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System for Converting Rules Cyclopedia Monsters to 5Eby Sheldon Morris
Alright, so here are the results of examining Basic D&D's XP values along with the balancing encounters system found in the RC (page 101), and the 5th edition D&D's CR system.
The first thing I wanted to do was to see how monsters in Basic D&D faired using the encounter balancing mechanism. This mechanism could be used to create a CR value that works similar to that in 5E. It essentially takes the HD value and adjusts that if any hps are added and if any power bonuses (asterisks) are present.
Once that was looked at, I also referenced XP values to see how well the Basic monster's "CR" value mapped to it. What I found was that there was too much CR range for a given XP value. In other words, it appeared as though there could be a significant difficulty range when looking at monsters of equal XP values. Long story short - I tweaked the encounter balancing mechanism to give more precise results, making the new CR values map closer to the XP values.
[If anyone is interested, my new "Individual Adjusted Hit Dice" for Basic D&D monsters formula is: HD + (additional hps x 0.2) + (power bonuses x HD x 0.25). To get the CR value, divide the adjusted HD by 0.70, which is a "challenging" encounter, and divide by 4 for the standard 4 PC party. For example, the Basilisk's HD is 6+1**, and its IAHD is 6+(1*0.2)+(2*6*0.25) or 6+0.2+3=9.2. Taking 9.2/0.7/4 gives us a CR of 3.3. The chart I have created shows that the Basilisk's 950 XP value translates to a 5E CR of 3. So in the Basilisk's case its power level is about the same between Basic and 5E D&D.]
Here's the resulting chart to use a Basic D&D monster's XP to find its suggested 5E CR:
XP CR 5 1/8 10 1/4 16 1/2 35 1 100 2 425 3 1,000 4 1,750 5 2,300 6 2,900 7 3,500 8 4,100 9 5,000 10 6,000 11 7,000 12 8,000 13 9,250 14 10,500 15 11,750 16 13,000 17 14,500 18 16,000 19 18,000 20 +2,000 +1
So far I have been using another chart that I found online but will be using this one going forward. The only creatures that would end up being different are the Adaptor (currently at CR 6 while this chart suggests it should only be CR 4), the Aerial Servant (CR 13 vs CR 8), and the Giant Ant (CR 1 vs CR 2). I believe that I will go back and adjust these creatures.