D6 vs D20: Dice Types, Probabilities and When to Use Each

D6 and D20: different ranges, the same uniform rule

Dice notation names the number of possible faces: a D6 produces integers from 1 to 6, while a D20 produces integers from 1 to 20. A fair single die has a uniform distribution, meaning every face has the same probability. A particular D6 face appears with probability 1/6, about 16.67%; a particular D20 face appears with probability 1/20, or 5%. Neither single die creates a cluster around its middle. The larger die simply offers more distinct outcomes and finer steps between probabilities.

Choose the die specified by your game's rules, rather than whichever die looks more dramatic. Board games often use six-sided dice for movement or resolving small ranges; many tabletop RPGs use a D20 for checks and several smaller dice for damage or healing. To try any of these examples, open the D&D dice roller, which offers D4, D6, D8, D10, D12, D20 and D100. This guide explains the mathematics; the roller is where you perform the rolls.

Reading notation: count, die type and modifier

The expression 1d20+5 means roll one twenty-sided die, then add five to its face value. A roll of 12 becomes a total of 17. The expression 2d6 means roll two independent six-sided dice and add them; its possible totals run from 2 to 12. The count is not the number of sides: 4d6 is four six-sided dice, not one twenty-four-sided die. A modifier changes the total after rolling, unless the rules explicitly specify a different procedure.

Do not confuse summing dice with keeping one result. Rolling two D20s and taking the higher number is different from adding them, and different again from rolling once with a bonus. Likewise, some ability-score methods say to roll four D6 and discard the lowest. Simply adding all four dice does not follow that rule. Read every instruction, including rerolls, discarded dice and minimum values, before interpreting a total.

Why 2d6 differs from a single die

Two-D6 totals are not equally likely. There are 36 ordered pairs of faces. Only one pair makes 2: (1,1). Six pairs make 7: (1,6), (2,5), (3,4), (4,3), (5,2) and (6,1). Only one pair makes 12: (6,6). Therefore 7 has probability 6/36, about 16.67%, while 2 and 12 each have probability 1/36, about 2.78%. The counts for totals 2 through 12 are 1, 2, 3, 4, 5, 6, 5, 4, 3, 2 and 1.

This is a discrete triangular distribution, not a bell curve. Adding many independent dice can produce a more bell-shaped pattern, but describing two-D6 sums as a bell curve obscures the exact counts. Replacing 2d6 with a uniform random number from 2 to 12 would change the game: every total would become equally likely. A single D20 remains uniform regardless of where its middle lies. The distinction matters for encounter tables, damage totals and classroom probability examples.

Thresholds and the real effect of a +5 bonus

Suppose success means a D20 total of at least 15, with no special automatic success or failure rules. Without a bonus, faces 15 through 20 succeed: 6 out of 20, or 30%. With +5, faces 10 through 20 succeed: 11 out of 20, or 55%. The increase is 25 percentage points. Relative to the original 30%, it is about an 83.33% increase, not a universal “25% improvement.” Those two ways of describing a change are not interchangeable.

Boundary effects also matter. Against a target of 3, an unmodified D20 succeeds on 18 faces, or 90%. Adding five raises that threshold-only chance to 100%, a gain of ten percentage points rather than twenty-five. Against a target of 30, +5 still cannot reach the target. Game-specific rules about natural 1s and 20s can change these examples, so apply the appropriate rulebook instead of treating a simple threshold calculation as a complete RPG resolution system.

Advantage and disadvantage are not extra damage dice

For the common D20 mechanic called advantage, roll two independent D20s and keep the higher face. Disadvantage keeps the lower. At a threshold where an ordinary roll succeeds 50% of the time, advantage succeeds 75% of the time: both dice fail only one quarter of the time. Disadvantage succeeds 25% of the time because both dice must succeed. These are concrete examples, not fixed bonuses that apply identically at every threshold.

More generally, if an ordinary roll succeeds with probability p, advantage succeeds with probability 1−(1−p)² and disadvantage with p². These formulas assume independent rolls and a simple success threshold. They do not describe summing two dice or stacking multiple sources of advantage. The roller shows the two faces and the kept result so you can inspect the calculation; check the game's rules for when the mechanic applies.

The rest of the dice set

D4, D8, D10 and D12 offer ranges of 1–4, 1–8, 1–10 and 1–12 respectively. In D&D, they often appear in weapon damage, spell effects, healing and class features, but those uses depend on the specific rules and edition. A small die can suit a short random table; a larger die permits more entries. The die type alone does not tell you whether the result represents damage, movement, a skill check or a choice of events.

A fair die with n sides has an average face value of (n+1)/2. That gives averages of 3.5 for a D6 and 10.5 for a D20. An average need not be a face you can actually roll. The average of a two-D6 sum is 7, but that does not guarantee that a particular pair will total 7. Expectations help compare repeated rolls; they do not predict the next result or override instructions about which die to use.

Percentile dice and the meaning of zero

Percentile rolls typically produce a number from 1 to 100 using two ten-sided dice. One represents tens, commonly marked 00, 10, 20 and so on; the other represents units, commonly marked 0 through 9. A tens face of 40 and a units face of 7 makes 47. Under the common 1–100 convention, a pair of 00 and 0 represents 100, not zero. Agree on the convention before rolling, since some systems explicitly use a different range.

The site's D100 roller returns a number directly from 1 to 100, so there is no need to interpret two physical markings. For a percentile table, confirm whether you need to roll under a skill value or select an entry by number; those are different uses of the same range. Read inclusive boundaries carefully, especially when a table lists intervals such as 01–05 and 06–10.

Critical results and fair play

A natural 20 means the die's face is 20 before modifiers. A total of 20 produced by 15+5 is not a natural 20. A natural 1 likewise refers to the unmodified face. In D&D attack rules, these faces have specific effects, but do not automatically apply those effects to every ability check, saving throw or game. House rules and other RPGs differ. Tell the table which rules you are using before a disputed roll occurs.

Physical dice offer a tactile experience; online dice offer convenience when a set is missing or players meet remotely. SpinGenerator uses the browser's cryptographic random number generator, and each face is selected with equal probability. The animation and roll log make the result visible, but they do not constitute independent certification of a competitive or legally audited draw. Use the appropriate rules, inspect the recorded faces and reserve a certified service for situations that require one.