The DM says "Dex save, with disadvantage, DC 15," and the new player at the end of the table picks up a d6. This glossary fixes that. Nine terms, each with what you roll, what you compare it to, and the odds.
Rules follow the 2024 rules as published in the free System Reference Document 5.2.1 (SRD 5.2) [1]. Where the 2014 rules (SRD 5.1) differ, the post says so. Every percentage comes from a short Python script, not memory, and assumes a fair die.
Normal Roll
One d20, add your modifier, done. The SRD calls every roll of this kind a "D20 Test": ability checks, saving throws and attack rolls. Roll, add the relevant ability modifier, add your Proficiency Bonus if you are trained in the thing, and compare the total to a target number [1, "D20 Tests"].
Each face of a d20 is worth exactly 5%. Needing a 15 means you win on 15 through 20: six faces, 30%.
Advantage
Roll two d20s and keep the higher [1, "Advantage/Disadvantage"]. The SRD's own example: roll an 18 and a 3, use the 18.
Two things surprise new players. Advantage never stacks: three reasons to have it still gets you two dice. And one source of disadvantage cancels any number of sources of advantage, leaving a plain single d20 [1].
Disadvantage
Roll two d20s and keep the lower. Same 18 and 3, and now you are stuck with the 3.
Here is what the second die does to your chances. It is the most useful table in this post.
| You need (on the die) | Normal | Advantage | Disadvantage |
|---|---|---|---|
| 5 or higher | 80.0% | 96.0% | 64.0% |
| 8 or higher | 65.0% | 87.8% | 42.3% |
| 11 or higher | 50.0% | 75.0% | 25.0% |
| 15 or higher | 30.0% | 51.0% | 9.0% |
| 18 or higher | 15.0% | 27.8% | 2.3% |
| Natural 20 | 5.0% | 9.8% | 0.3% |
Advantage pays most in the middle: a coin flip becomes three-in-four. Disadvantage on a hard roll is brutal: a 30% shot drops to 9%. Roll both on the 3D table with ADV and DIS and watch it happen.
Saving Throw (Save)
A save is your character trying not to get hurt by something: a fireball, poison gas, a spell poking around in your head. You don't choose to make one; the danger forces it [1, "Saving Throws"]. The SRD does let you choose to fail a save if you want the effect to land.
Each save is named for an ability, and the SRD's examples make them easy to remember:
| Save | You roll it to... |
|---|---|
| Strength | Physically resist direct force |
| Dexterity | Dodge out of harm's way |
| Constitution | Endure a toxic hazard |
| Intelligence | Recognize an illusion as fake |
| Wisdom | Resist a mental assault |
| Charisma | Assert your identity |
The target is a Difficulty Class (DC). For a spell, the caster's spellcasting ability and Proficiency Bonus set it. Meet or beat it and you usually take half damage or dodge the effect entirely. Fireball is the classic: Dexterity save, 8d6 fire on a fail, half on a success [1].
Damage
Damage dice turn "you hit" into "how much it hurts." Roll the dice the weapon or spell lists, add your modifier, and the target loses that many Hit Points. Some real numbers from SRD 5.2:
| Source | Dice | Average |
|---|---|---|
| Dagger (with +2 modifier) | 1d4 + 2 | 4.5 |
| Fire Bolt (cantrip, levels 1–4) | 1d10 | 5.5 |
| Fireball (3rd-level slot) | 8d6 | 28 |
| Adult Red Dragon Fire Breath | 17d6 | 59 (failed save) |
Stab a Goblin Warrior with that dagger, roll a 3, deal 5. It has 10 Hit Points, so it is still standing and deeply annoyed.
Drop to 0 Hit Points and you fall unconscious and start making death saving throws [1, "Damage and Healing"].
Healing
Healing runs damage in reverse: roll dice, get Hit Points back.
| Healing | Dice (SRD 5.2) | Average |
|---|---|---|
| Potion of Healing | 2d4 + 2 | 7 |
| Healing Word (1st level) | 2d4 + spellcasting modifier | 5 + modifier |
| Cure Wounds (1st level) | 2d8 + spellcasting modifier | 9 + modifier |
Edition trap: under the 2014 rules, Cure Wounds healed only 1d8 + modifier [2]. At a 2014 table the old number is right. If someone quotes 2d8 there, they are reading the new book.
Initiative
Initiative sets turn order when a fight starts. In the 2024 rules it is a Dexterity check: everyone rolls d20 + Dexterity modifier (plus anything that boosts it), and the GM ranks everyone from highest to lowest [1, "Initiative"]. The order stays the same every round.
Two details worth knowing. A group of identical monsters rolls once and shares one Initiative, so five goblins act together. And a surprised combatant rolls initiative with disadvantage [1]. That is the whole 2024 surprise rule: no lost turn, just a worse roll.
Example: the rogue totals 22, the goblins 15, the cleric 8. The rogue goes first, then all five goblins, then a cleric with plenty of time to regret that Dexterity score.
Ability Check
An ability check is the d20 for anything uncertain that isn't an attack or a save: kicking a door, spotting a trap, lying to a guard. Roll d20, add the ability modifier, and add your Proficiency Bonus if a skill you are trained in applies [1, "D20 Tests"]. "Make a Strength (Athletics) check" means Strength modifier, plus proficiency if you have Athletics.
The GM sets the DC. The SRD's typical numbers are worth memorising:
| Task | DC | Chance on a d20 with +0 |
|---|---|---|
| Very easy | 5 | 80% |
| Easy | 10 | 55% |
| Medium | 15 | 30% |
| Hard | 20 | 5% |
| Very hard | 25 | 0% (needs +5 to be possible) |
| Nearly impossible | 30 | 0% (needs +10 to be possible) |
Attack Roll
An attack roll decides whether you hit. Roll d20, add the ability modifier (Strength for melee, Dexterity for ranged, your spellcasting ability for spell attacks) and your Proficiency Bonus. Meet or beat the target's Armor Class (AC) and you hit [1, "Attack Rolls"].
Two faces break the maths. A natural 20 always hits and is a Critical Hit; a natural 1 always misses, whatever your bonus [1, "Rolling 20 or 1"]. On a crit you roll the attack's damage dice twice and add your modifier once. The SRD's example: a dagger crit is 2d4 plus modifier, not 1d4 [1, "Critical Hits"]. Sneak Attack dice double too.
Nat 20s and nat 1s on ability checks and saves do nothing special in the rules as written. If your table cheers a nat 20 on a History check, that is a house rule, and a lovely one.
Table Talk
James: I had three reasons for advantage. Why do I only get two dice?
Arthur: Because advantage never stacks. Two dice, however many reasons.
James: And one reason for disadvantage wipes out all three?
Arthur: Any number of them. You're back to one plain d20.
James: Harsh. But a nat 20 on a History check still gets a cheer.
Arthur: Cheer all you like. It's a house rule, not a Critical Hit.
James: Fine. Give the new player one tip.
Arthur: Before you roll, ask three things: which die, which modifier, what number to beat.
James: And if the answer is a d6, you're the player in the first paragraph.
Arthur: Everyone's been that player once. The table in this post is so you're only that player once.
References
- Wizards of the Coast. (2025). System Reference Document 5.2.1. CC-BY-4.0. Sections: "D20 Tests", "Advantage/Disadvantage", "Saving Throws", "Attack Rolls", "Initiative", "Critical Hits", "Damage and Healing"; entries Potion of Healing, Cure Wounds, Healing Word, Fire Bolt, Fireball, Goblin Warrior, Adult Red Dragon. https://www.dndbeyond.com/srd
- Wizards of the Coast. (2016; CC edition 2023). System Reference Document 5.1, "Cure Wounds". CC-BY-4.0. https://media.wizards.com/2023/downloads/dnd/SRD_CC_v5.1.pdf
