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 D6Difficulty of Task
10 0 (-6) +65% +490% 5 in 6Trivial-
11 0 (-5) +60% +485% 5 in 6Easy
12 0 (-4) +55% +380% 5 in 6Easy
13 0 (-3) +50% +375% 5 in 6Easy
14 0 (-2) +45% +270% 4 in 6Average
15 0 (-1) +40% +265% 4 in 6Average
16 0 +35% +160% 4 in 6Average
17 1 +30% +155% 4 in 6Average
18 2 +25% 050% 3 in 6Average
19 3 +20% -145% 3 in 6Difficult
20 4 +15% -140% 3 in 6Difficult
21 5 +10% -235% 2 in 6Difficult
22 6 +5% -330% 2 in 6Difficult
23 7 +0 -425% 2 in 6Difficult
24 8 -5% -520% 1 in 6Very Difficult
25 9 -10% -615% 1 in 6Very Difficult
26 10 -15% -710% 1 in 6Very Difficult
27 11 -20% -85% 1 in 6Very Difficult
28 12 -25% -90% 0 in 6Formidable
29 13 -30% -10-5% 0 in 6Formidable
30 14 -35% -11-10%  0 in 6Impossible+
31 15 -40% -11-15%  0 in 6Impossible+
32 16 -45% -12-20%  -1 in 6Impossible+
33 17 -50% -12-25%  -1 in 6Impossible+
34 18 -55% -13-30%  -1 in 6Godlike+
35 19 -60% -13-35%  -1 in 6Godlike+

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.


  1. Great stuff! I'll for sure be using this to try and "translate" things from other editions (from newer editions back into LL most likely).

  2. I can certainly find a use for this. Saves time. Thanks.

  3. I've tried to do something similar (All Systems)... You can find it at:


    I'm not sure if it as accurate as I'd like, but it is the beginnings of something that someone else can take and expand upon...



  4. Thanks for the table! Good stuff.

  5. I'd like to welcome all my french readers. I'm not sure what kind of board that is (I don't read french, only spanish and latin), but I hope this is helpful!

  6. This looks amazing, I never even thought of something like this but it makes so much sense.

  7. Just saw a link referring to this post and re-found it again. Wow. This is a super-handy resource. Thanks for assembling this table! Maybe this is what the guys at WotC are looking to produce for 5E?

  8. Thanks for providing this post. I will use this for my games, it has utility. I found your blog this week. I have been reading several OSR blogs over the last 18 months. They have made me want to start playing RPGs again. Your blog is very well written and fun to read. It is in my top five OSR blogs. Thanks for sharing your vision, It is interesting to read.

    Take care,

  9. Good stuff, thanks. Now, let's see if it helps with conversion for my Tunnels and Trolls campaign. :D


  10. I knew I had seen this, but wasn't finding it, until Alex Schroeder directed me to it.


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