Conventional wisdom tells us that Great Weapon Master (GWM) is the best feat for heavy-hitters, bar none. Indeed, GWM does cause the largest damage increase of any feat available to martial characters. However, that doesn’t mean it should be the first stop on your journey.
As it turns out, GWM can actually decrease your overall damage output if taken too soon. When you haven’t stacked a high enough attack bonus, the -5 to attack can be really punitive.
While GWM’s +10 damage benefit is responsible for much of the damage increase, that’s because it’s helped by other factors, including an increased chance to hit from proficiency bonus scaling and higher Strength investment via Ability Score Improvements (ASIs).
Since you can use GWM abundantly, it’s also helped by getting more attacks. You’ll probably get access to Extra Attack at some point as a martial character. You can also get a benefit from bonus action attacks. Since you can’t exactly pull off Two-Weapon Fighting with a polearm, the only way to unlock this ability is by taking the Polearm Master feat. While polearm master only provides a measly d4 damage attack, the real punch comes from adding your Strength modifier (up to +5) and GWM (+10).
No matter what character you’re playing, these numbers will eventually even out when you max your attacking ability score and take both feats. The real question is what road to take to get there. You don’t want to leave damage output behind, especially at squishier levels.
Damage Calculations
I’ve put together a calculator that shows the damage progression from level 1 to 20 for each different ASI route. I’ve run some charts for this analysis that focus on the damage output of the Fighter class with a standard race.
Note: if you want to run your own numbers for the Barbarian class or a Variant Human race, I’ve leaked the calculator on my patreon and the post will go public next week.
Based on the outputs you select, the chart determines the levels at which the PC will get a access to Strength ASIs and feats under each scenario.
These are all fed into tables which calculate the damage output for each scenario, then feed them into a master table from which we can graph the progression. Here we’re looking at an attack vs. a 16 Armor Class (AC) target.
Polearm Master First
As it turns out, taking the Polearm Master feat first is the best use of your ASI. The extra attack really boosts your damage output. It also serves as a great platform for tacking on another GWM attack.
Shockingly, the damage outputs are very similar whether you take an ASI or GWM after Polearm Master. Against the 16 AC target pictured above, GWM actually edges out Level 6 damage output by 0.03 damage/round. Not enough to ever make a noticeable difference. This doesn’t hold up against every AC. If the target is easier to hit, GWM comes out in the lead. If the target is harder to hit, GWM hurts your damage output. Here’s the same Fighter attacking a 12 AC target and a 20 AC target:
Now, you could always just not use GWM against those higher AC targets, but that isn’t helping your PC in a situation where a Strength boost certainly would.
Great Weapon Master First
Conventional wisdom tells us to take this feat first because it’s the best feat. Like much conventional wisdom, it doesn’t hold up when we run the numbers. Let’s start again vs. the AC 16 target:
Here you can see how pronounced the difference is for GWM against lower (AC 12) and higher (AC 20) armor class targets:
Conclusion
Against most armor classes, taking PAM first is going to give you the highest damage output. PAM comes out clearly in the lead against the vast majority of ACs you’ll fight on a standard adventuring day (AC 13 to AC 21).
Against lower AC targets (12-), there’s some variance where PAM is more helpful early (levels 4 & 5), but GWM is actually more helpful after (levels 6 & 7). This shouldn’t be surprising, since we all know GWM shines against low AC targets. However, this seems to favor PAM since you’re more likely to see those really low AC foes when you’re playing a low level PC.
Against very high AC targets (22+), there’s some variance where PAM is more helpful early (levels 4 & 5), but the ASI is actually more helpful after (levels 6 & 7). This isn’t really a consideration, though, since you likely won’t be fighting AC 22+ targets when you haven’t hit level 8. Also, against high AC targets, it’s best to take GWM dead last, even delaying it until after ASIs.
ThinkDM 
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