Roll D12
One twelve-sided die. Each face from 1 to 12 carries an 8.33% chance, and unlike two dice added together, a d12 is completely flat, so 12 is exactly as likely as 7.
1d12 1
Press roll.
Lowest possible 1, highest 12, average 6.5.
One twelfth a face
Twelve faces, 8.333% each, average 6.5. Nothing about the shape of the die favours any number. Twelve rolls will almost never show all twelve numbers: the chance of that is 0.005%, roughly one time in 18,600.
A d12 is not 2d6
Both stop at 12, and that is where the resemblance ends.
| d12 | 2d6 | |
|---|---|---|
| Average | 6.5 | 7 |
| Chance of 11 or 12 | 16.67% | 8.33% |
| Chance of 1 or 2 | 16.67% | 2.78% |
| Shape | flat | peaked at 7 |
The d12 swings harder in both directions and averages half a point lower. That is exactly the trade in fifth edition between a greataxe rolling 1d12 and a greatsword rolling 2d6: the axe is streakier, the sword is steadier and slightly better on average.
Repeats
In four rolls, the chance that some number appears twice is 42.7%. Expect around 37 rolls before every one of the twelve faces has appeared at least once. A number missing for a while is arithmetic, not a fault.
What it is good for
- Picking a month, or an hour on a clock face.
- Barbarian hit dice and heavy weapon damage.
- Any twelve-item table where each entry should be equally likely.
Compare it directly with 2d6, get the same flat spread without dice from 1 to 12, or change the sides in the dice roller.