Converting d20-based Skills to Fudge

by Mike Harvey (maharvey67 @ gmail .com)

From time to time people ask about converting between d20-based RPGs and Fudge. This article presents a simple, play tested method, for doing so. The emphasis here is on converting d20-based skills to Fudge, but the information here could just as easily be used to convert from Fudge to d20-based games.

The core of this approach is to "map" the Fudge trait ladder to the range of d20 results. At first glance this might seem absurd, since the trait ladder only has seven entries and a sharply curved probability distribution. But if you look at the middle of the range of the cumulative Fudge distribution (-2 to +2), you'll find that they correspond roughly to 20%-40%-60%-80%. Fudge is by nature inexact, and since around 85% of the dice results fall on one of these values, I feel comfortable just saying that each +1 on the Fudge Dice is roughly +20% probability.

From there it is very easy to construct an open-ended conversion, based on the idea that +1 Fudge level is +4 d20 ranks (20%). The tricky part is trying to decide the specific correspondence.

In Fudge, you have a 60% chance to roll your skill level or better. I take Poor as equal to +0 rank in d20, since both are the default skill when no training has been received. You have a 60% chance to roll 9 or better on d20, so DC 9 is the equivilant of a Poor (unskilled) character attempting a Poor task. The average attribute in d20 is 12-13 (based on 4d6 drop lowest). So the average character has a +1 skill bonus due to her ability modifier. Thus, he only needs to roll an 8 or better if his skill is untrained (0 ranks).

The ability modifier drops the d20 equivilant for a Poor task to a DC 8, which a completely unskilled character can succeed at 60% of the time. I consider this as the "minimum" -- you're guaranteed at least a 60% chance of success, regardless of skill. So, I'd say DC 8-11 corresponds to Poor. Thus,

This also seems a good intuitive fit for d20... most skill checks seem to be in the DC 15 to 25 range, which is Fair to Great.

We can also do a few reality checks here. A beginning adventurer can have +4 ranks in his starting skills, have an average of a +1 ability modifier, so his overall skill is +5; by taking an automatic 10 on his die roll, he can succeed at a Fair task, while he can succeed at a Great task by taking a 20.

In Fudge terms, he has Mediocre (+4) skill. I'd consider "taking 10" to be an automatic +0 Fudge result (both are close to 50% chances), and "taking 20" to be a +3 Fudge result (both close to 95% chances). Thus in Fudge, a character "taking 10" will succeed at a Fair task, and when "taking 20" he'll succeed at a Superb task. Not an exact match, but reasonably close.

Maximum natural human potential in d20 is character level 20, skill rank of +24, and an ability modifier of +5, for a total skill of +29. By adding a +2 skill focus, we get a skill of +31, which could succeed at a DC 51 task if you took 20. In Fudge, maximum human skill is Legendary, and the maximum die result is +4, so the ultimate achievement for a natural human is Legendary+4... which equates to DC 48-51. Perfect match!

That gives us our DC equivalence, but we can also determine a skill equivalence. If 24 ranks can at most succeed at DC 51, the maximum human potential, and each Fudge level is 4 ranks, than Legendary must equate to rank 21-24. Working backwards we can construct a chart of skill ranks and character levels:

A range is suggested for the Fudge equivalence, since a d20 character of a given level or skill rank may also have attribute and skill focus modifiers from -3 to +7. A 20th level character will only be Superb unless he also has the talent to push his skill to Legendary.

You'll note that the given level progression in the above table makes spell difficulties very easy to convert to Fudge difficulties as well:

(Zero-level cantrips would be Poor spells in Fudge)

As for attributes, since d20 abilities average to 12-13 with the recommended rolling method, I'd normalize them so that 12-13 = Fair. That puts 18 at Superb and 20 at Legendary, which seems right.

Unified Conversion Table