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.

[sir_pudding]

Originally Posted by JCurwen3

Thanks. I think I'll go with the earlier (DC-10)/2.

Shouldn't this be (-DC+10)/2? If you go with (DC-10)/2 you'd have negative modifiers for DCs <10 and positive modifiers for DCs >10 which is backwards. What am I missing?

[lexington]

Originally Posted by sir_pudding

Shouldn't this be (-DC+10)/2? If you go with (DC-10)/2 you'd have negative modifiers for DCs <10 and positive modifiers for DCs >10 which is backwards. What am I missing?

I was accidentally thinking that modifiers are always negative as thus "making a DC a modifier" automatically involves making it negative but I think the intent is easily decipherable anyway. You are correct that there ought to be a negative sign. I prefer "-(DC-10)/2" or "the negative of (DC-10)/2" personally. The mental math just feels easier.

[sir_pudding]

Originally Posted by lexington

I was accidentally thinking that modifiers are always negative as thus "making a DC a modifier" automatically involves making it negative but I think the intent is easily decipherable anyway. You are correct that there ought to be a negative sign. I prefer "-(DC-10)/2" or "the negative of (DC-10)/2" personally. The mental math just feels easier.

DCs 0-5 at least should correspond to positive modifiers in GURPS.

[Anthony]

Originally Posted by sir_pudding

DCs 0-5 at least should correspond to positive modifiers in GURPS.

Actually, the equivalent of an unmodified skill roll in GURPS is about a DC 15 task in d20 (something done half the time by a trained person), so it's more like (15-DC)/2, or +5 for a DC 5 task. Maybe even worse -- a task an untrained average person in GURPS can accomplish half the time is a +5 modifier, a task an untrained average person in D20 can accomplish half the time is a DC 11.

[Rasputin]

Originally Posted by Anthony

Actually, the equivalent of an unmodified skill roll in GURPS is about a DC 15 task in d20 (something done half the time by a trained person), so it's more like (15-DC)/2, or +5 for a DC 5 task.

That's the formula I use when converting D&D DCs, especially traps. Divide by 4 instead of 2 for DCs that will involve an active defense in GURPS, which almost always means Dodge.

[Pragmatic]

It's been recommended that DCs when converted to GURPS be kept to realistic items.

So, for instance, a lock shouldn't be arbitrarily DC 30. It's got to have a game world reason. You don't just increase the DCs for more powerful PCs, you have to have a reason.

In general, the situation should get modifiers (e.g., the guards are particularly alert), rather than a master adamantine clockwork lock.