Vol ranges are based on light novel.

Tier 10: 0

(Regular humans, animals, and other creatures with absolutely no magical power.)

Tier 9: 1-99

(Anyone from the weakest magic user (including but not limited to Issei when he was initially revived) to just below the average low-class devil. Can also apply to sorcerers who've just started their training.)

Tier 8: 100-199

(The varying power degrees of a normal low-class sorcerer, devil, angel, fallen angel, etc. Also applies to most magical creatures. Examples: Issei (Vol. I-II), Raynare (pre-revival), Kiba Yuuto (Vol. I-II), Koneko Toujou (Vol. I-III), Asia Argento (Vol. I-III), Genshirou Saji (Vol. I-III).)

Tier 7: 200-299

(The stage between low and middle class. Examples: Issei Hyoudou (Vol. III-V), Genshirou Saji (Vol. IV-VI), Koneko Toujou (Vol. IV-V).)

Tier 6: 300-399

Beings with middle-class magical power. Examples: Issei Hyoudou (Vol. VI), Kiba Yuuto (Vol. III-VI), Xenovia Quarta (Vol. III-VI), Irina Shidou (Vol. III-VII).)

Tier 5; 400-499

The stage between middle and high class. Examples: Akeno Himejima (Vol. I-V), Kiba Yuuto (Vol. VII-?), Xenovia Quarta (Vol. VII-?), Koneko Toujou (Vol. VI-VIII).

Tier 4: 500-999

High-class power. Examples: Rias Gremory (Vol. I-VIII), Sona Sitri (Vol. I-V) Akeno Himejima (Vol. VI-?), Issei Hyoudou (Vol. VII-?).)

Tier 3: 1,000-9,999

The stage between high and ultimate class. Examples:  Kuroka (Vol. II-?), Sona Sitri (Vol. VI-?), Sairaorg Bael (Vol. V-VII), Issei Hyoudou (Triaina forms).

Tier 2: 10,000-99,999

Ultimate-class. Examples: Sairaorg Bael (Vol. VIII-?), Issei Hyoudou (Cardinal Crimson Promotion).

Tier 1: 100,000-999,999

Satan/Seraph/Gov. General-like power. Examples: Serafall Leviathan, Azazel, Michael the Archangel, Gabriel, Metatron.

Tier α: 1,000,000-9,999,999

Gods, Super Devils, certain monstrous gods, various Lovecraftian creations that might make appearances in the story much later on, and the Heavenly Dragons. Examples: the Abrahamic God, the original Satans, Rizevim Livan Lucifer, Sirzechs Lucifer, Ajuka Beelzebub, the Leviathan God, Ddraig, Albion.

Tier Ω: 10,000,000-∞ minus 1

The amount of power reserved for those who are above the normal gods and can bring about the end of the timeline. Examples: Ophis, Great Red, Trihexa, Gaia, Tartarus, Ouranos.