D&D runs on one idea: roll a d20, add a number, beat a number. Everything else is a footnote to that sentence, and this post is the footnotes, with the odds worked out so you know when to push your luck.
Rules here are from the System Reference Document 5.2, the free version of the 2024 rules [1]. The 2014 rules (SRD 5.1) agree on everything below unless we say otherwise.
The seven dice and what each one does
| Die | Shape | What it rolls at the table |
|---|---|---|
| d4 | Tetrahedron | Dagger (1d4), Magic Missile darts (1d4 + 1 each) |
| d6 | Cube | Shortsword (1d6), Fireball (8d6), Greatsword (2d6) |
| d8 | Octahedron | Longsword (1d8), Cure Wounds (2d8 in 2024) |
| d10 | Pentagonal trapezohedron | Longsword two-handed (1d10), Fighter hit dice |
| d12 | Dodecahedron | Greataxe (1d12), Barbarian hit dice |
| d20 | Icosahedron | Every attack, check and save |
| d10 (tens) | Pentagonal trapezohedron | Paired with the d10 for a d100 roll |
Weapon and spell dice are from the SRD's Weapons table and spell list [1]. The seventh die is a second d10 marked 00 to 90: roll both, read the tens die then the ones die, and 00 with 0 is 100. The d4 is the one you step on at 2 a.m.
Look any spell up in our spell list to see which dice it wants.
The d20 Test: roll, add, compare
The SRD calls every d20 roll a D20 Test, and there are three kinds: ability checks, saving throws and attack rolls [1]. Each follows the same three steps:
- Roll 1d20. Two of them with Advantage or Disadvantage (below).
- Add modifiers: the relevant ability modifier, your Proficiency Bonus if you are proficient, and any bonus or penalty a spell or feature hands you.
- Compare to the target. "If the total of the d20 and its modifiers equals or exceeds the target number, the D20 Test succeeds" [1]. The target is a Difficulty Class (DC) for checks and saves, and Armor Class (AC) for attacks.
Ties go to the roller. Write that on your hand.
The DM sets the DC. The SRD's Typical Difficulty Classes table gives the scale [1]:
| Task | Very easy | Easy | Medium | Hard | Very hard | Nearly impossible |
|---|---|---|---|---|---|---|
| DC | 5 | 10 | 15 | 20 | 25 | 30 |
Which roll is which
- Ability check: you try something with a real chance of failure. Strength to force a stuck door, Dexterity (Stealth) to sneak past the guards, Intelligence (Investigation) to find the clue the DM hid badly.
- Attack roll: d20 + ability modifier + Proficiency Bonus against the target's AC. A skeleton has AC 14; a goblin warrior has AC 15.
- Saving throw: something bad happens to you and you try to shrug it off. Fireball asks for a Dexterity save; Hold Person asks for Wisdom [1].
- Death saving throw: the odd one out. A plain d20 with no modifier: 10 or higher succeeds, three successes and you are stable, three failures and you die. A natural 1 counts as two failures; a natural 20 gets you back up with 1 hit point [1].
Advantage and Disadvantage: worth about +5, sometimes much less
Advantage: roll two d20s, keep the higher. Disadvantage: keep the lower. They never stack, and one of each cancels to a plain roll, however many sources sit on either side [1].
Where they come from, straight from the SRD [1]:
| You get Advantage when | You get Disadvantage when |
|---|---|
| An ally takes the Help action for you | You are Prone and attacking |
| You attack a Prone target from within 5 feet | You attack a Prone target from more than 5 feet away |
| You attack a Blinded or Restrained target | You are Blinded or Restrained and attacking |
| The DM decides the circumstances favour you | The DM decides they don't |
What it does to your odds, computed for every d20 result:
| You need | Normal | Advantage | Disadvantage |
|---|---|---|---|
| 5+ | 80% | 96% | 64% |
| 8+ | 65% | 88% | 42% |
| 11+ | 50% | 75% | 25% |
| 14+ | 35% | 58% | 12% |
| 16+ | 25% | 44% | 6% |
| 18+ | 15% | 28% | 2% |
| 20 | 5% | 10% | 0.25% |
The swing is biggest in the middle: at a coin flip, Advantage adds 25 points, the same as a +5 bonus. At the edges it matters far less. If you need a natural 20, Advantage only takes you from 5% to 10%, so don't spend your Help action on the hopeless roll.
Damage rolls
When an attack hits, roll the weapon's or spell's damage dice and add the same ability modifier you used to attack [1]. A spell adds a modifier only when its text says so: Magic Missile's darts carry their +1, Fireball's 8d6 carries nothing.
2d6 or 1d12? The greatsword rolls 2d6 and the greataxe rolls 1d12 [1]. Close on average, not in feel:
| Greatsword (2d6) | Greataxe (1d12) | |
|---|---|---|
| Average | 7 | 6.5 |
| Chance of 3 or less | 8% | 25% |
| Chance of 10 or more | 17% | 25% |
| Most likely result | 7 (17%) | All equal (8% each) |
Two dice pile up in the middle; one die swings. The greatsword is the reliable one, the greataxe the one with stories.
Critical hits
Roll a 20 on the d20 for an attack and it hits no matter the target's AC. Roll a 1 and it misses no matter your bonus [1]. That rule belongs to attack rolls only: a natural 20 on an ability check is just a very good 20.
A Critical Hit rolls all the attack's damage dice twice and then adds modifiers once [1]. The SRD's example: a dagger crit is 2d4 plus your modifier. Extra dice ride along, so a rogue's Sneak Attack dice double too.
| Your crit range | Normal roll | With Advantage |
|---|---|---|
| 20 only | 5% | 9.75% |
| 19–20 (Champion Fighter, level 3) | 10% | 19% |
The Champion's Improved Critical is in the SRD [1]. Pair it with Advantage and nearly one swing in five crits.
Probability, and the people who ignore it
Every face of a fair d20 comes up 5% of the time, every roll, regardless of what the last three did. A +5 against DC 15 means you need a 10 or higher: 11 faces of 20, so 55%.
People have never quite believed this. In 1967 the sociologist James Henslin spent time with St. Louis cab drivers who shot craps, and recorded their shared principle that "a hard throw produces a large number, and a soft or easy throw produces a low number" [2]. They bet by the real odds and then threw by magic. You have seen the same player at your table, blowing on a d20 before the saving throw. It doesn't help. It doesn't hurt either.
The two numbers you add
Ability modifier: subtract 10 from the score, halve, round down [1].
| Score | 3 | 4–5 | 6–7 | 8–9 | 10–11 | 12–13 | 14–15 | 16–17 | 18–19 | 20 |
|---|---|---|---|---|---|---|---|---|---|---|
| Modifier | −4 | −3 | −2 | −1 | +0 | +1 | +2 | +3 | +4 | +5 |
Proficiency Bonus, by character level [1]:
| Level | 1–4 | 5–8 | 9–12 | 13–16 | 17–20 |
|---|---|---|---|---|---|
| Bonus | +2 | +3 | +4 | +5 | +6 |
That cap is the point. A level 20 hero with a 20 in the stat adds +11 to a proficient roll, and still fails a DC 15 lock on a 3 or less. The d20 stays the loudest number on the table from level 1 to level 20.
Tips for new players
- Write the totals on your sheet. "Stealth +5" beats working out Dex + proficiency every time the rogue wants to sneak. Our free character sheets do the sums for you.
- Say what you're rolling before you roll it. "Dexterity save, +3" saves the DM a question and you an argument.
- Ask when you're unsure. Every table has someone who loves explaining the rules. Let them.
- Know your crit. Roll the damage dice twice, add the modifier once.
- Hand the Help action to the teammate on a coin flip, not the one who needs a 20.
Want to practise? The 3D roller does modifiers, Advantage and Disadvantage, and keeps a roll log so you can check what you rolled when the DM asks.
Table Talk
Rohan: Advantage is worth +5. Done. Next.
Arthur: At a coin flip. If you need a natural 20, it takes you from 5% to 10%.
Rohan: So don't waste Help on the hopeless roll. That's what I said.
Arthur: It isn't, but I'll allow it. Two sources of Advantage and one of Disadvantage still cancel to a plain roll.
Rohan: Worst rule in the book.
Arthur: The SRD says so. Greatsword or greataxe?
Rohan: Greatsword. Average 7, and 3 or less only 8% of the time. The greataxe does that 25% of the time.
Arthur: The greataxe gets the stories.
Rohan: Stories don't drop the goblin.
Arthur: One more for your hand: ties go to the roller.
References
- Wizards of the Coast. System Reference Document 5.2 ("Playing the Game": D20 Tests, Typical Difficulty Classes, Rolling 20 or 1, Advantage/Disadvantage, Proficiency Bonus, Damage and Healing, Critical Hits, Death Saving Throws; "Character Creation": Ability Scores and Modifiers; "Rules Glossary": Blinded, Help, Prone, Restrained; "Equipment": Weapons; "Classes": Fighter, Improved Critical; "Spells": Cure Wounds, Fireball, Hold Person, Magic Missile; "Monsters": Goblin Warrior, Skeleton). 2025. CC-BY-4.0. https://www.dndbeyond.com/srd
- Henslin, James M. "Craps and Magic." American Journal of Sociology 73, no. 3 (1967): 316–330. https://doi.org/10.1086/224479
