Advancing Monsters

[alex1g]

I'm advancing the HD of a Treant and I use http://www.monsteradvancer.com/send/monster/initMonsterCustomization.ma site to advance. One of my players is saying that the advancer is wrong because the size is not being added to the CR using the monsteradvancer. So Monsteradvancer gives a Gargantuan Treant (Advanced Hit Dice) CR 12. The player is saying that the size modifier should make the CR 13 because of http://www.d20srd.org/srd/improvingMonsters.htm size CR modifier.
So Monster Advancer make my 21 HD Treant CR 12. Player is saying that it should be CR 13. Which one is it?

[InvisibleBison]

Your player is correct that increasing a creature's size (to large or larger) also increases its CR; however, the website probably does take that into account. The issue is that the advanced treant's CR has increased by 4.5 - 3.5 from gaining 14 plant HD, and 1 from increasing size to huge. The question is whether you should round up or down, and it seems that your player and whoever made that website disagree. I don't know if there's any explicit rules on the issue, but if I were you I'd err on the side of caution and round up.

[Crake]

Your player is correct that increasing a creature's size (to large or larger) also increases its CR; however, the website probably does take that into account. The issue is that the advanced treant's CR has increased by 4.5 - 3.5 from gaining 14 plant HD, and 1 from increasing size to huge. The question is whether you should round up or down, and it seems that your player and whoever made that website disagree. I don't know if there's any explicit rules on the issue, but if I were you I'd err on the side of caution and round up.

In dnd, the rule is always round down unless specified otherwise, regardless of there resulting decimal. So even if you had given the treant 15HD, and it's CR increase was to be 3.75, you still round that down to 3 (plus 1 for the size increase), since the rules on CR advancement don't specify otherwise. In that sense, for many creatures (Aberration, construct, elemental, fey, giant, humanoid, ooze, plant, undead, vermin) you can advance them by 3 HD without increasing their CR at all, and all the other types can get up to 2 HD without a CR increase except outsider and dragon, who can only get 1HD for "free".

This is actually a good method for ever so slightly increasing the challenge of encounters against higher optimization parties without increasing the CR, and thus not needing to award extra XP, because extra xp just results in a snowball effect.