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Hi, I'm Jack the Lad. You might know me from some analysis I wrote about skeletons and fighters and wizards and things around the release of D&D 5e. I've been taking a look at Pathfinder 2e over the past couple of days and doing some similar analysis.
One of the main things I've been trying to do is work out how martial classes can do the most damage, and in the process of doing that I've come to realise that unless you're attacking something with resistance, Power Attack will almost always do less damage than two normal attacks.
You can calculate the expected damage of an attack by multiplying its chance to hit by the damage it does on a hit; if an attack deals 10 damage and hits 50% of the time, you can expect it to do 5 damage per attack on average.
Critical hits deal damage over and above this, but because they usually deal additional damage equal to the original hit, you can generally account for them by simply adding your chance to crit to your chance to hit.
In the example above, if we crit 25% of the time (included in the 50% of attacks that hit), we can add that to our 50% chance to hit and know that we'll deal 75% of the "on hit" damage per attack on average, or 7.5 damage.
It's also worth noting that there is a lower floor on your expected damage at 10% of the total of your weapon damage and stat mod, since as per page 178 a natural 20 is always a critical success/critical hit.
Of the creatures we've seen so far in the bestiary, the Ox (and the Hidden Pit) have the joint lowest AC at 10 and the Titan and Devastator have the joint highest AC at 47. I've made some tables of a Fighter's expected damage against these and every intermediate AC, assuming they use a d12 weapon and upgrade the weapon and their Strength at every opportunity. Using the Character Wealth table 11-2 on page 348 of the playtest rulebook, that means increases to their Strength modifier at levels 10, 15 and 20 and:
- At level 5, a +1 magic weapon
- At level 9, a +2 magic weapon
- At level 10, a +2 magic weapon with +1d6 energy damage
- At level 13, a +3 magic weapon with +2d6 energy damage
- At level 17, a +4 magic weapon with +2d6 energy damage
As you can see in the table below, Power Attack becomes a damage decrease almost all of the time as soon as you start using a magic weapon:
Interestingly it's mostly the magic weapon(s) that account for this, as even a Fighter with 12 strength and therefore minimal stat mod double dipping capabilities (perhaps one with strong opinions on the relative merits of roleplay and rollplay) is almost always better off making regular attacks and a Fighter without a magic weapon is almost always better off making Power Attacks:
Power Attack also has significant downsides other than the obvious and purely numeric ones, too. Firstly and most obviously you only need to deal as much damage to an enemy as it has HP - overkill damage is wasted, while a weaker attack that kills an enemy lets you spend the rest of your turn doing something else and perhaps even killing another. Secondly and perhaps more importantly making two attacks dramatically increases your chances of dealing at least some damage on your turn rather than wasting it entirely with a whiff, which is important both in practical play terms and in player morale ones. Spending spend three or four turns trying to power attack something and missing is a very bad feeling.
Power Attack does improve your expected damage against enemies with resistances, though, against whom a single big hit is preferable to two smaller ones that each have a flat reduction applied to them, but it's not something people should blindly take and use assuming it will increase their damage, which is how it's mostly worked in other games and editions, and how I've mostly seen it being discussed so far.