc = parseFloat(dcEl.value); var ed = edEl.value;
// --- Validation --- if (modEl.value.trim() === '' || dcEl.value.trim() === '') { resEl.innerHTML = 'Please fill in all fields.'; return; } if (!Number.isInteger(mod) || !Number.isInteger(dc)) { resEl.innerHTML = 'Modifier and DC must be whole numbers.'; return; } if (mod 50) { resEl.innerHTML = 'Modifier must be between -20 and 50.'; return; } if (dc 60) { resEl.innerHTML = 'DC must be between 1 and 60.'; return; }
// --- Core maths --- // d20 faces: 1..20, each with probability 1/20 = 5% // Roll needed to succeed (before clamping): dc - mod // Success on roll >= (dc - mod), i.e. roll + mod >= dc // Natural 1 always fails (and may be critical failure in PF2e) // Natural 20 always succeeds (and may be critical success in PF2e)
var needed = dc - mod; // minimum d20 roll to succeed
if (ed === 'pf1e') { // PF1e / 3.5: nat 1 = auto-fail, nat 20 = auto-success, otherwise linear // Success rolls: max(2, needed) .. 20 (nat 1 always fails) // But if needed 20) { // Even nat 20 fails unless we treat nat 20 as auto-success // PF1e: nat 20 is auto-success on attack rolls but NOT on skill checks successRolls = 0; } else { // needed is 2..20 successRolls = 20 - needed + 1; // rolls needed..20 // nat 1 is already excluded because needed >= 2 if (needed === 1) successRolls = 19; // edge case guard } var pSuccess = (successRolls / 20) * 100; var pFail = 100 - pSuccess;
resEl.innerHTML = '### Results (Pathfinder 1e / D&D 3.5) ' + '' + 'OutcomeProbabilityd20 Rolls' + '✓ Success' + pSuccess.toFixed(1) + '%' + successRolls + ' of 20' + '✗ Failure' + pFail.toFixed(1) + '%' + (20 - successRolls) + ' of 20' + '' + 'Minimum d20 roll needed: ' + Math.max(1, needed) + ' (Natural 1 always fails on skill checks in PF1e.)
';
} else { // PF2e: four degrees of success // Critical Success : roll + mod >= dc + 10 OR nat 20 and roll + mod >= dc // Success : roll + mod >= dc AND NOT critical success // Failure : roll + mod = dc+10 -> CS, total >= dc -> S, total >= dc-9 -> F, else CF // nat 20 bumps degree up by one step (max CS) // nat 1 bumps degree down by one step (min CF)
var counts = {cs: 0, s: 0, f: 0, cf: 0};
for (var f = 1; f = dc + 10) degree = 3; else if (total >= dc) degree = 2; else if (total >= dc - 9) degree = 1; else degree = 0;
// Natural 20: bump up one step if (f === 20 && degree 0) degree -= 1;
if (degree === 3) counts.cs++; else if (degree === 2) counts.s++; else if (degree === 1) counts.f++; else counts.cf++; }
var pCS = (counts.cs / 20) * 100; var pS = (counts.s / 20) * 100; var pF = (counts.f / 20) * 100; var pCF = (counts.cf / 20) * 100; var pAnySuccess = pCS + pS; var pAnyFail = pF + pCF;
resEl.innerHTML = '### Results (Pathfinder 2e) ' + '' + 'OutcomeProbabilityd20 Faces' + '★ Critical Success' + pCS.toFixed(1) + '%' + counts.cs + ' of 20' + '✓ Success' + pS.toFixed(1) + '%' + counts.s + ' of 20' + '✗ Failure' + pF.toFixed(1) + '%' + counts.f + ' of 20' + '⚠ Critical Failure' + pCF.toFixed(1) + '%' + counts.cf + ' of 20' + 'Any Success (CS+S)' + pAnySuccess.toFixed(1) + '%' + (counts.cs + counts.s) + ' of 20' + 'Any Failure (F+CF)' + pAnyFail.toFixed(1) + '%' + (counts.f + counts.cf) + ' of 20' + '' + 'Minimum roll for Success: ' + Math.max(1, needed) + ' | Minimum roll for Critical Success: ' + Math.max(1, needed - 10) + '
'; } }
#### Formulas
Core roll: d20 + Total Modifier ≥ DC → Success
Pathfinder 1e / D&D 3.5 (simple pass/fail):
- Natural 1 always fails on skill checks; Natural 20 does not auto-succeed on skill checks in PF1e.
Pathfinder 2e (four degrees of success):
#### Assumptions & References
- 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.
- DC values: Common DCs in PF2e range from 14 (level 1) to 50 (level 25+); see the PF2e GM Core DC-by-level table.
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