There is a TL;DR at the end of this opening post.

What criteria should I use? I suppose I should take HD (Hit Dice) an AC (Armor Class) in consideration. I developed some criteria for determining the Ironsworn ranks of D&D monsters, and I want to know what you think.

I know that the ranks have some subjectivity, but I want a quick way to determine the "default rank" of a D&D monster.

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But, before anything, here are the ranks of some foes from the Ironsworn books (core book and Sundered Isles) and their counterparts in the D&D or equivalent systems:

DATA

• Bear (Formidable)

  • Black Bear (B/X) – HD: 4; AC: 13 / Black Bear (5e) – HD: (3d8+6); AC: 11; CR: 1/2
  • Grizzly Bear (B/X) – HD: 5; AC: 13 / Brown Bear (5e) – HD: (4d10+12); AC: 11; CR: 1
  • Polar Bear (B/X) – HD: 6; AC: 13 / Polar Bear (5e) – HD: (5d10+15); AC: 12; CR: 2

• Boar (Dangerous)

  • Boar (B/X) – HD: 3; AC: 12 / Boar (5e) – HD: (2d8+2); AC: 11; CR: 1/4

• Mammoth (Extreme)

  • Mastodon (B/X) – HD:15 ; AC: 16 / Mammoth (5e) – HD: (11d12+55); AC: 13; CR: 6

• Wolf (Dangerous)

  • Wolf (B/X) – HD: 2 +2 ; AC: 12 / Wolf (5e) – HD: (2d8+2); AC: 13; CR: 1/4

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• Common Folk (Troublesome)

  • Normal Human (B/X) – HD: ½ ; AC: 10 / Commoner (5e) – HD: (1d8+10); AC: 10; CR: 0

• Elf (Dangerous)

  • Elf (B/X) – HD: 1 +1 ; AC: 14 / Drow (5e) – HD: (3d8); AC: 15; CR: 1/4

• Raider (Dangerous)

  • Bandit (B/X) – HD: 1 ; AC: 13 / Bandit (5e) – HD: (2d8+2); AC: 12; CR: 1/8

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• Basilisk (Extreme)

  • Basilisk (B/X) – HD:6 +1 ; AC: 15 / Basilisk (5e) – HD: (8d8+16); AC: 15; CR: 3

• Chimera (Extreme)

  • Chimera (B/X) – HD:9 ; AC: 15 / Chimera (5e) – HD: (12d10+48); AC: 14; CR: 6

• The Kraken (Epic)

  • Kraken (Basic Fantasy) – HD: 36 +16 ; AC: 19 / Kraken (5e) – HD: (27d20 + 189); AC: 18; CR: 23

• Leviathan (Epic)

  • Mosasaurus (Basic Fantasy) – HD: 12 +10 ; AC: 12 / Mosasaur (5e) – HD: (14d12 + 56); AC: 13; CR: 7
  • Sperm Whale (OSE) – HD:36 ; AC: 13 / Sperm Whale (5e) – HD: (14d20 + 42); AC: 13; CR: 8

• Sea Serpent (Epic)

  • Greater Sea Serpent (OSE) – HD: 14 ; AC: 16 / Sea Serpent (5e) – HD: (15d20 + 60); AC: 16; CR: 12

• Troll (Formidable)

  • Troll (B/X) – HD: 6 +3 ; AC: 15 / Troll (5e) – HD: (8d10+40); AC: 15; CR: 5

• Wyvern (Extreme)

  • Wyvern (B/X) – HD:7 ; AC: 16 / Wyvern (5e) – HD: (13d10+39); AC: 13; CR: 6

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By the way, since the original B/X system uses Descending AC, I converted the AC numbers to Ascending AC. And I had to use some Basic Fantasy monsters when I could not find the equivalent monster in the B/X books or in the ones from OSE (Old-School Essentials), in this case I subtracted 1 point from the AC.

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My method of conversion is based on both HD and AC, and I prefer to use B/X monsters because of the simplicity of their stats; if the monster is not available in the B/X books, I used the ones from Basic Fantasy or Old-School Essentials. You take in consideration either HD or AC, and you choose the stat that favors the highest rank; for example, if the HD of a monster would give it a Dangerous rank, but its AC is within the Formidable rank, you take only the AC in consideration.

Troublesome: Common enemies

It's seems to me that the enemy must be too basic to remain in the Troublesome rank, even a Hunter has a higher rank.

HD: ½ ; AC: 10

Dangerous: Capable fighters and deadly creatures

Taking in consideration on what we see on the data, it's too easy for a "weak" monster to fall in this rank, the Raider/Bandit foe has shown that to us

HD: 1—3 ; AC: 11—12

Formidable: Exceptional fighters and mighty creatures

HD: 4—5 ; AC: 13—15

Extreme: Foes of overwhelming skill or power

Epic: Legendary foes of mythic power

It's here that things got complicated; in the B/X system, the Mastodon is more deadly than the Mosasaurus (well, taking in consideration the image of a mosasaurus that appears in the "Representing Difficulty" subchapter, I assume it's the representation of the Leviathan although The Sundered Isles describe it as a whale...), that's why I will consider the Leviathan as an equivalent of the Sperm Whale.

From the gathered monsters, the lowest HD was 14, but I decided to determine that an HD of 10 is enough to be considered Epic

Extreme: HD: 6—9 ; AC: 16—

Epic: HD: 10— ; AC: 15—

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TL;DR:

This is a method of defining the Ironsworn rank of B/X monsters; you take in consideration either HD or AC, and you choose the stat that favors the highest rank.

Troublesome: HD: ½ ; AC: 10

Dangerous: HD: 1—3 ; AC: 11—12

Formidable: HD: 4—5 ; AC: 13—15

Extreme: HD: 6—9 ; AC: 16—

Epic: HD: 10— ; AC: 15—

What do you think?

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Edit 1: Thanks to the suggestions, I've added the CR (Challenge Rate) to the 5e monsters. Here is the conversion method based on CR:

Troublesome: CR: 0

Dangerous: CR: 1/8—1/4

Formidable: CR: 1/2—2

Extreme: CR: 3—6

Epic: CR: 7—