1d13

1d13 (written 1D13 or 1d13) is a dice notation used in tabletop gaming to indicate rolling a single thirteen‑sided die. In the NdM convention, N is the number of dice and M is the number of sides; thus 1d13 yields one result in the range of 1 to 13. The concept is similar to more common dice like 1d6 or 1d20, but 13 is not one of the standard polyhedral dice that are widely manufactured.

Physical thirteen‑sided dice do exist, though they are relatively uncommon compared with dice of six, eight, ten, twelve, or twenty sides. The shapes used for a d13 vary, and not all designs are perfectly regular; manufacturers aim to ensure fair results, with each face carrying a unique value from 1 to 13. In practice, many tabletop players encounter 1d13 in homebrew systems or in scenarios that require a 13‑point outcome.

In terms of probability, a fair 1d13 gives each outcome from 1 to 13 an equal probability of 1/13. When a physical d13 is unavailable, players sometimes simulate a 1d13 with digital random number generators or by combining other dice, though such methods may introduce slight deviations from a uniform distribution.

Usage examples include determining a small number of distinct outcomes, table/chart lookups, or as a randomizer in custom rules. The NdM notation is widely used in online dice rollers and game documents for clarity and scripting compatibility.

See also: dice notation, polyhedral dice, probability distributions, random number generation.