Finer than 5% granularity on a 20-sided die

As a byproduct of the previous project, but perhaps useful on its own, here is a table of all the specific percentages you can get if you add advantage and disadvantage to a d20 roll.

Percent chance|Die expression -|- 0.25%|1/20 with advantage 1%|2/20 with advantage 2.25%|3/20 with advantage 4%|4/20 with advantage 5%|1/20 6.25%|5/20 with advantage 9%|6/20 with advantage 9.75%|1/20 with disadvantage 10%|2/20 12.25%|7/20 with advantage 15%|3/20 16%|8/20 with advantage 19%|2/20 with disadvantage 20%|4/20 20.25%|9/20 with advantage 25%|5/20 27.75%|3/20 with disadvantage 30%|6/20 30.25%|11/20 with advantage 35%|7/20 36%|4/20 with disadvantage 40%|8/20 42.25%|13/20 with advantage 43.75%|5/20 with disadvantage 45%|9/20 49%|14/20 with advantage 50%|10/20 51%|6/20 with disadvantage 55%|11/20 56.25%|15/20 with advantage 57.75%|7/20 with disadvantage 60%|12/20 64%|8/20 with disadvantage 65%|13/20 69.75%|9/20 with disadvantage 70%|14/20 72.25%|17/20 with advantage 75%|15/20 79.75%|11/20 with disadvantage 80%|16/20 81%|18/20 with advantage 84%|12/20 with disadvantage 85%|17/20 87.75%|13/20 with disadvantage 90%|18/20 90.25%|19/20 with advantage 91%|14/20 with disadvantage 93.75%|15/20 with disadvantage 95%|19/20 96%|16/20 with disadvantage 97.75%|17/20 with disadvantage 99%|18/20 with disadvantage 99.75%|19/20 with disadvantage 100%|20/20

Not saying use this always (it’s cumbersome af that the target number jumps up and down) but can sometimes (rarely, admittedly) be useful when you’re looking to port over a specific probability from some other system and want to leverage their playtesting. (If you have reason to suspect that a particular rule is not well playtested, then of course you don’t need to approximate their probability either. Just use what would make sense in your game instead.)

the previous project