Dice Notation Explained: d4, d6, d20, and How to Roll Online

If you have ever opened a tabletop rulebook and seen 2d6+3 or 1d20, you have met dice notation. It is a compact language for describing random rolls, and it is easy to learn.

Key takeaways

• In dice notation, the number before the d is how many dice to roll.
• The number after the d is how many sides each die has.
• A modifier is added to or subtracted from the total.
• Multiple dice create a bell-shaped distribution; a single die is flat.
• Digital dice can be uniform and fair even without a physics engine.

Reading dice notation

The basic pattern is simple: XdY+Z. The X is the number of dice, the Y is the number of sides, and the Z is a modifier. If X is omitted, it usually means one die.

• 1d20 means one twenty-sided die.
• 2d6 means two six-sided dice.
• 3d8+2 means three eight-sided dice, then add two to the total.
• 1d100 can mean a true hundred-sided die or two ten-sided dice used as percentile dice, depending on the system.

When a rulebook writes d20, it is naming the die, not the result. The result is whatever the roll produces on that die.

The standard dice

Tabletop games commonly use a set of polyhedral dice:

• d4: a tetrahedron, producing 1–4.
• d6: the familiar cube, producing 1–6.
• d8: an octahedron, producing 1–8.
• d10: a pentagonal trapezohedron, producing 1–10.
• d12: a dodecahedron, producing 1–12.
• d20: an icosahedron, producing 1–20.
• d100: used for percentile rolls and large random tables.

Not every game uses every die. Many systems rely on d6 pools, while d20 systems use the twenty-sided die as the central resolution mechanic.

Why 2d6 feels different from 1d12

A single d12 is flat: every result from 1 to 12 has the same probability, about 8.33 percent. Two d6 dice are different. The total can range from 2 to 12, but the middle values are much more likely because there are more ways to make them.

Seven is the most likely result when rolling 2d6 because it can appear as 1+6, 2+5, 3+4, 4+3, 5+2, or 6+1. Two and twelve each have only one possible combination. The distribution forms a triangle, and with more dice it approaches a bell curve.

This is why game designers choose different dice for different feelings. A d20 check is swingy and dramatic. A 2d6 roll is concentrated around the middle. Neither is more random; they shape the probability of outcomes.

Modifiers and averages

A modifier shifts every possible total by the same amount. Adding +3 to a d20 roll means the result still ranges from 4 to 23, but the chance of meeting any fixed target increases. The shape of the distribution does not change.

A useful shortcut is the average result. For a single die with N sides, the average is (N+1)/2. A d20 averages 10.5. Two d6 dice average 7. Add a modifier of +3 and the average total becomes 10. These averages help game masters set difficulty numbers and help players understand what a bonus is worth.

Advantage and disadvantage

Some d20 systems use advantage and disadvantage. With advantage, you roll two d20s and keep the higher result. With disadvantage, you keep the lower result. This changes the distribution without changing the die.

Advantage strongly favors high rolls. It is not the same as adding a flat bonus. A bonus helps every result equally, while advantage removes some of the low tail and increases the chance of a high result.

Are digital dice fair?

A digital die does not need to simulate the physics of a real die to be fair. It needs to choose each face with equal probability. The dice roller on this site uses a cryptographically secure random source and maps it to the die range without bias.

Real dice can be imperfectly weighted, worn, or rolled on an uneven surface. Digital dice remove those physical variables, though they introduce a different dependency: trust in the device and the code. For casual tabletop play, a well-implemented digital roller is a practical and fair alternative.

Dice notation in practice

In a d20 game, an attack might be 1d20+5 against an armor class. Damage might be 2d6+3. A spell might roll 8d6 for a fireball. The notation tells you exactly what to roll and what to add.

In a dice-pool system, you might roll a number of d6s equal to a skill rating and count successes. The notation may look different, but the underlying idea is the same: define the dice, define the success condition, and roll.

Bottom line

Dice notation is a compact way to describe randomness. Once you can read XdY+Z, most tabletop rolls become clear. The choice of die changes the shape of the probability, not just the range. Use the notation to understand the odds, and use a fair roller when the physical dice are out of reach.