D notation

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D notation, or Dice notation is a system to represent different combinations of dice in role-playing games. The system is borrowed from Dungeons & Dragons. Throughout articles in this Wiki, the D notation is used to show the chances of something.

Notation

The notation has the form of ndy, where n and y are variables:

  • n is the number of dice rolled
  • y is the number of faces on each die

For example, if an article states that a weapon has a damage of 1d24 (roll one die with 24 sides), it can do a damage between 1 and 24. If an article states that a weapon does a damage of 2d24 (roll two dice with 24 sides each), it can do a damage of (1..24) + (1..24).

If n is omitted, it is taken to be 1.

An additional notation that can be encountered is ndy x2, for the double damage property. For example, Grayswandir has 1d8 x2 +(1d20) damage. This means that a single die roll of 1d8, plus enchantment, is multiplied by 2 – the die is not rolled twice. The 1d20 is added separately without being multiplied.

Probabilities

d(n,x)
Probability mass function of a roll of 3d6 and probability density function of the normal distribution with the same mean and standard deviation.
Distribution uniform (n=1)
triangular (n=2)
approx. normal (n>2)
Mean n(x+1)/2
Standard deviation \sqrt{n(x^2-1)/12}

Rolling a single die gives any possible roll with equal probability, but multiple dice will tend to give many rolls in the middle of the range and few at the ends. Therefore, rolling 3d6 is not the same as rolling 1d16 and adding 2, even though both give a range from 3 to 18 and a mean (average) of 10.5.

A roll of many dice approximates a normal distribution or "bell curve".

This table gives the probabilities of each possible roll of 3d6:

Roll Number of Rolls Probability
3 1 0.00463
4 3 0.0139
5 6 0.0278
6 10 0.0463
7 15 0.0694
8 21 0.0972
9 25 0.116
10 27 0.125
11 27 0.125
12 25 0.116
13 21 0.0972
14 15 0.0694
15 10 0.0463
16 6 0.0278
17 3 0.0139
18 1 0.00463

Source

External links

For visualizing or seeing the results of other rolls, see any of the following calculators: