I'm working on a simplified rpg-system that only use D6 and I want a mechanic for fumble/critical fail.

Depending on how good the player is they have 1-5 die to roll and they have to beat a difficulty set by the player. I thought it would be fun to have players fail if they roll all 1:s but realized it makes it way to hard to fail if you have 5 die, and a bit to simple if you have 1. Is there some more linear way of defining critical fails?

This is what I get:

Nubmer of Die   Probability of all 1:s
1               16.67%
2               2.78%
3               0.46%
4               0.08%
5               0.01%

What I would like(approximately, exact numbers are not that important):

Nubmer of Die   Probability

1               18%
2               15%
3               12%
4               9%
5               6%

Unless you want those exact percentages, one simple possibility is that the player must roll a one on a d6. Then, if it's difficulty level 1, they're done; if it's difficulty level 2, they must roll the d6 again and get 1-5; if it's difficulty level 3, they must get 1-4; and so on. The actual percentages would then be 16.7%, 13.9%, 11.1%, 8.3%, and 5.6%, but it's close and in the right proportions.

ETA: Sorry, I just re-read the problem and realized this is for critical fails. So it's not really difficulty levels, but the same game action should work either way.


With one D6, you can only have multiples of $\frac16\approx 16.7\,\%$ as probabilities.

With two dice, $15\,\%$ of the $36$ outcomes is $\approx 4$ outcomes, so fail on "both are $\le 2$"

With three dice, you want to fail on about $26$ outcomes. It is easy to achieve $27$ outcomes: Fail on "each die is $\le 3$". In order to really have $26$ outcomes, try "... but succeed on three threes"

With four dice, you want about $117$ failing outcomes - I do not see a really simple description of such an event; similar for five dice and $\approx 467$ outcomes.


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