Converting d20 DCs to GURPS

[JCurwen3]

I have a couple of d20 supplements that list DCs for various things.

Are there any good rules on converting these into GURPS modifiers? Ideally those that would lead to similar probabilities of success and failure.

This has probably come up a lot but it's not an easily googlable thing to google.

[Blind Mapmaker]

As far as I know there is no easy way to just convert the the DCs straight over to GURPS skill modifiers. They are designed so that some feats are always impossible for low-level characters even on a natural twenty. The use of a d20 also means that every +1 has the same effect whereas the effect of a -1 on a GURPS modifier means different things depending on where on the bell curve the final check ends up.

If you want to do a quick and dirty conversion, I can offer the following:
DC 5-10: Task difficulty (B 345) Automatic to Easy. Keep in mind that low-level D&D characters will still fail these 50% of the time or more.
DC 11-15: Very Favourable to Favourable
DC 16-20: Average to Very Unfavourable
DC 21-30: Hard to Dangerous
DC 31+: Impossible

As I said most often it doesn't make sense to just convert things over without looking at how hard the check should be for the characters intended for the module. If you're lucky you can just follow the flavour text in the cases where it matters. When that doesn't help look at some sample characters of the intended levels and what they would have in their class skills.

When the sample level 10 rogue has Open Locks +12, a lock with a DC of 20 is not supposed to be a significant obstacle. The thief can just take 10 if he has enough time. A DC 30 going to require considerable bonuses or luck to be opened. So I would just give the first lock a -1 or -2 modifier and the second one a -8.

[Hai-Etlik]

Originally Posted by JCurwen3

I have a couple of d20 supplements that list DCs for various things.

Are there any good rules on converting these into GURPS modifiers? Ideally those that would lead to similar probabilities of success and failure.

This has probably come up a lot but it's not an easily googlable thing to google.

Although they have the same expected value, and similar ranges, 1d20 and 3d6 have very different probability distributions. +1 in 1d20 shifts the probability by a flat 0.05, until it goes beyond the range of the roll. +1 against a 3d6 roll varies dramatically with where you are on the curve.

I'd suggest ignoring the DC, looking at the situation, and coming up with a GURPS TDM based on that is a better choice than trying to convert D&D DCs mechanically.

If I absolutely had to do it, I'd start with something like TDM = (DC-10)*k. My initial though for a good k is 3/5 but it would take some testing and tweaking and might require a non-linear conversion function to get decent results.

[lexington]

Originally Posted by JCurwen3

I have a couple of d20 supplements that list DCs for various things.

Are there any good rules on converting these into GURPS modifiers? Ideally those that would lead to similar probabilities of success and failure.

This has probably come up a lot but it's not an easily googlable thing to google.

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.

[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.