Roll D100
A hundred-sided roll, 1 to 100. Every value is exactly 1% and the average is 50.5. A hundred options sounds like plenty, but repeats arrive far sooner than people expect.
1d100 1
Press roll.
Lowest possible 1, highest 100, average 50.5.
1% a value
Every number from 1 to 100 has the same one-in-a-hundred chance. The average is 50.5. Rolling 90 or above is 11%, and rolling in the top decile, 91 to 100, is 10%. Percentile tables are built on exactly this: the ranges in the table are the probabilities.
Repeats come sooner than a hundred slots suggests
This is the birthday problem with 100 days in the year, and the answer surprises people every time.
| Rolls made | Chance two of them match |
|---|---|
| 5 | 9.7% |
| 10 | 37.2% |
| 12 | 49.7% |
| 20 | 87.0% |
| 25 | 96.2% |
Twelve rolls is a coin flip on getting a duplicate. So if you roll on a hundred-entry encounter table a dozen times in an evening and hit the same entry twice, nothing has gone wrong. If you need every result to be distinct, cross entries off yourself, because the roll will not do it for you.
Going the other way, seeing all one hundred values at least once takes about 519 rolls on average. Coverage is slow.
Two d10 do the same job
A physical hundred-sided die exists and rolls forever, so most tables use two ten-sided dice instead, one for tens and one for units. That produces the identical flat 1% spread, which is why the d10 is the die people actually own.
For the same range in plain form, use a random number from 1 to 100, or change the sides in the dice roller.