D&D 5E (2014) DMG page 274.

[UngeheuerLich]

I will buy it no matter... I am just curious NOW^^.

I really believe, something either went wrong in the table (CR to hp ratio seems off) or something in the text about calculating defensive CR is either wrong or difficult to understand...

 

[Tormyr]

Yeah, regardless of whether it's average or maximum, the hp numbers in the table seem way higher than the vast majority of monsters of that CR are actually capable of in the MM. Even the Gargantuan, CR 30, Con 30 Tarrasque only has an average of 676 hp, equivalent to a Defensive CR of 27; seems like it would only get dragged up to Defensive CR 30 because of its AC (or more likely, its AC is only 25 because they wanted to justify a CR of 30).

It's almost as if the table is designed to focus on Offensive CR as the more important of the two. That the HP totals there aren't an accurate read of what a monster of that Defensive CR should have, but rather are listed as a tool to adjust the Offensive CR up or down by a limited degree.

(I'd happily share the paragraph with you, but I assume that wouldn't be appropriate, since it's such a big selling point for the book. Sorry. )

But the numbers are based off of effective HP, AC, DPR, and AB. The actual numbers are 676, 25, ~250, +19. As it stands
Defensive CR: 676 HP starts us at CR 27. The suggested AC is 19. The actual AC is 6 higher. 6/2 = 3 additional CR levels. So defensive CR is 30.
Offensive CR: DPR of ~250 starts us at CR 27. Suggested AB is +13. Actual is again 6 higher. Offensive CR ends at 30.

So Big T has a 30 even before traits. Then the traits kick in.

3 Legendary Resistance increase the effective HP by 90.
Magic Resistance: Increase effective AC by 2
Reflective Carapace: Not listed specifically, but it essentially gives T Immunity to magic missile, line spells and attack roll spells. Immunities at CR30 multiply effective HP by 1.25.
Seige Monster: Not listed specifically. This does not increase damage to PCs, so it probably does not affect CR.
Frightful Presence: Probably not going to be used against PCs of 10th level or less, this does not affect CR.

So the effective HP is (676 + 90) * 1.25 = 958 or 676 * 1.25 + 90 = 935. The effective AC is 27. Starting Defensive CR based on HP is off the chart. So let's call it 30. +4 for the increase in effective AC. So 34 might work for a Defensive CR. The Offensive CR did not change, but you could lower it to a 26 and still end up with a CR 30 monster.

 

[JEB]

But the numbers are based off of effective HP, AC, DPR, and AB. The actual numbers are 676, 25, ~250, +19. As it stands
Defensive CR: 676 HP starts us at CR 27. The suggested AC is 19. The actual AC is 6 higher. 6/2 = 3 additional CR levels. So defensive CR is 30.
Offensive CR: DPR of ~250 starts us at CR 27. Suggested AB is +13. Actual is again 6 higher. Offensive CR ends at 30.

So Big T has a 30 even before traits. Then the traits kick in.

3 Legendary Resistance increase the effective HP by 90.
Magic Resistance: Increase effective AC by 2
Reflective Carapace: Not listed specifically, but it essentially gives T Immunity to magic missile, line spells and attack roll spells. Immunities at CR30 multiply effective HP by 1.25.
Seige Monster: Not listed specifically. This does not increase damage to PCs, so it probably does not affect CR.
Frightful Presence: Probably not going to be used against PCs of 10th level or less, this does not affect CR.

So the effective HP is (676 + 90) * 1.25 = 958 or 676 * 1.25 + 90 = 935. The effective AC is 27. Starting Defensive CR based on HP is off the chart. So let's call it 30. +4 for the increase in effective AC. So 34 might work for a Defensive CR. The Offensive CR did not change, but you could lower it to a 26 and still end up with a CR 30 monster.

Wow, very nice breakdown of the Tarrasque! So that explains that particular case, at least.

Are you aware of any monsters in the MM with average HP that compares to the listed range for that CR on the table? (I'd check myself, but I'm busy prepping a game for Saturday.)

 

[aramis erak]

I will buy it no matter... I am just curious NOW^^.

I really believe, something either went wrong in the table (CR to hp ratio seems off) or something in the text about calculating defensive CR is either wrong or difficult to understand...

I don't think so. I will walk through the black bear.

Says CR 1/2

Step 4:
19 HP is CR 1/8
AC is 11, and that's two points lower, so down 1 line, for Def CR0

DPR is 12 (bite 5 claw 7) for Off CR1
Attack Bonus is +3, which is any line up to 2. So Off CR still 1
Save DC is immaterial; it causes no saves to be made.

Offense is 1, defense is 0, average is 1/2.

The HP also leads to HD. Size medium, so d8's, averaging 4.5 per HD, plus con bonus. 19/(4.5+2)=19/6.5=2.923, or round to 3, which matches the listed 3d8+6 12.5+6=19. Checks.

That's a match.

Brown Bear...
Says CR 1.

Step 4 calcs.
Def CR
34 HP, CR1/8, which says it should be AC13
AC 11 is 2 less than 13, so Def CR is down a step, to CR 0

Off CR.
DPR is 19, which is Off CR 2. CR 2 says AB +3
AB is +5, taking the Off CR to 3.

Total CR
Def CR 0 + Off CR 3 averages to CR1.5, which, per the rules may round up or down. They rounded down.

And, just for fun, Langderosa Cyanwrath, Blue Half-dragon.
Medium, CR4
HP:57 but due to having a resistance, we use double his HP for lookup 114 HP is Cr 3, and AC 13.
AC17 is 4 up from expected, so, Def CR goes up two, to Cr 5.

DPR: greatsword x2, every third round DC13 save breath weapon. (22+22+22)/3 is still 22... Off CR 3. But wait... Action Surge. He gets 44 in round 1. So... (44+22+22)/3=88/3, or 29 damage per round, pushing up to Off CR 4.

AB +6, save DC 13. Neither off by 2 points, so Off CR is still 4.

Total CR = (Def CR + Off Cr)/2 = (5+4)/2=9/2=4.5. Again, they rounded down; due to the resistance not being a commonly used one (It's lightning), that's probably the right call.

So, yeah, the math seems to work all right. Especially with the changes to encounter difficulty that the DMG makes from the DMBR. (it adjusts the multiplier for number of opponenents for # of PCs outside the range 3-5...

 

[Tormyr]

I don´t have the table, but the hp line, as you describe how it works is really off target if you compare it to the mm.

The clear trend there is:
about 7 hp: CR 1/8
about 11 hp: CR 1/4
about 15-22 hp: CR 1/2
about 30-45 hp: CR 1

I really like to read that paragraph, if something is missing here...

Just grabbing the first creatures I can find in Basic D&D.

CR1/8 Flying Snake. HP 5, AC 14, DPR 8, AB 6
Fly Speed + Flyby: effectively allows attacking at range because it can swoop in, hit and retreat without attacks of opportunity. This increases effective AC by 2
Defensive CR: Starts at 0. Effective AC is 16 which is 3 higher than suggested. So defensive CR goes up to 1/8 (I am assuming it goes up by 1 step rather than 1).
Offensive CR: Starts at 1/2. AB is 3 higher than recommended, so increase 1 level to CR of 1.
The average of 1/8 and 1 is 1/2. So there must be something else going on here, or the Flying snake was weaker than a CR 1/2 creature should be in play testing.

CR 1/4 Giant Frog. HP 18, AC 11, DPR ~7, AB 3. None of its traits change anything.
DCR: Starts at 1/8, goes to 0.
OCR: Starts at 1/2 and stays there
Avg CR is 1/4.

CR 1/2 Gnoll. HP 22, AC 15, DPR 6, AB 4
Rampage increases effective DPR by 2.
DCR: Starts at 1/8 and increases to 1/4.
OCR: Starts at 1/2 and stays there.
Average CR rounded to nearest level: 1/2.

CR 1 Animated Armor. HP 33, AC 18, DPR 10, AB 4.
Damage Immunities multiply effective HP by 2.
DCR: Starts at 1/2 and increases to 2.
OCR: Starts at 1 and stays there.
Average CR: Average rounded is 2, so play testing must have come up with a reason to keep it at 1.

I hope that gives at least a little insight.

 

[Tormyr]

Wow, very nice breakdown of the Tarrasque! So that explains that particular case, at least.

Are you aware of any monsters in the MM with average HP that compares to the listed range for that CR on the table? (I'd check myself, but I'm busy prepping a game for Saturday.)

Yeah, the Commoner.

The answer is: probably not. The creature would have to be incredibly vanilla for the hp to need to be all the way up where the table places it. Every creature either has good attack bonus, AC or damage, or special stuff to offset the lower hp.

 

[the Jester]

but due to having a resistance, we use double his HP for lookup...Again, they rounded down; due to the resistance not being a commonly used one (It's lightning), that's probably the right call.

You only apply the hp multiplier if the monster has multiple resistances, IIRC.

 

[Tormyr]

One thing that is really useful about this section and the table in particular is the ease in modifying existing creatures. In Age of Worms, there is an encounter where you stumble into a room with a bunch of skeletons in plate mail. The skeleton is CR 1/4 with an AC of 13. Changing the AC to 18 would increase the Defensive CR by 2 steps (regardless of where it was "supposed" to be, it is now 5 higher). Half of that change would carry over to the averaged result, so the new CR would be 1/2.

I did it the long way to make sure
Original: 13, 13, 5, 4 DCR 1/8 OCR 1/4 CR 1/4
Plate Mail: 13, 18, 5, 4 DCR 1/2 OCR 1/4 CR 1/2

Having this to eliminate some of the guesswork is going to be nice.

 

[UngeheuerLich]

Just grabbing the first creatures I can find in Basic D&D.

CR1/8 Flying Snake. HP 5, AC 14, DPR 8, AB 6
Fly Speed + Flyby: effectively allows attacking at range because it can swoop in, hit and retreat without attacks of opportunity. This increases effective AC by 2
Defensive CR: Starts at 0. Effective AC is 16 which is 3 higher than suggested. So defensive CR goes up to 1/8 (I am assuming it goes up by 1 step rather than 1).
Offensive CR: Starts at 1/2. AB is 3 higher than recommended, so increase 1 level to CR of 1.
The average of 1/8 and 1 is 1/2. So there must be something else going on here, or the Flying snake was weaker than a CR 1/2 creature should be in play testing.

CR 1/4 Giant Frog. HP 18, AC 11, DPR ~7, AB 3. None of its traits change anything.
DCR: Starts at 1/8, goes to 0.
OCR: Starts at 1/2 and stays there
Avg CR is 1/4.

CR 1/2 Gnoll. HP 22, AC 15, DPR 6, AB 4
Rampage increases effective DPR by 2.
DCR: Starts at 1/8 and increases to 1/4.
OCR: Starts at 1/2 and stays there.
Average CR rounded to nearest level: 1/2.

CR 1 Animated Armor. HP 33, AC 18, DPR 10, AB 4.
Damage Immunities multiply effective HP by 2.
DCR: Starts at 1/2 and increases to 2.
OCR: Starts at 1 and stays there.
Average CR: Average rounded is 2, so play testing must have come up with a reason to keep it at 1.

I hope that gives at least a little insight.

Thanks for the insights, but I still have a problem, that all low level critters seem to favour offensive CR over defensive according to the table. Imagine fighting against creatures at low levels that use average HP for CR 1/2 and average damage for CR 1/2. It seems very unintuitive, even though the monster´s overall CR is correct.

At defensive CR 1/8, 7-30 hp is a very very big gap. That is a range from 1 average hit needed to kill to 4 average hits at that low level. And in the MM we don´t see a single monster of that CR 1/4 or 1/2 that is balanced in offensive and defensive power.

I guess for higher level creatures everything works well enough. But the lower end seems rather a very rough estimate and it would lead to a game where fights last many rounds. Maybe it is the intention, for a 4e module, that you usually use more defensively orientated creatures...but for the game presented in the MM, it just seems off...

 

[Tormyr]

Thanks for the insights, but I still have a problem, that all low level critters seem to favour offensive CR over defensive according to the table. Imagine fighting against creatures at low levels that use average HP for CR 1/2 and average damage for CR 1/2. It seems very unintuitive, even though the monster´s overall CR is correct.

At defensive CR 1/8, 7-30 hp is a very very big gap. That is a range from 1 average hit needed to kill to 4 average hits at that low level. And in the MM we don´t see a single monster of that CR 1/4 or 1/2 that is balanced in offensive and defensive power.

I guess for higher level creatures everything works well enough. But the lower end seems rather a very rough estimate and it would lead to a game where fights last many rounds. Maybe it is the intention, for a 4e module, that you usually use more defensively orientated creatures...but for the game presented in the MM, it just seems off...

So why is it a problem?

One thing to remember is that the HP, AC, DPR, and AB are effective stats, not the actual stats for the creature. Even at higher levels, the creatures don't have that much HP. The Adult Red Dragon is 60 HP low. The Air Elemental is 50 HP low. The Yeti is 60 HP low.

Have you played much 5e yet? Combat using the low-level creatures in the MM generally goes very fast. It is usually just a few rounds. That probably is why creatures are skewed the way they are at the low levels. A CR 1/4 creature that had HP 40, AC 13, DPR 4, and AB 3 seems like it would not be very interesting. It would not have any CR altering traits. It would not do much damage. At level 1, the players would be hitting it regularly but not getting anywhere quickly.