DMG Table 55

Table 55: Dungeon Level
XP 	CreatureLevel
1-20 	1
21-50 	2
51-150 	3
151-250 	4
251-500 	5
501-1,000 	6
1,001-3,000 	7
3,001-5,500 	8
5,501-10,000	9
10,001 + 	10