I've been looking to scale up the CR of a few Ancient dragons for one of my settings, so I've been figuring out the offensive and defensive CR of each dragon to determine how my scaled-up dragons should compare. The thing is, by my calculations (as per DMG p. 274-281), most of these dragons (virtually all that are young or older) end up with a higher CR than is listed in the Monster Manual.
For example, a Young Red Dragon, listed at CR 10. By my calculations, its effective AC and HP come out to 22 and 356, respectively, making its defensive CR 21. Its average damage over 3 rounds is about 68, with an attack bonus of +10 and a DC of 17, giving it an offensive CR of 10 or 11. Regardless, its CR comes out as around 16. Ignoring the fire immunity and either rounding down or ignoring the fly speed (or the saving throw bonuses) gives a CR of 10, as written, but I can't figure out how exactly they arrived at their result.
For those who are requesting more complete details on calculations:
- Base AC is 18, +2 from a fly speed and a CR of 10 or lower, +2 from 4 saving throw bonuses, resulting in 22 effective AC
- Base HP is 178, x2 from fire immunity and a CR from 5-10, resulting in 356 effective HP
- 22 effective AC and 356 effective HP gives a DCR of 21
- DMG p. 280 says to assume that a breath weapon hits two creatures and that both fail their saving throws, giving 112 damage on turns it uses its breath weapon.
- DPR for the dragon's multiattack is 46
- The breath weapon recharges on a 5 or a 6, so it is usable for 1 out of every 3 rounds on average.
- Per DMG p. 278, the three round average thus comes out to 68.
- DPR of 68, an attack bonus of +10, and a saving throw DC of 17 gives an OCR of 11 or 10, respectively, based on if the attack bonus or saving throw is considered more used.