On an OSR Rosetta Stone

With no further ado, the OSR Rosetta Stone:

This is a tool that will quickly allow you to look at a published adventure and make adjustments so that it is compatible to your current gaming system. I've tried to set up this table so that a + always means that the skill is easier. In roll under systems (D100/NWP) this means the modifier is applied to the target number.

20 - ThAC0 = BAB
20 - BAB = ThAC0

20 - Descending AC = Ascending AC
20 - Ascending AC = Descending AC

D20 (DC)	C&C (CC)	D100	NWP	~% BSC	D6	Difficulty of Task
10	0 (-6)	+65%	+4	90%	5 in 6	Trivial-
11	0 (-5)	+60%	+4	85%	5 in 6	Easy
12	0 (-4)	+55%	+3	80%	5 in 6	Easy
13	0 (-3)	+50%	+3	75%	5 in 6	Easy
14	0 (-2)	+45%	+2	70%	4 in 6	Average
15	0 (-1)	+40%	+2	65%	4 in 6	Average
16	0	+35%	+1	60%	4 in 6	Average
17	1	+30%	+1	55%	4 in 6	Average
18	2	+25%	0	50%	3 in 6	Average
19	3	+20%	-1	45%	3 in 6	Difficult
20	4	+15%	-1	40%	3 in 6	Difficult
21	5	+10%	-2	35%	2 in 6	Difficult
22	6	+5%	-3	30%	2 in 6	Difficult
23	7	+0	-4	25%	2 in 6	Difficult
24	8	-5%	-5	20%	1 in 6	Very Difficult
25	9	-10%	-6	15%	1 in 6	Very Difficult
26	10	-15%	-7	10%	1 in 6	Very Difficult
27	11	-20%	-8	5%	1 in 6	Very Difficult
28	12	-25%	-9	0%	0 in 6	Formidable
29	13	-30%	-10	-5%	0 in 6	Formidable
30	14	-35%	-11	-10%	0 in 6	Impossible+
31	15	-40%	-11	-15%	0 in 6	Impossible+
32	16	-45%	-12	-20%	-1 in 6	Impossible+
33	17	-50%	-12	-25%	-1 in 6	Impossible+
34	18	-55%	-13	-30%	-1 in 6	Godlike+
35	19	-60%	-13	-35%	-1 in 6	Godlike+

Using this table.
So you're reading a module and you come across a DC 22 perception check to locate a trap. This translates to a +5% on a Find Traps or Obeservation roll, or a 2 in 6 chance of locating the trap. Simple, eh?

Playing 2nd edition and want to know if your alchemist can produce alchemist fire? Use that Alchemy NWP at -1 to find out!

Classic D&D is not included for skills. Mostly a determination should be made as to if the character possesses the skill, and then allow success or not based on circumstance.
D20 is any game using skill ranks and a target DC for skills
d100 refers to the modifer applied to the target number on any system using a roll under % system for skills
C&C is the challenge class rating for Castles & Crusades. The odds are calculated assuming primes. If not prime, simply subtract 8 to get the same chance for a non-prime.
NWP is the modifier for the non-weapon proficiency skill check. These modifiers are applied to the target number. (This is the way 2nd edition works, but not 1st)
Approximate % chance of success is for a 1st level character who has the skill in question. For each system this assumes different baselines.

• D20 is 1 rank + 3 class skills + 4 bonus in stat = +8
• C&C assumes 1 level +1 bonus from stat = +2
• D100 assumes 25% mastery at 1st level
• NWP assumes a base stat of 14 in the NWP

I selected these values because they are average baseline character creation values for core stats for each system in question (stat of 18 for point buy d20, stat of 14 for roll 3d6 in order drop lowest, and 25% for taking a skill multiple times in a system like Hackmaster). Modifiers for NWP rise slower because it is much more costly to become more effective at those skills (+5% every 3-4 levels). This makes the Approximate success chance for NWP inaccurate, but does keep it in line with the difficulty of the task. NWP modifiers should probably cap out around -8.

If this is useful, I'd be glad to create a printable .pdf. The following information will probably be contained in my Alchemy, Poison, and Herbalism guide.