The Area Level determines the item level of Gear that drops. The item level attached to gear determines what modifiers can roll on the item and the tiers of those modifiers. Plainly said, the higher your area level the more powerful items you will find.
The Area Level also determines the type of gear you will see. For example, early game items such as Wooden Swords become much more rare once you are far into the late game, but they can still drop.
The Overworld uses progression-based area levels. Boss Domains and Explorable Maps use the area level assigned by their map item.
The Overworld starts at Area Level 5. Each listed pre-hardmode progression boss adds 5 levels. Entering Hardmode sets the Overworld to at least Area Level 45. Hardmode progression bosses then add 5 levels each. Rows below are cumulative; if a listed boss has not been defeated, subtract 5 from later rows.
| Progression | Area Level |
|---|---|
| (DEFAULT) | 5 |
| King Slime defeated | 10 |
| Eye of Cthulhu defeated | 15 |
| Eater of Worlds defeated | 20 |
| Brain of Cthulhu defeated | 25 |
| Queen Bee defeated | 30 |
| Deerclops defeated | 35 |
| Skeletron defeated | 40 |
| Hardmode / Wall of Flesh defeated | 45 |
| Queen Slime defeated | 50 |
| The Twins defeated | 55 |
| The Destroyer defeated | 60 |
| Skeletron Prime defeated | 65 |
| Plantera defeated | 70 |
| Golem defeated | 75 |
| Lunatic Cultist defeated | 80 |
| Moon Lord defeated | 85 |
| Domain | Area Level |
|---|---|
| King Slime | 5 |
| Eye of Cthulhu | 15 |
| Eater of Worlds | 20 |
| Brain of Cthulhu | 25 |
| Queen Bee | 30 |
| Deerclops | 35 |
| Skeletron | 40 |
| Wall of Flesh | 45 |
| Queen Slime | 51 |
| The Twins | 53 |
| The Destroyer | 55 |
| Skeletron Prime | 57 |
| Plantera | 59 |
| Golem | 61 |
| Duke Fishron | 63 |
| Empress of Light | 65 |
| Lunatic Cultist | 67 |
| Moon Lord | 69 |
Explorable Map area level is calculated as 48 + (Tier * 2), clamped between 50 and 72.
| Tier | Area Level |
|---|---|
| 1 | 50 |
| 2 | 52 |
| 3 | 54 |
| 4 | 56 |
| 5 | 58 |
| 6 | 60 |
| 7 | 62 |
| 8 | 64 |
| 9 | 66 |
| 10 | 68 |
| 11 | 70 |
| 12+ | 72 |