Spell Attack Bonus Calculator

Formula used

The normal spell attack formula is:

Spell Attack Bonus = Spellcasting Ability Modifier + Proficiency Bonus + Applicable Bonuses − Applicable Penalties

The related saving throw formula is:

Spell Save DC = 8 + Spellcasting Ability Modifier + Proficiency Bonus + Applicable Save DC Bonuses

An ability modifier is normally calculated with:

Ability Modifier = floor((Ability Score − 10) ÷ 2)

The calculator treats a natural 1 as a miss. A natural 20 hits. An expanded critical range can also create a critical hit when an applicable feature allows it. Advantage, disadvantage, custom d20 pools, rerolls, and attack bonus dice are evaluated with exact discrete probability distributions.

Expected damage is calculated with:

Expected Damage = Noncritical Hit Chance × Average Hit Damage + Critical Chance × Average Critical Damage

For several independent attack rolls, the displayed combined expectation multiplies expected damage per attack by the entered attack count.

How to use this calculator

1. Select the ruleset, creature mode, class, and spell source.
2. Enter the total character level and spellcasting ability.
3. Use automatic proficiency or provide a manual value.
4. Add only modifiers that explicitly affect spell attack rolls.
5. Enter the target AC, cover, and current roll condition.
6. Add spell damage dice when expected damage is useful.
7. Enable comparison profiles for multiclass or alternate sources.
8. Press Calculate spell attack and review every warning.
9. Export the result, print it, or copy a shareable link.

For a quick answer, only level, ability score, target AC, and roll mode are required. Advanced fields may remain at their defaults.

Example spell attack data

These examples are demonstrations, not automatic character-building decisions. A particular item, feat, subclass, or spell source may use different wording.

Understanding spell attack bonuses

Start with the spellcasting source

A character can receive spells from several places. A class spell usually uses that class’s listed spellcasting ability. A feat, species trait, subclass feature, or magic item can specify a different ability or a fixed number. Multiclass characters therefore may have several valid spell attack bonuses at once. Do not combine those abilities into one universal value. Select the source used by the specific spell.

Separate attack rolls from saving throws

A spell attack asks the caster to roll against Armor Class. A saving throw asks the target to roll against the caster’s save DC. Some spells make several attacks. Other spells affect several creatures through saving throws. A spell may also use neither mechanic. The calculator displays both common caster statistics, but only use the statistic required by the spell description.

Check every bonus carefully

Magic items and features use precise wording. A bonus to weapon attack rolls does not automatically affect spell attacks. A bonus to spell attack rolls may not raise spell save DC. A general bonus to attack rolls may be broader. Temporary dice can also apply only under defined circumstances. Enter a modifier only after confirming that its wording covers the current roll.

Use fixed values for monsters and items

Published monster statistics often provide a ready spell attack bonus and save DC. Certain items can also provide fixed values. In those cases, fixed mode prevents accidental double counting. The ability, proficiency, and modifier fields remain useful for comparison, but they do not change the fixed result.

Interpret advantage and disadvantage

Advantage increases the chance of high d20 results. Disadvantage increases the chance of low results. When both apply under the standard approach, they cancel and one d20 is rolled. Multiple ordinary sources do not usually create extra d20s. Custom pools are available for special features and homebrew play. The calculator shows their mathematical effects without deciding whether a feature is legally applicable.

Understand bonus dice and rerolls

A flat bonus shifts every eligible total equally. A bonus die creates a distribution of possible adjustments. Therefore, a die’s average alone does not fully describe its effect near a target number. This calculator combines each d20 result with every possible bonus-die result. Rerolls are also modeled as one reroll of each qualifying die, with the second result retained.

Review cover and distance

Cover can raise Armor Class against an attack. Total cover commonly prevents direct targeting rather than merely increasing AC. Distance is checked against the range entered for the spell. The page displays warnings instead of forcing a universal ruling because line of effect, unusual spell wording, creature size, and table rulings can change the situation.

Read expected damage correctly

Average damage on a hit is not the same as expected damage per attack. Expected damage includes misses and critical hits. Resistance, vulnerability, and immunity alter the displayed damage expectation. The result is a planning average across many similar attacks, not a prediction of one die roll. Separate rays can target different creatures, so tactical outcomes may differ from a simple combined average.

Use profile comparisons for multiclass characters

A wizard and cleric multiclass can use Intelligence for wizard spells and Wisdom for cleric spells. A feat might use Charisma. An item might use its own fixed bonus. Profile comparison keeps those sources separate and places their attack bonuses, save DCs, and hit chances in one table. It is also useful when comparing an ability score improvement against a magic item bonus.

Confirm table-specific rules

The 2014 and 2024 labels help organize a character, but the mathematical core is configurable. Supplements, errata, optional rules, virtual tabletops, third-party material, and homebrew features can change details. Use the custom fields for known adjustments and keep notes beside the calculation. The final authority remains the rule text and the game’s adjudication.

Class ability reference

Frequently asked questions

What is a spell attack bonus?

It is the modifier added to a d20 when a spell calls for a spell attack roll. The total is compared with the target’s Armor Class.

How do I calculate my spell attack bonus?

Add the spellcasting ability modifier, proficiency bonus, and every applicable spell attack modifier. Subtract applicable penalties. Do not add bonuses that only affect weapon attacks.

Is spell attack bonus the same as spell save DC?

No. The caster rolls a spell attack against AC. A target rolls a saving throw against spell save DC. They share ability and proficiency components but use different formulas.

Which ability does a multiclass spell use?

Use the spellcasting ability associated with the class or source that grants the spell. A multiclass character can have several spell attack bonuses.

Does a magic weapon bonus improve spell attacks?

Not automatically. The item must state that its bonus applies to spell attacks, general attacks, or the specific spell effect being used.

Do multiple sources of advantage stack?

Ordinary advantage generally does not become three dice merely because several sources apply. Use a custom pool only when a feature or table rule explicitly changes the number of dice.

How does the calculator handle natural 1 and natural 20?

A natural 1 is treated as a miss. A natural 20 is treated as a hit and critical result. An expanded critical range can be selected for applicable features.

Why can a bonus die change hit chance differently than its average?

A bonus die has several possible results. Each result affects borderline attack totals differently. The calculator evaluates the complete probability distribution instead of adding only the die’s average.

Can I calculate a monster’s spell attack?

Yes. Choose monster mode and fixed attack mode. Enter the published spell attack bonus and fixed save DC to avoid rebuilding included statistics.

Does critical damage double fixed modifiers?

The calculator doubles damage dice, not ordinary fixed modifiers. Extra critical-only dice can be entered separately when a feature adds them.

What does expected damage mean?

It is the long-run average after accounting for misses, normal hits, critical hits, and the target’s damage response. A single attack can vary widely.

Can I save or share a setup?

Yes. Local save stores form data in the browser. The share button creates a URL containing the current non-secret inputs. Review the URL before sharing notes.

Accuracy and privacy notes

This tool performs mathematical calculations and provides configurable rules reminders. It does not replace a spell description, character feature, official clarification, or game master ruling. Local profiles and history use browser storage. Share links place entered values in the URL, so remove private notes before sharing. Server calculations validate numeric ranges and escape displayed text.