View Full Version : PF: Calculating statblock AC line

2014-01-09, 01:15 AM
A few days ago, I built a monster to test my monster building skills and posted it, asking for critiques ...

Turns out I don't have much skill in monsterizing. :/

Below is an attempt, after going back to the books and really reading them, to lay out in a concise manner how to calculate the statblock AC (as opposed to the effective AC during combat, etc.). Hopefully, this will be useful to others, though there's nothing new in it.

Of course, I probably goobered this up too, but perhaps not as much as the full monster attempt. Please let me know if you find anything wrong, I'll update the post.

Calculating the AC line

The analysis below is for the statblock value and includes nothing pertaining to in-play rules.

The standard format for the AC line is:

AC n, touch n, flat-footed n (n_mod mod_name, ...)

where the list of all modifiers is in alphabetical order, all lowercase except for Dex.

According to the Core Rulebook page 179 (CR179) the value for AC itself is simply:

AC: 10 + armor bonus + shield bonus + Dex modifier + other modifiers

but, of course, the Devil/Daemon/Demon is in the details.

The arithmetic is easy, but determining exactly what the various bonuses are, especially the vague 'other modifiers', takes a bit of work. d20pfsrd has a list of bonuses (is this list in the rulebook in this form somewhere? I can't find it, the information appears all spread out). Only some of these affect AC:

armor, deflection, Dex modifier, dodge (self-stacks), enhancement*, insight, luck, natural, resistance, shield, size

None of these bonuses self-stack, i.e. stack with other bonuses of the same type, except Dodge.

* Enhancement bonuses apply to armor/shield/natural directly and so affect Touch AC

The touch component is (CR179): AC - armor bonus - shield bonus - natural armor bonus

Flat-footed is (CR178): AC - Dex - dodge

And that's it, which isn't so complicated after all.

Post script:

The Bestiary page 291 (B291) has a table (table 1-1) of average statistics for balanced monsters. Not being a fan of tables (heresy, 'tis, but true) a little fiddling suggests that the relation between AC and CR is pretty easy to calculate:

CR = (AC - 12) * 5 / 6 and then rounded to the nearest integer

or, what is possibly easier to calculate on the fly,

CR = (AC - 12) * 10 / (2 * 2 * 3) and then rounded to the nearest integer

This gives the same answer as the table for all but CR 1/2, 1, 3, and 19. If you memorize 11 and 12 for CR 1/2 and 1, then the other two are immaterial since the table gives average numbers anyway so being off by one (as the formula is) isn't important.

Now a glance at a stat block allows a quick test of coherent AC/CR values. Cthulhu, for one of many instances, is AC 49 CR 30, exactly what the equation predicts. But a Hungry Fog is AC 5 CR 6, way off. As discussed in the Bestiary Appendix 1, this suggests that it will have some compensating strength. My guess is the long list of Defensive Abilities fits that bill.

If there's interest, I'll add a similar analysis for the other statblock elements.

added dodge to flat-footed calculation

2014-01-09, 07:07 AM
Flatfooted AC also skips dodge bonuses.