Monster Attack & Damage Modifier Calculator

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.calc-container { max-width: 640px; margin: 2rem 0; padding: 1.5rem; background: #fff; border: 1px solid #ddd; border-radius: 8px; box-shadow: 0 1px 3px rgba(0,0,0,0.06); font-family: system-ui, -apple-system, sans-serif; } .calc-container h3 { font-family: Georgia, serif; font-size: 1.15rem; color: #1a1a1a; margin-bottom: 1rem; padding-bottom: 0.5rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-row { display: flex; align-items: center; gap: 0.75rem; margin-bottom: 0.75rem; flex-wrap: wrap; } .calc-row label { min-width: 160px; font-size: 0.9rem; color: #333; font-weight: 500; } .calc-row input[type="number"], .calc-row select { flex: 1; min-width: 120px; max-width: 200px; padding: 0.5rem 0.6rem; border: 1px solid #ccc; border-radius: 4px; font-size: 0.9rem; font-family: system-ui, sans-serif; color: #1a1a1a; background: #fafaf8; } .calc-row input:focus, .calc-row select:focus { outline: none; border-color: var(--ac, #3d5a80); box-shadow: 0 0 0 2px rgba(26,74,138,0.12); } .calc-row .unit { font-size: 0.82rem; color: #888; min-width: 30px; } .calc-btn { display: inline-block; margin-top: 0.5rem; padding: 0.55rem 1.5rem; background: var(--ac, #3d5a80); color: #fff; border: none; border-radius: 4px; font-size: 0.9rem; font-weight: 600; cursor: pointer; font-family: system-ui, sans-serif; } .calc-btn:hover { opacity: 0.9; } .calc-result { margin-top: 1.25rem; padding: 1rem 1.25rem; background: #f0f6fc; border-left: 3px solid var(--ac, #3d5a80); border-radius: 0 6px 6px 0; display: none; } .calc-result.visible { display: block; } .calc-result-label { font-size: 0.78rem; text-transform: uppercase; letter-spacing: 0.06em; color: #666; margin-bottom: 0.25rem; } .calc-result-value { font-size: 1.6rem; font-weight: 700; color: var(--ac, #3d5a80); } .calc-result-detail { font-size: 0.85rem; color: #555; margin-top: 0.5rem; line-height: 1.5; } .calc-note { margin-top: 1rem; font-size: 0.8rem; color: #888; font-style: italic; } .calc-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 0.75rem; margin-top: 0.75rem; } .calc-grid-item { padding: 0.6rem 0.8rem; background: #f8f9fa; border-radius: 4px; border: 1px solid #eee; } .calc-grid-item .label { font-size: 0.75rem; color: #888; text-transform: uppercase; letter-spacing: 0.04em; } .calc-grid-item .value { font-size: 1.1rem; font-weight: 600; color: #1a1a1a; } @media (max-width: 720px) { .calc-row { flex-direction: column; align-items: flex-start; gap: 0.3rem; } .calc-row label { min-width: auto; } .calc-row input[type="number"], .calc-row select { max-width: 100%; width: 100%; } .calc-grid { grid-template-columns: 1fr; } } .calc-chart { margin: 1rem 0; text-align: center; } .calc-chart svg { max-width: 100%; height: auto; } .calc-chart-legend { display: flex; flex-wrap: wrap; justify-content: center; gap: 0.6rem 1.2rem; margin-top: 0.6rem; font-size: 0.8rem; color: #555; } .calc-chart-legend span { display: inline-flex; align-items: center; gap: 0.3rem; } .calc-chart-legend i { display: inline-block; width: 10px; height: 10px; border-radius: 2px; font-style: normal; } .calc-related { max-width: 640px; margin: 2rem 0 1rem; padding: 1.25rem 1.5rem; background: #f8f9fa; border: 1px solid #e8e8e8; border-radius: 8px; } .calc-related h3 { font-family: Georgia, serif; font-size: 1rem; color: #1a1a1a; margin: 0 0 0.75rem; padding-bottom: 0.4rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-related-list { list-style: none; padding: 0; margin: 0 0 0.75rem; display: grid; grid-template-columns: 1fr 1fr; gap: 0.4rem 1.5rem; } .calc-related-list li a { font-size: 0.88rem; color: var(--ac, #3d5a80); text-decoration: none; } .calc-related-list li a:hover { text-decoration: underline; } .calc-browse-all { margin: 0.5rem 0 0; font-size: 0.9rem; font-weight: 600; } .calc-browse-all a { color: var(--ac, #3d5a80); text-decoration: none; } .calc-browse-all a:hover { text-decoration: underline; } @media (max-width: 720px) { .calc-related-list { grid-template-columns: 1fr; } }

Monster Attack & Damage Modifier Calculator

Calculate a monster's attack bonus and damage modifier based on its ability score, proficiency bonus, and challenge rating (CR). Uses standard D&D 5e rules.

Ability Score (STR or DEX, 1–30)

Challenge Rating (CR, 0–30)

Proficient with Attack?

Yes No

Extra Attack Bonus (magic weapon, etc.)

Damage Dice (e.g. 2d6)

Extra Flat Damage Bonus

Calculate Fill in the fields and click Calculate.

function monCalc() { var resultDiv = document.getElementById('mon-result');

var abilityScore = parseFloat(document.getElementById('mon-ability-score').value); var cr = parseFloat(document.getElementById('mon-cr').value); var proficient = parseInt(document.getElementById('mon-proficient').value); var extraBonus = parseFloat(document.getElementById('mon-extra-bonus').value); var damageDice = document.getElementById('mon-damage-dice').value.trim(); var extraDamage = parseFloat(document.getElementById('mon-extra-damage').value);

// Input validation if (isNaN(abilityScore) || abilityScore 30) { resultDiv.innerHTML = 'Error: Ability Score must be between 1 and 30.'; return; } if (isNaN(cr) || cr 30) { resultDiv.innerHTML = 'Error: CR must be between 0 and 30.'; return; } if (isNaN(extraBonus) || extraBonus 10) { resultDiv.innerHTML = 'Error: Extra Attack Bonus must be between -10 and 10.'; return; } if (isNaN(extraDamage) || extraDamage 20) { resultDiv.innerHTML = 'Error: Extra Flat Damage must be between -10 and 20.'; return; }

// Validate damage dice format (e.g. 2d6, 1d8, 3d4) var diceMatch = damageDice.match(/^(\d+)d(\d+)$/i); if (!diceMatch) { resultDiv.innerHTML = 'Error: Damage Dice must be in format NdM (e.g. 2d6, 1d8).'; return; } var numDice = parseInt(diceMatch[1]); var dieSize = parseInt(diceMatch[2]); if (numDice 20 || dieSize 20) { resultDiv.innerHTML = 'Error: Dice must be between 1d2 and 20d20.'; return; }

// Step 1: Ability Modifier = floor((score - 10) / 2) var abilityMod = Math.floor((abilityScore - 10) / 2);

// Step 2: Proficiency Bonus from CR (D&D 5e table) // CR 0-4: +2, CR 5-8: +3, CR 9-12: +4, CR 13-16: +5, CR 17-20: +6, // CR 21-24: +7, CR 25-28: +8, CR 29-30: +9 var profBonus; if (cr = 0) ? "+" + attackBonus : "" + attackBonus; var damageModStr = (damageModifier >= 0) ? "+" + damageModifier : "" + damageModifier; var abilityModStr = (abilityMod >= 0) ? "+" + abilityMod : "" + abilityMod;

var profLine = proficient
? "Proficiency Bonus (CR " + cr + "): +" + profBonus + ""
"Proficiency Bonus: Not applied";

resultDiv.innerHTML = "### Results " + "" + "Ability Score (" + abilityScore + ")Modifier: " + abilityModStr + "" + "" + profLine + "" + "Extra Attack Bonus" + (extraBonus >= 0 ? "+" : "") + extraBonus + "" + "Total Attack Bonus" + attackStr + "" + "Damage Roll" + damageDice + " " + damageModStr + "" + "Average Damage per Hit" + avgDamage.toFixed(1) + "" + ""; }

#### Formula

Ability Modifier = ⌊(Ability Score − 10) / 2⌋

Proficiency Bonus (by CR): CR 0–4 → +2 | CR 5–8 → +3 | CR 9–12 → +4 | CR 13–16 → +5 | CR 17–20 → +6 | CR 21–24 → +7 | CR 25–28 → +8 | CR 29–30 → +9

Attack Bonus = Ability Modifier + Proficiency Bonus (if proficient) + Extra Bonus

Damage Modifier = Ability Modifier + Extra Flat Damage

Average Damage = (Number of Dice × (Die Size + 1) / 2) + Damage Modifier

#### Assumptions & References

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References

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