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Old 10-23-2017, 03:21 PM   #1
Rhino
 
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Default Converting DND 3.5 DC to DFRPG

There is a particular DND 3.5 adventure (Necropolis) I want to run using DFRPG. I would like to avoid having to convert everything ahead of time. I will be winging it on the monsters or using the closest DFRPG analog I can find. However, for traps, locked doors, etc., I wanted to see if there was a way I could convert the DND 3.5 Difficulty Classes ("DC") for these tasks to a (range of) DFRPG difficulty modifiers for the analogous exploit.

The problem is the DC scale is linear, while the DFRPG has a bell curve. Has anyone done such an analysis and arrived at a conversion of DND 3.5 DC to DFRPG difficulty modifiers? Thanks.
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Old 10-23-2017, 04:03 PM   #2
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Default Re: Converting DND 3.5 DC to DFRPG

The easiest way is probably just compare the Task Difficultly Modifiers to the generic DC table and match similar descriptors. So:
DC 0=Very Easy=+6 or +7
DC 5=Easy=+4 or +5
DC 10=Average=+0
DC 15=Tough=Unfavorable=-1
DC 20= Challenging= Hard=-4 or - 5
DC 25 =Formidable= Very Hard =-6 or - 7
DC 30= Heroic= Dangerous = -8 or -9
DC 40 = Nearly Impossible = Impossible = -10

The difficulty with a more rigorous approach might be to say that to have a 50 % of DC 10 you need skill 0 which would therefore be statistically equivalent to Skill-10, but that means that untrained skills in D&D are better than GURPS defaults by about 40-48%. If instead you assume that default use is equivalent, "Average" difficulty in D&D is actually equivalent to Very Easy to Easy in GURPS, which makes it non-trivial to assess what the equivalent of Skill-10 is. At any rate once you have decided on what Skill-10 is equivalent to you can use that baseline and the fact that each +1 to DC is - 5% chance of success and compare that the Probability of Success table and then subtract 10 to get a modifier.

Assuming +0=Skill-10 then:
DC 5=75%=~Skill-12=+2
DC 10=50%=Skill-10=+10
DC 15=25%=~Skill-8 = -2
DC 20=5%=~Skill-5 = -5
And then you run into the problem that nothing higher is even possible to roll on 1d20 whereas 3d6 goes down to half a percent.

Last edited by sir_pudding; 10-23-2017 at 04:16 PM.
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Old 10-23-2017, 10:13 PM   #3
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Default Re: Converting DND 3.5 DC to DFRPG

Thanks, sir pudding. I like the simplicity of your first suggestion and will give it a try.
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Old 10-23-2017, 11:52 PM   #4
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Default Re: Converting DND 3.5 DC to DFRPG

Maybe this is overly simplistic, but I treat a DC 15 task as skill +0, and add +2/-2 to skill for each 5 points above or below that. Easy enough to remember:

DC 5: +4
DC 10: +2
DC 15: 0
DC 20: -2
DC 25: -4
DC 30: -6
DC 35: -8
DC 40: -10

Then I'll add or subtract another +1 for the numbers which are more than 1 DC off from a multiple of 5. So DC 19 and 21 are -2 just like DC 20, but DC 17 and 18 are just -1, and DC 22 and 23 are -3.

Note: I haven't played much D&D 3.5, so I can't say if these are exactly right, but the module I converted seems to have a lot of DCs in the 15 to 25 range, and this yields modifiers in the 0 to -4 range, which seems like a reasonable challenge for Dungeon Fantasy characters, who tend to have pretty good ability and skill levels. And it has occasional DCs in the 40 range for tasks that are supposed to be very hard, and a -10 does reflect very hard (but not impossible with high skill) pretty well. So I've been pretty happy with this simple system.
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Old 10-24-2017, 03:08 AM   #5
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Default Re: Converting DND 3.5 DC to DFRPG

Note that a 1st level D&D 3.5 character with a Stat of 10-11 and max ranks (4) in a skill for a total of +4 has a 50% chance of making a DC15 check, and DC15 is what d20 systems tend to think of as a average check. Thus +0 for DC15 is about right - someone with some training an no great talent will have about skill-10 in GURPS.

D&D 3.5 tends to consider an situation that would give a modifier worth a +-2, so where something would give that, give a +-1 in GURPS (masterwork non-combat gear does this,m for example, while 'good' equipment gives a +1 in GURPS).

Jumps in DC in D&D usually come in 5-point chunks unless part of some formula, while B343-344 uses 2-point increments for difficulty, so each +5DC would give a -2 penalty (as other posts also suggest). If this doesn't seem to make life hard enough, just go to -3 per step.
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Old 10-24-2017, 12:17 PM   #6
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Default Re: Converting DND 3.5 DC to DFRPG

Quote:
Originally Posted by Rupert View Post
Note that a 1st level D&D 3.5 character with a Stat of 10-11 and max ranks (4) in a skill for a total of +4 has a 50% chance of making a DC15 check, and DC15 is what d20 systems tend to think of as a average check. Thus +0 for DC15 is about right - someone with some training an no great talent will have about skill-10 in GURPS.
In 3.5, DC 10 was defined as "Average"; DC 15 was "Tough".
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Old 10-24-2017, 01:14 PM   #7
Rhino
 
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Default Re: Converting DND 3.5 DC to DFRPG

Quote:
Originally Posted by dripton View Post
Maybe this is overly simplistic, but I treat a DC 15 task as skill +0, and add +2/-2 to skill for each 5 points above or below that. Easy enough to remember:

DC 5: +4
DC 10: +2
DC 15: 0
DC 20: -2
DC 25: -4
DC 30: -6
DC 35: -8
DC 40: -10

Then I'll add or subtract another +1 for the numbers which are more than 1 DC off from a multiple of 5. So DC 19 and 21 are -2 just like DC 20, but DC 17 and 18 are just -1, and DC 22 and 23 are -3.

Note: I haven't played much D&D 3.5, so I can't say if these are exactly right, but the module I converted seems to have a lot of DCs in the 15 to 25 range, and this yields modifiers in the 0 to -4 range, which seems like a reasonable challenge for Dungeon Fantasy characters, who tend to have pretty good ability and skill levels. And it has occasional DCs in the 40 range for tasks that are supposed to be very hard, and a -10 does reflect very hard (but not impossible with high skill) pretty well. So I've been pretty happy with this simple system.
Thanks. I'll have to compare the module's DC settings to the character's skill levels and figure out which conversion chart yields the most playable results.
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Old 10-24-2017, 02:44 PM   #8
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Default Re: Converting DND 3.5 DC to DFRPG

Quote:
Originally Posted by sir_pudding View Post
In 3.5, DC 10 was defined as "Average"; DC 15 was "Tough".
And in GURPS, a difficulty +/-0 task is a typical task for adventurers.
Quote:
Originally Posted by p. B345
0 - Average. Most adventuring tasks, and the majority of skill use under stress. Example: A Driving roll in a car chase.
Quote:
Originally Posted by d20SRD
Average (10). Hear a guard approaching (Listen)
Tough (15). Rig a wagon wheel to fall off (Disable Device)
Speaking as someone who has done this conversion many times, I go with D&D DC 15 = GURPS task difficulty 0.
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Old 10-24-2017, 04:32 PM   #9
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Default Re: Converting DND 3.5 DC to DFRPG

If you are converting adventures meant for low level characters for 250 point delvers having tasks be -1 instead of +0 is probably a feature.
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Old 10-24-2017, 08:19 PM   #10
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Default Re: Converting DND 3.5 DC to DFRPG

Quote:
Originally Posted by sir_pudding View Post
If you are converting adventures meant for low level characters for 250 point delvers having tasks be -1 instead of +0 is probably a feature.
The adventure I plan on converting (Necropolis) is for level 10 to 15 DND characters.
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