Calculating CRs: Help with Vorpal Tribble's CR Calculator

[Doctor Despair]

Hello, friends! I know the CR is famously unhelpful, and I've heard Tribble's formula is a helpful tool for calculating encounter difficulty. I'm in the process of planning my first campaign, I was wondering if folks could help me evaluate some creatures as I go to make sure I am challenging my party appropriately (while not putting them at unnecessary risk of a TPK).

For those unfamiliar, this is Tribble's formula:

#1. Divide creature's average HP by 4.5 to 6.5.


4.5 for 5 HD or lower,
5 for 6-10 HD,
5.5 for 11-15 HD,
6 for 16-20 HD,
6.5 for 20-25 HD.



#2. Add 1 for each 5 points above 10 its AC is, subtracting 1 for every 5 below.

#3. Add 1 for each special attack (+2 to +5 or more if its got a decent number of spells in its spell-like abilities).

#4. Add 1 for each special quality unless you deem it worthy of more. Add 1 for each resistance and 10 points of DR it has, and 2 for each immunity. Subtract 1 for each vulnerability.

#5. Add 1 for every 2 bonus feats it has.

#6. Divide total by 3.

Consider an unarmed human skeleton, for example.

HP: 1d12, 6hp (/4.5 = 1.34)
AC: 13 (+0)
Special Attacks: None (+0)

Special Qualities: Damage reduction 5/bludgeoning, darkvision 60 ft., immunity to cold, undead traits [immunity to mind-affecting, poison, sleep, paralysis, stunning, disease, death, critical hits, nonlethal damage, ability drain, energy drain, physical ability damage, massive damage, fortitude saves that don't affect objects, healed by negative energy, damaged by positive energy, destroyed at 0hp, mindless, cannot heal naturally] (I'm unsure of how to value undead traits since there's just so much. Mindlessness is definitely a relevant negative trait at any level; I'm not sure how many of the immunities will be relevant at this level, but there are at least 14 of them. Should Undead Traits just be a generic +2 as a source of immunities, then -1 for mindlessness? And +1 for darkvision if the encounter is in the dark? In that case, this would be +2)

Bonus Feats: Improved Initiative (+0)
Total: +3.34
CR: 1.111
Listed CR for the same skeleton armed with a heavy steel shield: 0.34

Did I overvalue undead traits, or is a 1hd unarmed skeleton a tough fight for a level 1 party? Or does the formula just not do well with CRs less than 1?

[Biggus]

I always treat "undead traits" as a single special quality, you get more accurate results that way. As you say, there are both positives and negatives to being undead, especially if they're mindless.

And yes, I've found that the calculator doesn't work well for very low CRs, or very high ones for that matter.

[Doctor Despair]

I always treat "undead traits" as a single special quality, you get more accurate results that way. As you say, there are both positives and negatives to being undead, especially if they're mindless.

And yes, I've found that the calculator doesn't work well for very low CRs, or very high ones for that matter.

Fair enough; perhaps the human skeletons aren't a very good example then. Let me try a few more at a few CRs, and let's see how they look.

HP: 2d8, 9hp (/4.5 = 2)
AC: 16 (+1)
Special Attacks: - (+0)
Special Qualities: Damage reduction 10/magic, darkvision 60 ft., scent (+2; scent and darkvision overlap a little)
Bonus Feats: Alertness, Track (+1)
Total: 6
CR: 2
Listed CR:3

https://static.wikia.nocookie.net/forgottenrealms/images/7/76/Grick.jpg/revision/latest?cb=20200311072227

HP: 5d8+5, 27hp (/4.5 = 6)
AC: 14 (+0)
Special Attacks: Poison, web (+2)
Special Qualities: Low-light vision (I'll give this a +0 since it won't often be relevant)
Bonus Feats: Great Fortitude, Multiattack (+1)
Total: 9
CR: 3
Listed CR: 3

https://media-waterdeep.cursecdn.com/avatars/thumbnails/0/339/1000/1000/636252776771842998.jpeg

HP: 3d8+3, 16hp (/4.5 = 3.56)
AC: 17 (+1)
Special Attacks: Improved grab, constrict 1d3+3 (+2)
Special Qualities: Darkvision 60 ft., quickness (+2)
Bonus Feats: Improved Initiative, Lightning Reflexes, Stealthy (+1)
Total: 9.56
CR: 3.19
Listed CR: 2

https://static.wikia.nocookie.net/eberron/images/4/4f/MM35_PG35a.jpg/revision/latest/top-crop/width/360/height/450?cb=20120113210910

HP: 6d8+9, 36hp (/5 = 7.2)
AC: 17 (+1)
Special Attacks: Constrict 1d6, disease, improved grab (+2, as the disease shouldn't affect the encounter)
Special Qualities: Darkvision 60 ft., scent (I'll give this a +1, as they both are essentially filling the same "detection" role)
Bonus Feats: Alertness, Toughness, Weapon Focus (tentacle) (+1)
Total: 12.2
CR: 4.1
Listed CR: 4

https://static.wikia.nocookie.net/forgottenrealms/images/1/1b/Otyugh_4e.jpg/revision/latest?cb=20200310215408

HP: 6d8+18, 45hp (/5 = 9)
AC: 19 (+1)
Special Attacks: Moan, engulf (+2)
Special Qualities: Darkvision 60 ft., shadow shift (+2)
Bonus Feats: Alertness, Combat Reflexes, Improved Initiative (+1)
Total: 15
CR: 5
Listed CR: 5

https://static.wikia.nocookie.net/forgottenrealms/images/9/9b/Cloaker-5e.jpg/revision/latest?cb=20161119181500

HP: 4d8+24, 42hp (/4.5 = 9.34)
AC: 19 (+1)
Special Attacks: Gibbering, spittle, improved grab, blood drain, swallow whole, ground manipulation (+6)
Special Qualities: Amorphous, damage reduction 5/bludgeoning, darkvision 60 ft. (+3)
Bonus Feats: Lightning Reflexes, Weapon Finesse (+1)
Total: 20.34
CR: 6.78
Listed CR: 5

https://static.wikia.nocookie.net/forgottenrealms/images/3/3d/Gibbering_mouther.jpg/revision/latest?cb=20151208233231

Does it seem like I'm using the formula appropriately?

[Biggus]

Does it seem like I'm using the formula appropriately?

Yes, except for one thing: you only count bonus feats (those marked with a 'B', such as the Choker's Improved Initiative), not the regular feats they get.

Some thoughts about estimating CRs:

The grick is a good example of a creature that the formula misrepresents. It has five attacks per round, and while each one is pretty low damage, that's a heck of a lot for a CR 2-3 creature. Also, it has damage reduction 10/magic, which will negate all damage except critical hits from many level 1-2 characters. According to the WBL guidelines, you can't afford a +1 weapon until level 3 at the earliest. Putting all that together, the Grick is definitely not CR 2.

Which illustrates why personally I'm a bit dubious about the formula as written; there are several important features a creature may have which it doesn't take into account at all. I find that before making the final calculation, you need to look at the creature and ask "does it have any other major advantages or disadvantages?" and add or subtract points for those.

Another useful (but also incomplete) tool which can be used in conjunction with the calculator is the average monster stats table found here (https://forums.giantitp.com/showthread.php?623578-3-5-Average-Monster-Stats-UPDATED-TABLE&highlight=average+monster+stats). Both the table and the calculator can be helpful, but ultimately you still have to eyeball it, especially with regard to questions like "does the creature have any attacks or defences which require a particular spell or item to overcome that may not be available yet?" and "is it easily shut down by any common tactics that most parties of its level will have access to?".

[Saintheart]

Nitpicking, but I think your count of scent and darkvision as only a +1 together for the grick is wrong. Scent defeats invisibility at 5 feet, that's fairly significant if the party mage blunders into combat (or the beatstick is closing to range with some form of concealment, which might be knocked out by the fact scent allows you to 'pinpoint' the creature's location) - and darkvision has no overlap with that.

[Elves]

As the discussion is showing, seems like the kind of formula that is vague enough in application that you can always blame a misaligned result on application instead of the formula itself.

Monsters vary so much that the only simple+consistent way to calculate CR is to eyeball it. Start with its HD and then ask whether its abilities make it significantly weak or strong for its HD.

[Saintheart]

As the discussion is showing, seems like the kind of formula that is vague enough in application that you can always blame a misaligned result on application instead of the formula itself.

Monsters vary so much that the only simple+consistent way to calculate CR is to eyeball it. Start with its HD and then ask whether its abilities make it significantly weak or strong for its HD.

Pretty much yeah, CR really isn't much of a tool to assess how tough a monster is beyond a very, very rough guess, whether you're using Tribble's calculator or the numbers WOTC came up with. It isn't even a question of whether its abilities make it significantly weak or strong for its HD, weakness or strength is also dependent on the party's abilities. Take the discussion about the Vrock above and DR 10/magic. For a level 2 party that's no easy task to get over, not when they're likely not all sporting +1 weapons at that point. I think in that respect the Tribble's calculator is a bit off simply because significant DR is far harder on a party at low levels than it is around, say, the 6-10 sweet spot. But in Tribble's defence, if not that of WOTC, it really isn't possible to do much better with the system as written.

[Doctor Despair]

I took a second look at those creatures, edited parts of the formula, added notes and considerations, and added a few new creatures. Are these fair, do you think, friends?

HP: 2d8+2, 11hp (/4.5 = 2.45)
AC: 13 (+0)
Special Attacks: Rake 1d6+2 (+1)
Special Qualities: Darkvision 60 ft., amphibious (+2)
Bonus Feats: Alertness (+0)
Total: 5.45
CR: 1.81
Listed CR: 2

Notes: The maximum HP on a CR1 encounter is 31 (avg 12.24) and maximum AC is 21 (avg 15.54), so this seems weak by those standards. The issue is the Skum's attacks: the skum has a +5 bite that does 2d6+4 (6-16, avg 11), and 2 +0 claws that deal 1d4+2 each (6-12, avg 9). If all three attacks hit, that has the potential to kill the party wizard even at minimum damage, or even to one-shot the party barbarian at max. On top of that, they get two more rake attacks while swimming (+0, 1d6+2, 6-16 avg 11...) It's a tough level 1 fight if played intelligently, and they have 10 intelligence, so it's not out of the question. I'd imagine Wizards specifically left their organization without a solitary option to compensate for this damage. Is their CR2 appropriate in the event you made it solitary? Maybe, if encountered out of the water; it'd still be a tough fight. In the water, it's probably closer to a CR3 or 4.

https://i1.wp.com/www.wizards.com/dnd/images/MM35_gallery/MM35_PG228.jpg

HP: 3d8+3, 16hp (/4.5 = 3.56)
AC: 17 (+1)
Special Attacks: Improved grab, constrict 1d3+3 (+2)
Special Qualities: Darkvision 60 ft., quickness (+2)
Bonus Feats: Improved Initiative(B), Lightning Reflexes, Stealthy (+0)
Total: 8.56
CR: 2.86
Listed CR: 2

Notes: The Choker is interesting; it recommends ambush gameplay on a squishy party member, it gets an extra action per round, and it has reach. The damage is medium on two successful attacks and one successful grapple (12-18), as is its hp for its CR; if it passes its grapples, it gets essentially three low-attack-bonus attacks per round, but it's a sitting duck for opponents. More notably (and probably fitting for the creature statistics), the creature would make an AOO on the creature in the back of the party, then use its second standard to move to pin to silence the party member. Either way, provided the creature doesn't successfully pull the party member away unnoticed, I think a CR2 party could probably beat this thing, although one or two party members may go down -- a CR3 party certainly could.

https://static.wikia.nocookie.net/eberron/images/4/4f/MM35_PG35a.jpg/revision/latest/top-crop/width/360/height/450?cb=20120113210910

HP: 2d8, 9hp (/4.5 = 2)
AC: 16 (+1)
Special Attacks: - (+0)
Special Qualities: Damage reduction 10/magic, darkvision 60 ft., scent (+3; scent and darkvision overlap a little)
Bonus Feats: Alertness, Track(B) (+0)
Total: 6
CR: 2
Listed CR:3

Notes: As pointed out in the comments, while the stats might see the Grick as a CR2, the DR10/magic relegates it to CR3 except against parties with native access to Magic Weapon or Magic Fang. Additionally, it has five attacks per round, albeit relatively low-damage ones, but a few successive hits on a 2HD creature could start to present issues. CR3 seems fair. With a magic weapon, it probably goes down relatively easy to that CR3 party.

https://static.wikia.nocookie.net/forgottenrealms/images/7/76/Grick.jpg/revision/latest?cb=20200311072227

HP: 5d8+5, 27hp (/4.5 = 6)
AC: 14 (+0)
Special Attacks: Poison, web (+2)
Special Qualities: Low-light vision (I'll give this a +0 since it won't often be relevant)
Bonus Feats: Great Fortitude, Multiattack (+0)
Total: 8
CR: 2.67
Listed CR: 3

Notes: This CR came out close to accurate, but the poison has me concerned. Web makes fleeing difficult, and that's less of an issue than the 3d6 dex poison. Let's be fair though -- the encounter will probably be over before the secondary damage hits, leaving time to wait out the ability damage. In this case, a full round of hits ranges from 7-20 damage with the 1d6 dex damage rider. That is less damage than the Skum, which we tentatively put at 2, when out of the water, but the poison could disable a character before the damage does. CR3 is probably about right, I think.

https://media-waterdeep.cursecdn.com/avatars/thumbnails/0/339/1000/1000/636252776771842998.jpeg

HP: 6d8+9, 36hp (/5 = 7.2)
AC: 17 (+1)
Special Attacks: Constrict 1d6, disease, improved grab (+2, as the disease shouldn't affect the encounter)
Special Qualities: Darkvision 60 ft., scent (2)
Bonus Feats: Alertness, Toughness, Weapon Focus (tentacle) (+0)
Total: 12.2
CR: 4.1
Listed CR: 4

Notes: The Otyugh comes out relatively even with its listed CR, and at this level it does seem whelming. It is pretty equivalent to the Choker, but with slightly higher damage numbers. The disease is what sets it apart, but as said in the statblock, that won't be relevant for three days after the Otyugh is dead. CR4 is probably fine.

https://static.wikia.nocookie.net/forgottenrealms/images/1/1b/Otyugh_4e.jpg/revision/latest?cb=20200310215408

HP: 6d8+18, 45hp (/5 = 9)
AC: 19 (+1)
Special Attacks: Moan, engulf (+2)
Special Qualities: Darkvision 60 ft., shadow shift (+2)
Bonus Feats: Alertness, Combat Reflexes, Improved Initiative (+0)
Total: 14
CR: 4.67
Listed CR: 5

Notes: This thing is scary. It is another ambush predator (as so many aberrations seem to be), and it has a very synergistic attack routine. Its moan has a lot of DC15 abilities that target different saves, and while according to this chart (https://www.enworld.org/threads/list-of-standard-dcs-by-level-for-3-5e.328665/#post-6005459) 15 is relatively average for this CR, it's still a lot of save-or-suck effects that disable for 2-5 rounds. Thankfully it takes a standard to use a moan, so if it's moaning, it's not attacking. In shadowy areas, it can get a miss chance as a standard. Its actual damage isn't too bad at 9-17, but it does get a grapple mode that lets it disable one person while attacking another. Fortunately, its hp is low for CR4 or CR5; it seems appropriately CR'd.

https://static.wikia.nocookie.net/forgottenrealms/images/9/9b/Cloaker-5e.jpg/revision/latest?cb=20161119181500

HP: 7d8+21, 52hp (/5 = 10.4)
AC: 15 (+1)
Special Attacks: Adhesive, crush (+2)
Special Qualities: Darkvision 60 ft., immunity to acid, mimic shape (+4)
Feats: Alertness, Lightning Reflexes, Weapon Focus (slam) (+0)
Total: 17.4
CR: 5.8
Listed CR: 4

Notes: A famous, gimmicky creature, the formula claimed it was a little under-CR'd. It damage isn't great, but it is an ambush creature (like most low-level aberrations, I'm seeing), it has a damaging grapple mode (as many aberrations do), and it has a defensive ability that can reduce party damage output. It has close to average HP for CR4, or a little lower than average for CR5. Its damage isn't phenomenal either... CR5 would probably be fine. As the CR system dictates, a CR6 party would probably stomp a mimic.

https://i.pinimg.com/originals/ee/b1/f4/eeb1f41f241a694b9e2b7a7098891cc9.jpg

HP: 4d8+24, 42hp (/4.5 = 9.34)
AC: 19 (+1)
Special Attacks: Gibbering, spittle, improved grab, blood drain, swallow whole, ground manipulation (+6)
Special Qualities: Amorphous, damage reduction 5/bludgeoning, darkvision 60 ft. (+3)
Bonus Feats: Lightning Reflexes, Weapon Finesse (+0)
Total: 20.34
CR: 6.78
Listed CR: 5

Notes: This is potentially a very challenging fight. The free-action gibbering could hypothetically disable the whole party, or cause them to fight each other, while it uses its free-action spittle to lob acidic spit into the party's eyes. That is strong enough, but it can use Ground Manipulation to prevent the party from closing in, and if one does, it can use Improved Grapple/Swallow Whole to disable that one opponent... It's a very synergistic creature. I'd err high here and say CR7 so that the party will probably pass the will save against the Gibbering at least.

https://static.wikia.nocookie.net/forgottenrealms/images/3/3d/Gibbering_mouther.jpg/revision/latest?cb=20151208233231

[RNightstalker]

to make sure I am challenging my party appropriately (while not putting them at unnecessary risk of a TPK).


There are a few ways to handle this in game. There are plenty of movies/tv shows where the hero is screwed but the bad guy lets them live, only for them to come back later to save the day: Bruce Wayne in The Dark Knight Rises, Gandalf in The Fellowship of The Ring, to name a couple. It wouldn't be too hard to incorporate that into the story at a moment's notice, especially if the bad guys start landing a bunch of crits. There is also something to be said for running to fight another day.

[Biggus]

Nitpicking, but I think your count of scent and darkvision as only a +1 together for the grick is wrong. Scent defeats invisibility at 5 feet, that's fairly significant if the party mage blunders into combat (or the beatstick is closing to range with some form of concealment, which might be knocked out by the fact scent allows you to 'pinpoint' the creature's location) - and darkvision has no overlap with that.

Agreed. I'd give at least half a point for a special sense which partly overlaps another but still adds some unique functionality.


I took a second look at those creatures, edited parts of the formula, added notes and considerations, and added a few new creatures. Are these fair, do you think, friends?

HP: 2d8+2, 11hp (/4.5 = 2.45)
AC: 13 (+0)
Special Attacks: Rake 1d6+2 (+1)
Special Qualities: Darkvision 60 ft., amphibious (+2)
Bonus Feats: Alertness (+0)
Total: 5.45
CR: 1.81
Listed CR: 2

Notes: The maximum HP on a CR1 encounter is 31 (avg 12.24) and maximum AC is 21 (avg 15.54), so this seems weak by those standards. The issue is the Skum's attacks: the skum has a +5 bite that does 2d6+4 (6-16, avg 11), and 2 +0 claws that deal 1d4+2 each (6-12, avg 9). If all three attacks hit, that has the potential to kill the party wizard even at minimum damage, or even to one-shot the party barbarian at max. On top of that, they get two more rake attacks while swimming (+0, 1d6+2, 6-16 avg 11...) It's a tough level 1 fight if played intelligently, and they have 10 intelligence, so it's not out of the question. I'd imagine Wizards specifically left their organization without a solitary option to compensate for this damage. Is their CR2 appropriate in the event you made it solitary? Maybe, if encountered out of the water; it'd still be a tough fight. In the water, it's probably closer to a CR3 or 4.

https://i1.wp.com/www.wizards.com/dnd/images/MM35_gallery/MM35_PG228.jpg

HP: 3d8+3, 16hp (/4.5 = 3.56)
AC: 17 (+1)
Special Attacks: Improved grab, constrict 1d3+3 (+2)
Special Qualities: Darkvision 60 ft., quickness (+2)
Bonus Feats: Improved Initiative(B), Lightning Reflexes, Stealthy (+0)
Total: 8.56
CR: 2.86
Listed CR: 2

Notes: The Choker is interesting; it recommends ambush gameplay on a squishy party member, it gets an extra action per round, and it has reach. The damage is medium on two successful attacks and one successful grapple (12-18), as is its hp for its CR; if it passes its grapples, it gets essentially three low-attack-bonus attacks per round, but it's a sitting duck for opponents. More notably (and probably fitting for the creature statistics), the creature would make an AOO on the creature in the back of the party, then use its second standard to move to pin to silence the party member. Either way, provided the creature doesn't successfully pull the party member away unnoticed, I think a CR2 party could probably beat this thing, although one or two party members may go down -- a CR3 party certainly could.

https://static.wikia.nocookie.net/eberron/images/4/4f/MM35_PG35a.jpg/revision/latest/top-crop/width/360/height/450?cb=20120113210910

HP: 2d8, 9hp (/4.5 = 2)
AC: 16 (+1)
Special Attacks: - (+0)
Special Qualities: Damage reduction 10/magic, darkvision 60 ft., scent (+3; scent and darkvision overlap a little)
Bonus Feats: Alertness, Track(B) (+0)
Total: 6
CR: 2
Listed CR:3

Notes: As pointed out in the comments, while the stats might see the Grick as a CR2, the DR10/magic relegates it to CR3 except against parties with native access to Magic Weapon or Magic Fang. Additionally, it has five attacks per round, albeit relatively low-damage ones, but a few successive hits on a 2HD creature could start to present issues. CR3 seems fair. With a magic weapon, it probably goes down relatively easy to that CR3 party.

https://static.wikia.nocookie.net/forgottenrealms/images/7/76/Grick.jpg/revision/latest?cb=20200311072227

HP: 5d8+5, 27hp (/4.5 = 6)
AC: 14 (+0)
Special Attacks: Poison, web (+2)
Special Qualities: Low-light vision (I'll give this a +0 since it won't often be relevant)
Bonus Feats: Great Fortitude, Multiattack (+0)
Total: 8
CR: 2.67
Listed CR: 3

Notes: This CR came out close to accurate, but the poison has me concerned. Web makes fleeing difficult, and that's less of an issue than the 3d6 dex poison. Let's be fair though -- the encounter will probably be over before the secondary damage hits, leaving time to wait out the ability damage. In this case, a full round of hits ranges from 7-20 damage with the 1d6 dex damage rider. That is less damage than the Skum, which we tentatively put at 2, when out of the water, but the poison could disable a character before the damage does. CR3 is probably about right, I think.

https://media-waterdeep.cursecdn.com/avatars/thumbnails/0/339/1000/1000/636252776771842998.jpeg

HP: 6d8+9, 36hp (/5 = 7.2)
AC: 17 (+1)
Special Attacks: Constrict 1d6, disease, improved grab (+2, as the disease shouldn't affect the encounter)
Special Qualities: Darkvision 60 ft., scent (2)
Bonus Feats: Alertness, Toughness, Weapon Focus (tentacle) (+0)
Total: 12.2
CR: 4.1
Listed CR: 4

Notes: The Otyugh comes out relatively even with its listed CR, and at this level it does seem whelming. It is pretty equivalent to the Choker, but with slightly higher damage numbers. The disease is what sets it apart, but as said in the statblock, that won't be relevant for three days after the Otyugh is dead. CR4 is probably fine.

https://static.wikia.nocookie.net/forgottenrealms/images/1/1b/Otyugh_4e.jpg/revision/latest?cb=20200310215408

HP: 6d8+18, 45hp (/5 = 9)
AC: 19 (+1)
Special Attacks: Moan, engulf (+2)
Special Qualities: Darkvision 60 ft., shadow shift (+2)
Bonus Feats: Alertness, Combat Reflexes, Improved Initiative (+0)
Total: 14
CR: 4.67
Listed CR: 5

Notes: This thing is scary. It is another ambush predator (as so many aberrations seem to be), and it has a very synergistic attack routine. Its moan has a lot of DC15 abilities that target different saves, and while according to this chart (https://www.enworld.org/threads/list-of-standard-dcs-by-level-for-3-5e.328665/#post-6005459) 15 is relatively average for this CR, it's still a lot of save-or-suck effects that disable for 2-5 rounds. Thankfully it takes a standard to use a moan, so if it's moaning, it's not attacking. In shadowy areas, it can get a miss chance as a standard. Its actual damage isn't too bad at 9-17, but it does get a grapple mode that lets it disable one person while attacking another. Fortunately, its hp is low for CR4 or CR5; it seems appropriately CR'd.

https://static.wikia.nocookie.net/forgottenrealms/images/9/9b/Cloaker-5e.jpg/revision/latest?cb=20161119181500

HP: 7d8+21, 52hp (/5 = 10.4)
AC: 15 (+1)
Special Attacks: Adhesive, crush (+2)
Special Qualities: Darkvision 60 ft., immunity to acid, mimic shape (+4)
Feats: Alertness, Lightning Reflexes, Weapon Focus (slam) (+0)
Total: 17.4
CR: 5.8
Listed CR: 4

Notes: A famous, gimmicky creature, the formula claimed it was a little under-CR'd. It damage isn't great, but it is an ambush creature (like most low-level aberrations, I'm seeing), it has a damaging grapple mode (as many aberrations do), and it has a defensive ability that can reduce party damage output. It has close to average HP for CR4, or a little lower than average for CR5. Its damage isn't phenomenal either... CR5 would probably be fine. As the CR system dictates, a CR6 party would probably stomp a mimic.

https://i.pinimg.com/originals/ee/b1/f4/eeb1f41f241a694b9e2b7a7098891cc9.jpg

HP: 4d8+24, 42hp (/4.5 = 9.34)
AC: 19 (+1)
Special Attacks: Gibbering, spittle, improved grab, blood drain, swallow whole, ground manipulation (+6)
Special Qualities: Amorphous, damage reduction 5/bludgeoning, darkvision 60 ft. (+3)
Bonus Feats: Lightning Reflexes, Weapon Finesse (+0)
Total: 20.34
CR: 6.78
Listed CR: 5

Notes: This is potentially a very challenging fight. The free-action gibbering could hypothetically disable the whole party, or cause them to fight each other, while it uses its free-action spittle to lob acidic spit into the party's eyes. That is strong enough, but it can use Ground Manipulation to prevent the party from closing in, and if one does, it can use Improved Grapple/Swallow Whole to disable that one opponent... It's a very synergistic creature. I'd err high here and say CR7 so that the party will probably pass the will save against the Gibbering at least.

https://static.wikia.nocookie.net/forgottenrealms/images/3/3d/Gibbering_mouther.jpg/revision/latest?cb=20151208233231

Yeah, those look about right to me.