Converting d20 DCs to GURPS
 

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.

Re: Converting d20 DCs to GURPS
 

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.

Re: Converting d20 DCs to GURPS
 

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.

Re: Converting d20 DCs to GURPS
 

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.

Re: Converting d20 DCs to GURPS
 

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.

Re: Converting d20 DCs to GURPS
 

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?

Re: Converting d20 DCs to GURPS
 

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.

Re: Converting d20 DCs to GURPS
 

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

Re: Converting d20 DCs to 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.

Re: Converting d20 DCs to GURPS
 

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.

Re: Converting d20 DCs to GURPS
 

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.

Re: Converting d20 DCs to GURPS
 

I thought you were arguing to the contrary? Did you change your mind? Or did I fail to understand you in the first place?

Re: Converting d20 DCs to GURPS
 

Uh... Wait... that quote is three years ago. It's entirely possible for an individual to change their opinions about things in much shorter amounts of time. I'm trying to lurk here, but that... pulled me aside and slapped me across the face.

As for the actual topic... uh... I wrote a good thousand words on an attempt to comment a half hour or so ago then erased it. It was summed up to:

Since the games use different design approaches, converting stats doesn't always provide desirable results. Although it's not exactly applicable to the topic, converting concepts does work brilliantly.

With each trait GURPS assumes a default action that should have a standardized chance of success. For example, to punch someone typically is easier a task than to conduct a surgical procedure upon them. So GURPS applies a regulator (skill difficulties in this case) to regularize the roll when performing a standard action.

DnD assumes that because punching a dude is easier it will have a lower DC. Which means before you start with any equation, you might consider... oh darn, I'm rambling on again. Before I just backspace this into the oblivion that contains all my other posts.

[TL;DR]Consider assigning difficulty modifiers case by case/whimsically as a GM and use the suggested DR's as a sort of... feeling gauge? Does that sound like a thing?[/TL;DR]

Re: Converting d20 DCs to GURPS
 

Why? Bloated DCs in d20 are no more realistic than bloated difficulties in GURPS.

Re: Converting d20 DCs to GURPS
 

Changing your mind isn't a bad thing, unless you are a politician. I'm just wondering if I a) misunderstood him or b) he changed his mind.

Re: Converting d20 DCs to GURPS
 

I didn't say *I* recommended it. :-) I said that it's been beaten into my head that it doesn't make sense to have locks (or other challenges) based on the skill of the party, rather than in-game-world realities.

As if dungeon fantasy actually had to make sense...

In any event, I'm always in favor of "normalizing" anything converted from D&D to something more realistic. (E.g., Elminster, in my mind-space, is a decent mage, a pretty good sage, and a hell of a self-promoter...)