For your and my convenience, a table to convert any dice for any game rolls you may encounter, percentages of that outcome and at (>=) or over (>) the number specified:
|Quality||D100 >||%||D20 >=||%||D10 >=||%||2D6 >=||%|
Do the percentages match? No. Is it worth it to be exact? Also no – these are for use in games of fun and non-monetary chance, each no more than ~10% from one another. As I once heard from a lead game designer (in paraphrase): “If a change isn’t 10% or more, it doesn’t matter.”
Note: Some games start the player’s characters strong, or the goals are to instill a dire feel in play. That just means to up the tiers of quality by 1, e.g. easy rolls as if medium, medium becomes hard, etc.
What About Other Dice?
Anything smaller than a single D10 or two D6 is really hard to make work intuitively.
Going back to the ~10% difference, look at a D6 -> a ‘very hard’ roll would be a 6 or more on a D6, a 17% chance of happening with a single die. We violate the ~10%-or-less rule. Could we make it happen? … Yes, if a roll requires a final number greater than the faces of the die (e.g. a 25 on D20), but now we are getting into some severe nitty-grittiness.
Don’t Forget the Stats!
If using a system specifically designed for a ‘DX’ roll but a player brings a ‘DY’ to the party, make sure to convert the stats any fictional character uses from ‘X’ to ‘Y’.
Example: A D10 is used for a D20 system – stats for the D20 system need to be cut in half in how they would add or subtract from the D10 roll.
An example for the example: A +4 Toughness stat would add to a normal D20 roll in the game. However, since using a D10, only +2 (half of +4) would be used when D10 is rolled.
Here is another: A D6 is brought to a D20 system. Stats would thereby be about one-third as applicable (+3 becomes a +1).
Hope this helps if needing to get people playing quickly at your table with any dice for any game. Might put this into something printable for an index card – let me know if I ought get that going sooner than later 👍
Cheers to you rolling what you need to 🎲🎲