Note

The Challenge Ratings are based on the idea that you'll be with a party.

If you're going solo, it's suggested you add 2-3 levels to the CR (in many cases (Areas), you are better off adding 4-5 Levels...depending on your Build, and especially whether or not your Build utilizes Magic (divine/arcane) or UMD (considering how easily dispellable UMD is from Scrolls).

The stated CR of some areas is quite wildly different from the real difficulty.

Distribution

Levels Area Count
00-05 20
05-10 18
10-15 16
15-20 15
20-25 15
25-30 6

Surface Level 0-5

CR Area
0.5 Soubar Well
1 Chionthar East, from Outer City -> Wyrms Crossing -turn left before crossing the bridge.
1 Mausoleum (from Graveyard)
1.5 BG Outer City, Graveyard
2 BG North, Bandit Cave
2 Hilltop Ruins Level 1
2,5 Mausoleum Floor 2
2.5 Hilltop Ruins Level 2
2.5 BG, East Gate Sewers
2.5 Trade Way, Road south of BG
3 Boareskyr Bridge, Bear Cave
3 Gibberling Cave
3 Chionthar East, Ruins
4 BG, Palace District Sewers
4 Cloakwood East
4 Boareskyr Bridge
4,5 Underground Ruin (Mausoleum Floor 3, PARTY RECOMMENDED. Lots of offensive spellcasters therein.)
5 Northern Tradeway, Road south of Soubar

Surface Level 6-10

CR Area
6 Lion's Way, Road south of FAI
6 Northern Tradeway, Ant Burrow
6 North of Beregost, Road
6 Cloakwood East, Mine
6 Ulgoth's Island
7 North of Beregost, Goblin Mine
7 Thundar's Ride (CR 8 at night)
7 Boareskyr Bridge, Bridgefort
7 Sea Cave Trolls, Next to Candlekeep
8 North of Beregost, Overgrown Abandoned Chapel
8 BG, Sewer Bowels
8 Thundar's Ride, Water Cave
8 High Hedge
8 Woods of Sharp Teeth and Orc Cave
8 Lion's Way, Gnoll Cave
8 Cloakwood Central
9 Dragonspear Exterior and Castle
10 Cloakwood Central, Minotaur Cave
10 BG, Undercity

Surface Level 11-15

CR Area
12 Cloakwood Central, Hag Cave
12 Cloakwood North-West
12 Chionthar West, Next to Ulgoth's Beard
13 Uldoon's Trail, North of Nashkel
13 Cloakwood North-West, Haunted House Level 1
13 Skuldask's Cave (probably higher)
13 High Moor Ruins (careful very dangerous)
14 Cloakwood North-West, Haunted House Level 2
14 High Moor South
14 Ulgoth's Island, Crab Cave
14 Ulgoth's Beard, Pirate Cave
14 Xvarts
14 Fields of the Dead
15 The Troll Claws, Elder Ogre Stronghold
15 Gullykin, Crypt
15 South of Beregost, Wyverns
15 Winding Waters

Surface Level 16-20

CR Area
16 Fields of the Dead, Crypt
16 Troll Claws, Ogre Cave
16 Corm Orp
16 Winding Waters Temple
16 Durlag's Exterior and Tower
16 Cloud Peaks (Nashkel) Foothills
16 South of Beregost Mines (formerly Nashkel's Iron Mine); CR increases to 18 one descends the mines. Boss is CR 21.
17 Nashkel's Iron Mine Exterior
18 Troll Claws
18 Ulcaster Ruins
19 Durlag's Labyrinth Levels 1-2
19 Cloud Peaks
20 Troll Claws, Troll Cave
20 Ulcaster Shadow Ruins
20 Yuan-ti Hills

Surface Level 21-25

CR Area
21 Searing Tunnels and Searing Caverns
21 Cloakwood North-West, Haunted House Level 3
22 Crystal Cave (entrance at exterior Naskhel Iron Mines map)
22 Durlag's Labyrinth Level 3
22 Durlag's Labyrinth Level 4
23 Trollclaws, Old Mine
23 High Moor, Spirit Troll Cave
23 Wyrm's Cave
23 Yuan-ti Temple
23 Reaching Woods
23 Lizard Cave
23–24 Elder Ogre Stronghold (Trollclaws)
24 Stone Giants (not currently accessible)
24 Cloudpeaks - Lich Cave
24 The Forsaken Catacombs - First Floor
24 Forest of Wyrms
24–25 Reaching Woods, Talis
25 The Forsaken Catacombs - Second Floor

Surface Level 26-30

CR Area
26 Ziggurat
26+ Cliffside Keep (Note: CR increases as you descend interior levels)
27 Frost Giants Castle
28 Speartop
29–30 The Forsaken Catacombs - Third Floor and "Fourth" floor called Depths of the Undying
29 Netherese Maze
30 Lyran's Hold
30 Graypeaks

Upper Dark Areas

CR Area
5 South of Grottoes
6 Gauth Grottoes
8 Abandoned Mines
10 Titanfist Mines
17(?) Blackrock

Underdark Level 0-5

CR Area
1 Sewers
1 Varalla's Passage
1 Wine Cellar & Herb Garden
1 S'samath Tombs
2 Mushroom Grove
3 South of Varalla's
4 West of Varalla's
5 Goblin Lake
5 Spore Grove

Underdark Level 6-10

CR Area
7 River Rift
9 Northwest of River
9 Upper Goblin Cave
9 Lower Goblin Cave
10 Southwest of River

Underdark Level 11-15

CR Area
12 Tarpits
13 Blackrock
13 Large Crypt
14 North of Tarpits
14 Illithid Mine Cavern
14 Illithid Mines (IG map states CR 15)
15 Ophidian Tunnels
15 Kro's Labyrinth

Underdark Level 16-20

CR Area
16 Illithid Hive (questionable rating, lots of illithids make it considerably harder)
16 South of Tarpits
16 Myconid Circles (says CR 17 on IG map)
18 Kro's Descent
20 East of Mistlake (questionable rating...more like CR 17)

Underdark Level 21-25

CR Area
21 Duergar Compound
22 Formian Ruins
22 The Wilds
24 The Wilds Descent
24 Lava Cave
25 Oghrann's Ruins
?? Illithid Mines (Deep) [Is this even a thing, or is it referring to the Illithid Hive?]

Underdark Level 26-30

CR Area
26 North of Mist Lake
28 Oghrann Great Halls
30 Netherese Ruins

And Avernus...

Exists too. Not.