d20 math — deliberate failure

A lot of games are clever about deliberately failing rolls “You need to deliberately fail an empathy check to shoot him” etc.

But if I don’t want to be clever. If I just want to calculate the inverse. To get a “hardness” roll derived from your empathy. Or whatever. Because it feels so weird to roll to fail.

Let’s say there’s a +4 mod vs DC 10 chance of making a wisdom save (for dropping a torch without it going out). And I want to flip it to get the chance for dropping a torch and putting it out? For when you want to do that?

Do two things.
The new DC: 22 minus the old DC.
The new mod: 0 minus the old mod.

Or, say, “Roll vs DC 12 but minus your wis save instead of adding it.” (The 12 is 22-10.)

Or if you want to obfuscate & be illusionist.
I mean I don’t see the point. But.
The new DC: 22, minus the old DC, but plus twice the mod.

You need to know the mod (I usually don’t know my PC’s mods) and it becomes contingent on which PC is making the save which is unblorby.


With a +4 wis save mod, the new DC becomes therefore… 20.

22 -10 +4 +4 = 20.

(And also this is yet another reason why I want to switch from d20 + mod vs DC to an ACKS style “throw” thac10 system. Because to flip any “roll-over” roll you just do 22 - it.)


  • Nicely done! This is very slick.
  • Thanks! I should clarify that the method that I want to use is:

    flip the DC by subtracting it from 22 and flip the mod.
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