About This Tool
### Should You Risk It?
In Dungeons & Dragons, understanding the math behind the dice can save your character's life. "I have a +5, the DC is 15... what are my odds?"
**D20 Probability Calculator** tells you the exact percentage chance of success.
### Advantage vs Disadvantage
* **Normal**: 1d20 + Mod. Standard flat probability (5% per number).
* **Advantage**: Roll 2d20, keep highest. This drastically boosts your success rate on mid-level tasks.
* **Disadvantage**: Roll 2d20, keep lowest. This makes even easy tasks risky.
Use this tool to decide if you should burn that Inspiration point!
Key Features
- **Math made Visible**: Takes the guesswork out of checks.
- **Advantage Logic**: Calculates the complex 'keep highest' probability curves.
- **Disadvantage Logic**: Shows how punishing 'keep lowest' really is.
- **Any Modifier**: Handles huge buffs (Bardic Inspiration, Pass Without Trace, etc).
Frequently Asked Questions
How does Advantage change the odds?
Advantage makes you much more likely to roll avg/high numbers. For a target of 11+ on the die (50% base), Advantage boosts your odds to 75% (+25% increase!).
Does a Natural 20 always succeed?
By default rules: Only on Attack Rolls. For Skill Checks, a Nat 20 is just a 20 (plus modifiers). If the DC is 30 and you have +5, you still fail.