Converting 5e CR to Classic HD

Challenge rating (CR) in Dungeons & Dragons, Fifth Edition determines basically everything you need to know about your monster: armor class, hit points, and average damage per round. This makes conversion between 5e and classic editions of D&D like B/X quite difficult, not only because of different conventions but also different expectations of how monsters and their math should function. Hits deal more damage than in classic D&D because characters add their ability bonus to the dice roll, and hit dice are kind of arbitrary and calculated retrospectively because what really matters is hit points by challenge rating (e.g.: a giant monster might have d12-sized hit dice whereas a medium monster might have d8-sized hit dice, but a CR 10 monster needs to have around 160 hit points either way). What do you if you're writing an adventure that you want to support both systems, but you don't want to work from scratch? My friend Nova found herself in this situation while writing stats for her forthcoming book Bridewell, and here was how I approached a solution.

In classic D&D, hit dice measure a monster's longevity with respect to that of an average person (having 1 hit die). This is sort of complicated in AD&D, but let's not think about that. Challenge rating in 5e measures of monster strength relative to a party of 4 characters, so a CR 1 monster is an appropriate challenge for a party of 4 level-1 characters. You might think, then, that converting challenge rating to classic hit dice is as easy as multiplying CR by 4:

HD = 4 × CR

However, B/X scales differently than 5e, and of course also has different expectations for monsters and how they compare to player-characters. An ancient red dragon in 5e has CR 24 and 546 hit points, but an elderly red dragon in OD&D (B/X isn't quite the same) has 10 hit dice and 6 hit points per die, or 60 total (about 17 hit dice, relative to monsters who don't have the weird age rules). How do you convert between systems with such different expectations, especially with regards to scaling power relative to characters? Here's a formula that I really liked, at first:

HD = 2 × √CR

The outcome is a CR ¼ monster, equal in strength to 1 level-1 character in 5e, has 1 hit die in classic D&D; meanwhile, a CR 25 monster has 10 hit dice (rather than 100!). This formula is not perfect, especially because it's a little conservative when converting high-CR monsters, but it's very manageable. It sort of even resembles the pre-release scale that the original draft of D&D, Beyond This Point Be Dragons, had for monsters, with a maximum of 10 hit dice (convenient for use with Necropraxis' house rules). Here's how some common monsters, between classic D&D and 5e, compare:

Monster	Classic HD	5e CR	Converted HD
Berserker	1d+1	2	2.8d
Dryad	2d	1	2.0d
Ghoul	2d	1	2.0d
Gnoll	2d	½	1.4d
Goblin	1d–1	¼	1.0d
Hobgoblin	1d+1	½	1.4d
Kobold	½d	⅛	0.7d
Medusa	4d	6	5.0d
Minotaur	6d	3	3.5d
Mummy	5d+1	3	3.5d
Ogre	4d+1	2	2.8d
Orc	1d	½	1.4d
Owlbear	5d	3	3.5d
Skeleton	½d or 1d	¼	1.0d
Spectre	6d	1	2.0d
Treant	8d	9	6.0d
Troll	6d+3	5	4.5d
Vampire	7d to 9d	13-15	7d to 8d
Wight	3d	3	3.5d
Wraith	4d	5	4.5d
Zombie	1d or 2d	¼	1.0d

Something apparent right away is that monsters often have different roles in 5e compared to earlier editions. Medusae have a higher challenge rating (and higher hit points) than minotaurs do! Berserkers are also treated as minibosses for a level 2 party, as opposed to just slightly stronger human beings with +1 hit point (and +1 to attack). The undead are just kinda all over the place from not being hierarchical.

The hierarchy has also been shaken up a bit with regards to what I call "fellkin", who maintain their pecking order but are scaled differently. 1 goblin is considered to be about as strong as 1 level-1 character, whereas an orc is considered twice as strong. Using our formula, this translates to goblins having 1 hit dice, orcs having ~1.4 hit dice, and kobolds having ~0.7 hit dice. This means is that these monsters are about 1.4 (√2) times stronger than their classic D&D counterparts! We can change our formula, moving the 2 under the radical, for these numbers to appear more in line; however, this results in the stronger monsters having less hit dice.

Monster	Classic HD	5e CR	Converted HD
Berserker	1d+1	2	2.0d
Dryad	2d	1	1.4d
Ghoul	2d	1	1.4d
Gnoll	2d	½	1.0d
Goblin	1d–1	¼	0.7d
Hobgoblin	1d+1	½	1.0d
Kobold	½d	⅛	0.5d
Medusa	4d	6	3.5d
Minotaur	6d	3	2.5d
Mummy	5d+1	3	2.5d
Ogre	4d+1	2	2.0d
Orc	1d	½	1.0d
Owlbear	5d	3	2.5d
Skeleton	½d	¼	0.7d
Spectre	6d	1	1.4d
Treant	8d	9	4.2d
Troll	6d+3	5	3.2d
Vampire	7d to 9d	13-15	5d to 6d
Wight	3d	3	2.5d
Wraith	4d	5	3.2d

So, this is obviously very not generous to the high-CR monsters, but we know that the multiplier should be lower for lower CR monsters and higher for higher CR monsters. In other words, this shouldn’t be treated as a constant but as basically another function. This is what I came up with, by calculating a best-fit line and fiddling with the values:

HD = (1.5 + 0.05 × CR) × √CR

So our table becomes:

Monster	Classic HD	5e CR	Converted HD
Berserker	1d+1	2	2.0d
Dryad	2d	1	1.5d
Ghoul	2d	1	1.5d
Gnoll	2d	½	1.0d
Goblin	1d–1	¼	0.8d
Hobgoblin	1d+1	½	1.0d
Kobold	½d	⅛	0.5d
Medusa	4d	6	4.4d
Minotaur	6d	3	2.8d
Mummy	5d+1	3	2.8d
Ogre	4d+1	2	2.3d
Orc	1d	½	1.0d
Owlbear	5d	3	2.8d
Skeleton	½d	¼	0.8d
Spectre	6d	1	1.5d
Treant	8d	9	5.8d
Troll	6d+3	5	3.9d
Vampire	7d to 9d	13-15	7.8d to 8.7d
Wight	3d	3	2.8d
Wraith	4d	5	3.9d

Notably, an ancient red dragon converts from CR 24 to HD 13 (aligning with B/X), an adult red dragon converts from CR 17 to HD 10, and a tarrasque converts from CR 30 to HD 16. This is not perfect with regards to individual monsters just because of the different roles they play between B/X and 5e, but a good way to think of it is that it converts a monster’s challenge rating (not the monster itself) to B/X terms. It's still better!

That was my best shot. It's not often that anyone translates anything from 5e to classic D&D, but maybe this all serves as a helpful heuristic for anyone who would subject themselves to that mess. Below is the original table I made for Nova using my original equation along with values from classic monsters wherever they were available, which makes the whole thing seem maybe more consistent than it really is.

I highlighted what I felt were obvious outliers. I also have two columns for the values I calculated versus the ones I suggested in the end (though not without caveats, like "I think swarms should be more a pain to hit than they are").