The standard set of 7 dice
Every D&D player needs seven polyhedral dice: d4, d6, d8, d10, d12, d20, and d100.
d4 — the caltrop
Four sides, pyramid shape. Used for small weapon damage (daggers, handaxes: 1d4), Healing Word (2d4), and low-level cantrip damage dice.
d6 — the classic cube
Six sides. Used for short swords (1d6), sneak attack damage (multiple d6s), and many spells (Fireball: 8d6).
d8 — the octahedron
Eight sides, used for longswords and battleaxes (1d8), and the hit dice for clerics, druids, rogues, and bards.
d10 — the pentagonal trapezohedron
Ten sides, used for heavy crossbows (1d10), polearms, and the hit dice for fighters, paladins, and rangers.
d12 — the dodecahedron
Twelve sides. Greataxes deal 1d12 damage. Barbarians use d12 as their hit die.
d20 — the icosahedron
The star of the game. Used for ability checks, attack rolls, and saving throws. Rolling a natural 20 is a critical hit.
d100 (percentile) — two d10s
Rolled as two d10s where one represents tens and one represents units. Used for wild magic surge tables and random encounter tables.
How to roll ability scores (4d6 drop lowest)
Roll four six-sided dice, discard the lowest die, and sum the remaining three. Repeat six times. The average result per stat is approximately 12.2.
Understanding advantage and disadvantage
Advantage: roll 2d20, take the higher result (+3.3 to average). Disadvantage: roll 2d20, take the lower result (−3.3 from average).
Critical hits: double the damage dice
On a natural 20, roll all the damage dice twice and add modifiers once. A Rogue dealing 4d6 sneak attack rolls 8d6 on a crit.