Converting d20 DCs to GURPS

[JCurwen3]

Originally Posted by lexington

IIRC a normal person in d20 rolls 1d20+0 to complete a task in what GURPS would consider a an adventuring situation.

By probabilities then:
DC10 is about -0
DC13 is about -1.
DC16 is about -2.
DC18 is about -3.
DC19 is about -4.
DC20 is about -5.

This, however, is distorted by GURPS using 3d6 which makes it too granular early on and too fine later. It is probably simpler to convert DCs to modifiers as (DC-10)/2. This makes the d20srd explanation of DCs fit the Basic Set description of TDMs pretty well. However, the level for "nearly impossible" doesn't fit as nicely since DC40 become -15 rather than -10.

That could reflect higher expected abilities in d20 characters than in GURPS characters or suggest that typical doesn't quite line up with 10 in both systems due to skills. Alternately you could force them to line up with a crazy progression like (DC-10)^(1/1.5), rounding up.

Thanks. I think I'll go with the earlier (DC-10)/2. It seems clean. I know this won't be perfect and modifiers will seriously skew things, but I'm happy to have a rough approximate conversion and then tweak that in situations where the results clearly need to be different based on intent / realism.