The three d20 rolls
Almost every "does it work?" question in D&D is a d20 plus a modifier against a target number. Rulebooks write it 1d20, so an attack with a +5 bonus is 1d20+5:
- Attack rolls against the target's Armor Class.
- Saving throws against a spell or effect's save DC.
- Ability checks, skills included, against a DC the Dungeon Master sets. Initiative is a Dexterity check.
Your chance to hit
Every face of a d20 is 5%, so each point of bonus is worth 5%. To find your chance, take the number you need on the die (the DC or AC minus your bonus) and look it up:
| You need on the die | Chance |
|---|---|
| 2 or higher | 95% |
| 5 or higher | 80% |
| 8 or higher | 65% |
| 10 or higher | 55% |
| 11 or higher | 50% |
| 12 or higher | 45% |
| 15 or higher | 30% |
| 18 or higher | 15% |
| 20 | 5% |
A +5 attack against AC 15 needs a 10 on the die: 55%. The advantage roller has the same table for two dice, and the dice calculator works out any roll.
Natural 20 and natural 1
On an attack roll, a natural 20 (a "nat 20") hits whatever the target's AC, and it is a critical hit: roll all the damage dice twice and add them together. A natural 1 misses whatever your bonus.
The 2014 rules give no automatic success or failure on ability checks or saving throws, though plenty of tables play it that way. Say which rule you use before the session, not after the roll.
Death saving throws
At 0 hit points you roll a d20 at the start of each turn. 10 or higher is a success, lower is a failure. Three successes and you stabilise; three failures and the character dies. A natural 20 brings you back with 1 hit point. A natural 1 counts as two failures.
Rolling with advantage
Two d20s, keep the higher. It raises your chance of a natural 20 from 5% to 9.75%. It has its own roller.
Questions
What are the odds of rolling a natural 20?
1 in 20, or 5%, on every roll. With advantage it is 9.75%. Two nat 20s in a row is 1 in 400 (0.25%).
How many rolls until I get a nat 20?
Twenty on average, but the die has no memory. You have a better than even chance of at least one in 14 rolls (51%), 64% in 20 rolls and 90% in 45.
What does 1d20 mean?
One twenty-sided die. The number in front is how many dice, the number after the d is how many sides, so 1d20+5 is one d20 with 5 added. The Dice Notation Guide covers the rest.
What is the average roll on a d20?
10.5, halfway between 1 and 20. Each point of modifier moves the average and every result up by one.
Is this d20 roller random and fair?
Yes. Each roll comes from your browser's cryptographic random number generator, the same kind used for passwords, with no bias towards any face. Every face is exactly 5%.
Can I roll more than one d20 at once?
Yes. Use the stepper to roll several d20s and add a modifier, or roll two and keep one on the advantage roller. The main roller takes any mix of dice.