On an Updated Rosetta Stone

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There are a lot of OSR type games.

Each game features a unique variant of some basic math. Each has a few assumptions about how these numbers play and scale, and each edition carries its own variation affecting play. For the most part converting between systems is pretty trivial. However for some of the subsystems, such as converting 1st edition style non-weapon proficiencies to 3e skill DC's, or 5th edition skills back to 3rd edition skill DC's it can get a little more complicated. A few years ago, I wrote a tool that helped me convert Hackmaster to Rolemaster to D20 seamlessly, allowing me to more objectively use books and oddball skill systems with whatever I happened to be playing.

Now that ACKS and 5th edition is out, it seemed like it was time to update.

Without further ado, the updated OSR Rosetta Stone:

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

Table Key

D20: This is the D20/3.5/Pathfinder Skill system. It assumes constantly scaling bonuses and difficulties.
5th (DC): This is the 5th edition "bounded accuracy" system. It assumes a maximum proficiency bonus of +6, a maximum statistic bonus of +5, giving your average 20th level character a +11 on rolls. Expertise can double the proficiency bonus and magic can add up to +3, making the maximum value a character can achieve +20 on a roll.
C&C: This is the challenge class rating for Castles & Crusades. The odds are calculated assuming primes. If not prime, simply subtract 6 to get the same chance for a non-prime. A prime stat has a challenge value of 12 for checks, a non prime 18. Challenge class is added to the base (prime/non-prime) difficulty. Statistic bonuses range from -4 to +3, your level is added if applicable to the check.
ACKS Throw: This is the player facing value the roller must succeed against in the Adventurer Conqueror King system. Modifiers are applied to the die roll. You may alternately align these numbers with the Base Success Chance value, but the proficiencies make it clear that average tasks succeed on a 11+, where as formidable tasks (using healing to cure disease for example) still have a chance of success.
D100: This refers to the modifier applied to the target number on any system using a roll under % system for skills, such as Hackmaster 4th edition. For open ended percentile systems like Rollmaster, these can modify the percentile result (but if they modify the result over 95% or under 5%, they will not cause the roll to become open ended.)
NWP: This is the modifier applied to Dungeons & Dragons 2nd edition Non-Weapon Proficiency system. These modifiers are applied to the target number. 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.
BSC: This is the base success chance. It is simply a representation of the percentage chance of success at any task in the abstract. Consider this the baseline of the table for comparison.
D6: Using a system like Skills: The Middle Road or Lamentations of the Flame Princess? This is a d6 representation of core chances for success.
Difficulty of Task: This is a word description of the difficulty of the task being accomplished, like the base chance of success, intended to act as a baseline.

Monster Conversions

This process is somewhat complicated and also very forgiving. When converting monsters between systems, it's important to remember the following:
Original Dungeons and Dragons: Monsters all use D6 for hit dice.
Basic Dungeons and Dragons: Monsters use D8 for hit dice, but weapon damage and player hit point totals remain at OD&D levels, making monsters much more deadly.
1st/2nd Edition: Monsters use D8's for hit dice, but player hit point totals are higher.
All the above versions of D&D use a +X modifier following monster hit points to indicate that the monster hits "above it's class" gaining a bonus to ThAC0.
3rd Edition: Monster hit points vary by monster type and have Constitution bonuses applied. When converting back to earlier editions, eliminate the Constitution bonus and roll the appropriate die for hit dice. When converting forward use the appropriate die type for the monster type and consider increasing hit points based on hit die due to increased player damage output. Armor classes must also be capped if converting backwards, eliminating dexterity bonuses and adjusting for scaling increases.
Hackmaster 4e: This is functionally equivalent to 1st edition if the hit point kicker is removed.
ACKS monsters are equivalent to B/X monsters.
C&C monsters are close to 1st edition style monsters, requiring only armor class conversion.

5th Edition Conversion notes:
5th edition monsters are the most difficult to convert. The difficulty comes in having to deal with needing to inflate the hit point totals and deal with bounded accuracy. +Surf Archer has a detailed breakdown of 5th edition monster design if you're looking to accurately transfer or design monsters. Here are some guidelines for on the fly conversion.

On the fly quick and dirty conversion from d20/PF to 5th edition:

• Ability Scores—Use 1/2 Fortitude, Reflex and Will saves for ability bonuses. 
• Alternately the bonus is +1/2 CR. 
• Proficiency bonus is as Character level to Hit Dice, i.e. 1-4 Hit die is +2, 5-9 Hit die is +3.
• To hit bonus—is 1/2 CR + Proficiency Bonus.
• no penalties for multi-attack
• damage should remain the same
• Skills—Use 1/2 appropriate save bonus + proficiency bonus. Double this bonus if trained.
• Armor Class—It bounded. Use AC 10-12 for lightly armored, AC 13-15 for medium armor, and AC 16-19 for heavy armor.
• Hit points—CRx10+20. If not, adjust up only.
• Abilities—keep and use the rules for 5th edition, i.e. Magic Resistance gives advantage on saves.

Baseline Assumptions

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
• 5th assumes a +2 proficiency and +3 statistic bonus = +5

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 4th edition).

Any comments or suggestions or additions to the table are appreciated.

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