Monster Math: Ultimate 5e/Classic Conversion

Wasn't happy with my last conversion because CR is not a pure measure of longevity like HD. It accounts for AC as well. Since then, though, I've come up with an actual measure for OD&D that counts both HD and AC! So we can do some mad science now. I mapped CR values to virtual HD values (virtual HP / 3.5), and found a really tight function for CR ≥ 1 and VHD ≥ 3.

CR = 0.5 × VHD – 0.5

VHD = 2 × CR + 1

Let's put this funky function to the test. Below is a table converting monsters from 5e to OD&D and vice versa. Keep in mind that dragons of different ages do not truly have different HD values; instead, they have different hit points per HD. This turns out so that a red dragon with "10 HD" can have the equivalent of from 3 to 17 HD (which is really from 9.5 to 56.6 VHD with an AC of 2; ibid.). For the purposes of this, I'm interpreting a wyrmling as a "very young" dragon and an ancient dragon as an "old" but not "very old" dragon.

Monster 0e VHD 5e CR 0e-to-5e 5e-to-0e
Balor 33 19 16 39
Basilisk 12 3 6 7
Chimera 18 6 9 13
Cockatrice 7 ½ 3
Dragon, Red, Wyrmling 10 4 5 9
Dragon, Red, Young 19 10 9 21
Dragon, Red, Adult 38 17 19 35
Dragon, Red, Ancient 48 22 24 45
Gargoyle 7 2 3 5
Giant, Frost 20 8 10 17
Gorgon 27 5 13 11
Manticore 12 3 6 7
Medusa 4 6 2 13
Minotaur 9 3 4 7
Mummy 13 3 6 7
Hydra 17 8 8 17
Ochre Jelly 5 2 2 5
Ogre 7 2 3 5
Purple Worm 21 15 10 30
Troll 14 5 6 11
Werewolf 7 3 3 7
Wraith 10 5 5 11
Wyvern 18 6 9 13
Vampire 26 13 13 27

Notable outliers included specters, gorgons, cockatrices who are all relatively weaker in 5e than in OD&D since they serve straight-up different fictional purposes. Specters are simply ghosts rather than evil twisted poltergeists, and both gorgons and cockatrices seem to serve as lower-level bosses. Hydras' heads are however more individually powerful, more like two level-1 PCs worth rather than just one. This makes the 5-headed 5e hydra equivalent to the 10-headed 0e hydra. Medusae are also seemingly less powerful in Oe than 5e, but their VHD does not account for their petrification ability.

The percent difference across the board is –1% with the medusa (the worst outlier) or 8% without.

What About Mooks & Grunts?

The formula just does not work if CR < 1. That's basically where the "grounded" numbers of 0e clash most directly with the "superheroic" numbers of 5e. What's sort of funny is that that's a distinction that 0e already makes between regular and fantastic figures. How about a twist to that old question?

Is a non-fantastic figure one with less than 2 HD (around ~1 HD), or one with less than 4 HD (ranging from ½ to 3)? Taking our analyses of both 0e and 5e, we can confidently say neither. A non-fantastic figure could therefore be one whose CR is less than 1, or whose VHD is less than ~3-4. Would you look at that intergenerational continuity!

This has no actual basis except that the math adds up in a funny, coincidental way that agrees with the basic fiction of what constitutes a "regular" figure.


  1. One factor in your "outliers" is that petrification, paralyzation and level drain have been hugely nerfed in 5e compared to 0-2e.

    1. that's true, but VHD does not account for those factors (only HD and AC). if it did, the difference between 0e and 5e would be even more pronounced!

    2. actually, now that i look at it, the 5e-to-0e conversion for the medusa puts it much closer to the VHD of other level 5 monsters (~13). it's almost as if the conversion can also serve to clarify in some cases!


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