D&D Dice and Checks Explained: d20, DC, Advantage and Saving Throws

Dungeons & Dragons and the games built on its rules look complicated from the outside, with their bags of oddly shaped dice and numbers on every line of a character sheet. The core is simple. When an action could fail and failure would matter, you roll a twenty-sided die (d20), add a modifier that reflects how good your character is at it, and compare the total to a target number. Meet or beat it and you succeed. Nearly everything else on this page is a variation of that one rule. The page explains the dice a beginner actually needs, in the order you meet them at the table, and then shows how Thavernia's AI Game Master runs them.

The short answer#

• The d20 decides success or failure. Roll it, add your modifier, compare to the Difficulty Class (DC). Total equal to or higher than the DC means success.
• The other dice decide how much. d4, d6, d8, d10 and d12 are mostly for damage and healing, for example 1d8+3 for a longsword hit.
• Three kinds of d20 roll: ability checks (you try something), saving throws (you resist something) and attack rolls (you hit something, against its Armor Class).
• Advantage means roll two d20s and keep the higher; disadvantage means keep the lower.
• A natural 20 on an attack always hits and doubles the damage dice; a natural 1 always misses.

The dice set: d4 to d100#

Dice are named with a d followed by the number of sides. A standard set has seven dice, and each one has a typical job:

The d100 is not usually a hundred-sided ball. You roll two ten-sided dice: one marked 00, 10, 20 and so on gives the tens, the ordinary d10 gives the ones. A 30 and a 7 read as 37; a 00 and a 0 read as 100.

Reading notation like 2d6+3#

Rules and character sheets write dice in a compact form: XdY+Z. X is how many dice you roll, Y is how many sides they have, and Z is a flat number added at the end. Some examples:

• 1d20+5: roll one d20 and add 5. The result runs from 6 to 25.
• 2d6+3: roll two six-sided dice, add them up, add 3. The result runs from 5 to 15, most often around 10.
• 1d8-1: roll one d8 and subtract 1. The result runs from 0 to 7.
• 8d6: roll eight six-sided dice and add them all. No bonus is added.

One thing worth knowing early: more dice make results more predictable. A d20 gives every number from 1 to 20 the same 5 percent chance. Two d6 cluster around the middle: a 7 comes up six times more often than a 2 or a 12. That is why a single d20 roll can feel so dramatic, and why a pile of damage dice rarely surprises anyone.

The core rule: d20 + modifier vs DC#

Every uncertain action follows the same three steps:

• 1. You say what you do. "I try to climb the wet wall of the well."
• 2. The Game Master decides whether a roll is needed, which ability it tests, and how hard it is. A dry staircase needs no roll. A slippery wall might be a Strength (Athletics) check against DC 15.
• 3. You roll a d20 and add your modifier. You roll a 9, your modifier is +4, the total is 13. Thirteen is below 15, so you slip. A total of 15 or more would have been a success.

Two details make this fair. First, ties go to the roller: equal to the DC is a success. Second, the Game Master should only call for a roll when failure is possible and interesting. If nothing is at stake, or the action is clearly impossible, there is no roll at all, the story simply moves on.

What a modifier is#

A modifier is the number you add to the die to show what your character is good or bad at. In 5e-style games it usually has two parts.

The ability modifier. Characters have six ability scores: Strength, Dexterity, Constitution, Intelligence, Wisdom and Charisma, usually between 3 and 20. The modifier is the score minus 10, divided by 2, rounded down. A score of 10 or 11 gives +0, 14 gives +2, 18 gives +4, 20 gives +5, and 8 gives -1. You add the modifier, not the score.

The proficiency bonus. If your character is trained in what they are doing (a skill, a weapon, a type of saving throw), you also add a proficiency bonus. It starts at +2 at level 1 and grows to +6 at the highest levels. An untrained character adds only the ability modifier.

So a level 1 rogue with Dexterity 16 (+3) who is trained in Stealth rolls d20+5 to sneak. The same rogue trying to lift a portcullis with Strength 8 rolls d20-1. Situational bonuses from magic items, spells or a clever plan can be added on top.

The Difficulty Class scale#

The Game Master picks the DC. Most 5e-style games use the same six steps, and knowing them helps you judge your odds before you commit:

In practice most checks sit between 10 and 15. A DC of 25 or 30 is meant for heroes at high levels with large modifiers; at level 1 it is effectively out of reach, and a good Game Master signals that before you roll.

Ability checks, saving throws, attack rolls and Armor Class#

All three are a d20 plus a modifier. What changes is who starts the roll and what number you have to beat.

Armor Class is a fixed number that says how hard a creature is to hit. Without armour it is 10 plus the Dexterity modifier; light armour adds a little, heavy plate reaches 18, and a shield adds 2. An attacker rolls d20 plus their attack bonus; equal to or above your AC means a hit. Your AC does not change from roll to roll, which is why defence in 5e is passive: you do not roll to dodge an ordinary sword swing, the attacker rolls against your AC.

The six abilities map to the six kinds of saving throw. A Dexterity save is for getting out of the way (a fireball, a falling ceiling), Constitution for enduring (poison, disease), Wisdom for resisting the mind being bent (charm, fear). If you fail the save, the full effect hits; often a success halves the damage or avoids the effect completely.

Advantage and disadvantage#

Instead of a long list of small bonuses and penalties, 5e uses a single switch. Advantage: when the situation clearly favours you (the target cannot see you, an ally is distracting it, the guard is asleep), roll two d20s and keep the higher. Disadvantage: when it clearly works against you (you are blinded, poisoned, shooting at long range), roll two and keep the lower.

• It does not stack. Three reasons for advantage still mean two dice, not three.
• They cancel out. If you have any advantage and any disadvantage at the same time, you roll one d20, no matter how many sources each side has.
• It is worth a lot. On a roll that needs 11 or more, advantage lifts your chance from 50 to 75 percent. Your chance of a natural 20 nearly doubles, from 5 to 9.75 percent; with disadvantage it drops to 0.25 percent.

Natural 20 and natural 1#

"Natural" means the number the die itself shows, before you add anything. Under the standard 5e rules:

• Natural 20 on an attack roll: the attack hits no matter how high the target's AC is, and it is a critical hit (more on damage below).
• Natural 1 on an attack roll: the attack misses no matter how large your bonus is.
• Ability checks and saving throws: under the standard rules a natural 20 or natural 1 has no automatic effect; only the total counts. A modifier of +2 against DC 25 cannot succeed even on a 20.

That last point is one of the most commonly house-ruled parts of the game. Many tables treat a natural 20 as an automatic success and a natural 1 as an automatic failure on every roll, because it feels right and keeps every throw exciting. Neither approach is wrong; what matters is that the table knows which one it uses before the dice hit the table.

Damage rolls and critical hits#

Once an attack hits, you roll damage: the weapon's or spell's dice plus, for weapons, the same ability modifier you used to attack. A longsword wielded with Strength 16 deals 1d8+3; a dagger thrown with Dexterity 14 deals 1d4+2; a fireball deals 8d6 to everyone in its area, halved for those who make their Dexterity save. Damage is subtracted from hit points (HP), and a character at 0 HP falls unconscious.

A critical hit doubles the damage dice, not the modifier. A longsword crit is 2d8+3, not 2d8+6. The difference matters most for spells and sneak attacks with many dice, which is why a rogue's critical is the one players remember.

Why dice make better stories#

You could play a roleplaying game with no dice at all, and some games do. D&D keeps them for reasons that have little to do with maths.

• Uncertainty. If the hero always succeeds, a plan is just a description. A 30 percent chance makes every attempt a small story with a real ending.
• Stakes. A failed roll is not a dead end; it is a new situation. The lock holds and the guards' footsteps are getting closer. Some of the best scenes at any table start with a natural 1.
• Shared authority. Nobody at the table, the Game Master included, decides the outcome alone. The die is the neutral party that everyone agreed to trust.
• Earned moments. A natural 20 at the right moment is remembered for years precisely because nobody could have arranged it.

All four depend on one thing: the number has to be genuinely random and the result has to be honoured. That is exactly where a chatbot playing Game Master tends to fail, because a language model asked to "roll a d20" is predicting a likely-looking number, not generating a random one. The detail is on the playing D&D with ChatGPT page.

How Thavernia handles the dice#

Thavernia is a tabletop RPG where the Game Master is an AI. Its checks run on the d20 in the way this page describes, with a few deliberate differences. Everything below is how the game works today.

Two-phase rolling: the server decides, you throw, the GM honours

• Plan. After you write your action, a referee step decides which checks the turn needs: who rolls, which ability, the DC, whether it is an attack and on whom, and whether it has advantage. At the same moment the server generates each d20 with a cryptographically secure random number generator and stores it. Your browser is not told the number.
• Throw. The 3D d20 appears on your screen and you throw it. It lands on the stored number. Everyone at the table watches the same die, one roll at a time, in turn order. AI companions' dice are thrown automatically.
• Resolve. Only when the last die has landed does the Game Master write the turn, from the committed results. A total that meets or beats the DC is a success, ties included. If the narration contradicts a result (a door flying open after a failed Strength check, for example), the turn is caught and rewritten once with the outcomes spelled out.

Walking away does not change anything either: if a die is left unthrown, the server eventually throws it itself, and because the number was fixed when the check was set, that gives nobody an edge. Why this design exists, with the line of code that generates the number, is on the is the AI Game Master fair? page.

What the roll badge shows

Every check leaves a badge in the story: the ability, the raw d20, your modifier, the total, the DC, success or failure and a few words on what the roll was for. Bonus from equipped gear is shown separately in gold so you can see what your sword actually added. Consequences sit on the same badge: damage dealt, HP lost or healed, XP earned, a foe killed. A natural 20 and a natural 1 are marked.

Where Thavernia's rules differ from the standard ones

• Natural 20 and natural 1 apply to every roll, not only attacks: a 20 succeeds and a 1 fails whatever the DC. A few special actions are set so that only a natural 20 can succeed, for example a desperate melee lunge at a foe who is out of reach on the tactical board.
• DCs lean low. The referee is told that routine actions sit at 8 to 10, moderate challenges at 11 to 13, hard feats at 14 to 16, and 17 to 20 is kept for the rare near-heroic attempt. The server keeps every DC between 5 and 30. Checks against a boss use a raised 14 to 18 band.
• The modifier has diminishing returns. Up to a score of 20 it follows the standard formula (+5 at 20). Above that, each 4 points add only +1, up to a ceiling of +7 at 30. There is no separate proficiency bonus.
• Equipped gear adds a fixed bonus by rarity: uncommon +1, rare and epic +2, legendary +3; your best equipped weapon on attacks, your best equipped armour on defence and saves. A weapon sitting in your pack adds nothing.
• You roll to defend. When an enemy attacks, you often make a defence roll (a dodge, a block, a braced stance) instead of the enemy rolling against a fixed Armor Class. A successful defence means no real wound that turn.
• No separate damage dice. For actions from your action kit, the damage range is shown on the action and the blow lands inside it according to how high the d20 came up; a natural 20 pushes past the top of the range, a miss deals 0. Large foes have per-blow damage caps.

Advantage, a helping hand, and chained rolls

Advantage works as in the standard rules: two dice appear on screen, both are thrown, and the one that does not count is dimmed. It comes from the story (the referee grants it for a clear edge the fiction established, such as striking from hiding), from conditions (blessed gives advantage, frightened gives disadvantage), from attacking a foe that is prone, which the server enforces, and from an armed class ability. Any advantage and any disadvantage cancel out. A natural 20 counts if either die of an advantage pair shows 20.

In a group, there is a Lend a hand button: a teammate with a living hero at the table can help another player's roll before it is thrown, turning it into an advantage roll. Each player can do this once per round of dice, and only on a roll that has no advantage or disadvantage yet.

Chained rolls handle actions that depend on each other. If you write "I leap the chasm and drive my sword into the golem", the strike is linked to the leap. If the leap fails, the strike is cancelled: that die is never thrown, the screen shows why, and the story treats the blow as something that never happened. Links can also point the other way between players: if one hero tries to banish a corpse and another tries to loot it, the looting only happens if the banishing fails.

Dice skins

The d20 can wear different skins. Three are free on every account (Gold, Obsidian and Jade) and more are in the shop. Other players at the table see your throw in your skin. Skins are purely cosmetic: they never change the number.

Try it yourself#

The fastest way to understand a d20 check is to make one. The trial needs no account and no card and gives you three turns as a guest; a free account then gives you 20 turns in total. Try something risky, look at the DC on the badge before you throw, and see what a low roll does to your hero.

Frequently asked questions#