dnd combat calculator

What this DnD combat calculator does

This calculator is designed for quick, practical combat math. Instead of running a full simulation, it uses expected value to estimate your average output over many turns. You enter your attack bonus, damage profile, crit settings, and target stats, then get immediate results for:

• Overall hit chance
• Critical hit chance
• Expected damage per attack
• Expected damage per round (DPR)
• Estimated rounds to reduce a target to 0 HP

How the math works

1) Hit probability

For each possible d20 result, the calculator checks whether that roll hits the target AC after adding your attack bonus. A natural 1 always misses. If you select advantage or disadvantage, probabilities are adjusted using the max/min of two d20 rolls.

2) Critical probability

Any result in your crit range (for example, 20 only or 19–20) is treated as a critical hit. Crits are included inside total hit chance, then broken out separately so you can see exactly how much they contribute to DPR.

3) Expected damage

Average die value is calculated as (die size + 1) / 2. Then expected damage uses this formula:

• Expected per attack = (non-crit hit chance × normal damage) + (crit chance × crit damage)
• Expected DPR = expected per attack × attacks per round

For 5e-style crits, only damage dice are doubled. If your table doubles the entire damage result, switch the crit rule dropdown.

When to use this tool

• Character optimization: Compare weapon choices, feat impact, and advantage uptime.
• Encounter planning: Estimate how fast monsters or PCs can burn through HP pools.
• Tactical decisions: Understand whether boosting hit chance or damage is more valuable right now.

Example interpretation

If your result says 18 DPR into a 72 HP enemy, the rough expectation is about 4 rounds to defeat that target. Actual combat will vary because of luck, action economy shifts, control effects, resistance, and healing—but this gives a reliable baseline for planning.

Limitations and assumptions

This is intentionally a clean baseline calculator. It does not automatically include:

• Saving throw spells and half-damage effects
• Resistance, immunity, and vulnerability
• On-hit riders (smite, sneak attack timing rules, poison saves, etc.)
• Resource spikes (Action Surge, limited-use features, nova rounds)
• Conditional effects like bless, bane, cover, or target debuffs

Still, for a fast DPR estimate in a D&D encounter, this approach is excellent for side-by-side comparisons.

Bottom line

A solid dnd damage calculator helps you make better combat decisions with less guesswork. Use this page as your quick combat math companion: test builds, stress-test encounter pacing, and make your turns more intentional.