When should you take the -5 on your attack for the bonus damage? We can solve this with the amazing POWER OF MATH!!!!!!!!!! 

Let me Save you the Trouble with the algebra:

(16 + your attack Modifier) Subtract (1/2 the AVERAGE DAMAGE of your HEAVY WEAPON) 

For Instance: My hexblade Dreslin DeVir's Glaive attack modifier is +7, and he deals 10 damage on average (1d10+5). He should only take the -5 to hit when the enemy's AC is less than or equal to 18 (16 + 7 - 10 ÷ 2)

Note: Don't ever take the penalty if you have disadvantage on the attack roll or if you can one-shot the creature with a normal weapon attack. 

If you have ADVANTAGE on the attack roll the equation changes to (14 + your attack modifier). 

So: if Dreslin has Advantage then he should take the penalty if the enemy's AC is 21(14 + 7) or less.

Record this number for yourself somewhere on your character sheet and don't forget to update it if your attack modifier or damage change. Happy Gaming and Keep Rolling those Dice. Also you can use these same equations for the Sharpshooter Feat

Or you can use this Analysis here. http://wizardofthetavern.com/great-weapon-master-5e-when-to-use-it/

Quote from Lyxen >>

Thanks for the equation, I would just like to point out that this probably involves a bit of metagaming as, in general, you should not know your target's AC. You might guess a range for the AC based on the physical characteristics and, after a few rounds, you might narrow it down, but...

 While it is sort of metagaming, you could explain it in character. For example, you could say that your character knows to make more risky, but more effective strikes at an opponent if they can tell that they are not defending themself well. 

Quote from Lyxen >>

Thanks for the equation, I would just like to point out that this probably involves a bit of metagaming as, in general, you should not know your target's AC. You might guess a range for the AC based on the physical characteristics and, after a few rounds, you might narrow it down, but...

Kinda of. Its "Meta-Adjacent" (also the name of my Youtube). The DM isn't going to just tell you the targets AC (some might but in my experience most don't and I don't). But similar to what you said, most players through the course of combat are going to figure out which attacks hit the enemy and which ones don't. Adding to what Mr. Walrus said above, in-game characters during the course of combat naturally figure out how hard a target is to hit. Real combatants do the same thing in their heads instinctually when they have to fight.

But the point of Great Weapon Master is that it's supposed to be a gamble. You don't 100% know what the target's AC is, even if you could make a VERY educated guess. Like a game of chess or poker, you size up your opponent, and make a decision based on what you think of them and what your own skills are. Is the risk (-5) worth the reward of extra damage. But there is a catch. Most gamers don't actually have personal experience with real fighting or small group tactics or effective weapon use. Some do, but for most real people, it's not a common skill set. The equation is to help more combat inexperienced players bridge that gap. Because their "Character" IS experienced in combat. Their character would know this stuff and be able to make a judgement on whether they want to take that risk in-game. The above equation, at the game table, is a way to compensate for the "Characters Skill and Experience" vs the "Players lack-there-of". Ultimately it's just a tool for people to use (or not). Think of it as "training wheels" to help players more effectively use this feat until they have gotten more experienced using it. Taking the risk and succeeding (or flubbing horribly) helps create the highs and lows you want in a dynamic combat. And you don't want any player to miss out just because they aren't experienced in using the feat enough yet.

So admittedly it is Meta-adjacent, but its for the purpose of teaching and making the gaming experience more fun for people. But every table is different. So long as you and your party are having a fun time, that's all I care about. Cheers.

Okay, now that you gave me the short version... how about the long one showing the math? Why 16? 

Quote from Chicken_Champ >>

Okay, now that you gave me the short version... how about the long one showing the math? Why 16? 

 16 minus half the damage of the weapon versus 14 with advantage (both plus the to hit). That's the interesting part to me. Indeed, very curious. 

Quote from Chicken_Champ >>

Okay, now that you gave me the short version... how about the long one showing the math? Why 16? 

 If you want to see the math, sure. But I don't know how much it will help you or your players. It's SUPER BORING. And Im no math professor so my ability to explain all of it is pretty limited so I hope you will bear with me. But I applaud your inquisitiveness. You can plug the DPR inequality of using the feat into a venerable wolfram alpha, and it simplifies it to the formulas below:

• solve [1-0.05*(x-y-1)]*z < [1-0.05*(x-(y-5)-1)]*(z+10) for x.     Where x is the AC, y is your attack bonus and z is average damage
• Result for above : x < (y - (z/2) + 16)
• with advantage the equation changes to: x < 0.5*(2*y + √(z^2+10*z+1600) - z - 8)  
• and, if you are curious, with elven accuracy it becomes: solve [1-(0.05*(x-y-1))^3]*z < [1-(0.05*(x-(y-5)-1))^3]*(z+10) for x

But honestly, don't get too hung up on the exact mathematical formulas. Just use the simplified versions in the original post. 

An even simpler method if you are interested in another option is: use great weapon master if you believe the targets AC is 16 or lower, without advantage. And always use it with advantage. Never use it if you can one-shot the creature with a normal attack. It's not exact by any means but it is a simple and easy method that will work just fine. They are just options on how to guide you on using the feat.

Technically, you don't even need to know what the targets exact AC is to determine when to use the feat -- you just need to know if it's below a certain number. You can find that out by attacking it a few times. One BIG exception: If a target has AC equal to or greater than 20 + your normal attack bonus, always use GWM. There is no downside in this circumstance since you will only hit on a natural 20 anyways. But it rarely (if ever) comes up.

I hope that makes some sense for you. There are much smarter people than me with this kind of math who could probably give a more in-depth explanation of the formulas. It's just one option intended to help give players a bit of direction. The link Rojo35 posted is also a fantastic option to use as well.  http://wizardofthetavern.com/great-weapon-master-5e-when-to-use-it/. Experiment, play around with it, and ultimately so long as you and your friends are having fun with it, thats all I care about. Cheers. 

Is the old rule of thumb 2/3, or 75% :p

For martial classes with multiple attacks per round, I usually hear 60% as a reasonable level-appropriate hit chance. Hitting on a 9 or better feels pretty competent... I’ve never heard anyone suggest that if you aren’t hitting on a 6 or better you’re taking a “bad” risk. 

Thanks for the glimpse at the formula, I’m bad with math but may play with that a little, I have an idea for (an exhausting) way to do analysis from a different angle...

Quote from Chicken_Champ >>

Is the old rule of thumb 2/3, or 75% :p

For martial classes with multiple attacks per round, I usually hear 60% as a reasonable level-appropriate hit chance. Hitting on a 9 or better feels pretty competent... I’ve never heard anyone suggest that if you aren’t hitting on a 6 or better you’re taking a “bad” risk. 

Thanks for the glimpse at the formula, I’m bad with math but may play with that a little, I have an idea for (an exhausting) way to do analysis from a different angle...

 Sorry that was totally my fault. Thats part of a different answer on a different question. It's super late here and the brain isn't working at full speed.   I'll edit my post to try and explain it better. Thanks for catching it. 

Quote from Ajexton >> 

Because their "Character" IS experienced in combat. Their character would know this stuff and be able to make a judgement on whether they want to take that risk in-game. The above equation, at the game table, is a way to compensate for the "Characters Skill and Experience" vs the "Players lack-there-of".

 This is well-said and really strikes to the heart of a lot of meta-gaming accusations that I take issue with. I once advised our fighter, who was new to 5e, to use Dodge when we were fighting something that was shooting us from beyond the range of anything we had. I was told that my Reaction was used up by doing that. But in that moment I wasn't the voice of my character, I was telling the player what the character absolutely knew.

I see the immersion-breaking problem with it, and in that game I have just learned to keep my mouth shut and give advice later outside of the game out of respect for the DM. But in the games I run I will always take great pains to make sure the players always know the stuff the characters would know, because it breaks my immersion when a character makes a mistake it wouldn't ever reasonably make due to the player's lack of information.

Quote from Jhfffan >>

Quote from Chicken_Champ >>

Okay, now that you gave me the short version... how about the long one showing the math? Why 16? 

 16 minus half the damage of the weapon versus 14 with advantage (both plus the to hit). That's the interesting part to me. Indeed, very curious. 

 The 14 is a little different, and well out of my ability to explain fully, but I can do the very simple version to help your curiosity.

It has to do with the way advantage on a d20 roll works. The equivalent modifier to your dice roll that you get from advantage is +5 on an unmodified d20 roll of between 8-14. Rolls lower and higher rolls than that start to reduce that equivalent bonus. So the highest unmodified dice roll you can make and still get the maximum equivalent bonus from advantage is 14. With that +5 from advantage canceling out the -5 from GWM it kind of acts like a straight roll. If GWM + Advantage is essentially the same as a straight roll, then it's worth trying for the extra damage. But an unmodified dice roll greater than 14 to hit the target starts suffering diminishing returns and stops being worth it. There is a LOT more math to it than that, but it starts to give you an idea where the 14 comes from. 

Remember that it's just one guideline for you to use (or not) as you see fit. And there are a few different guides for this available to players. As you get more experience with the feat you will develop your own instincts on when you want to use it or not, and what works for you and how you play your character. If you are not comfortable with (14 + your attack bonus) as a simple guideline check out the chart in  http://wizardofthetavern.com/great-weapon-master-5e-when-to-use-it/ titled "Highest AC to Use GWM per Level". The second column accounts for advantage on the hit. The author even covered if you have disadvantage on your rolls (which I did not). The numbers given there will work fine as a guide and they tend to be a lower AC threshold, making it more reliable. 

Hope that helps. Im certainly not the perfect person in the world to explain the calculations. But they work. Try em out for yourself and hopefully it will help take a little of the analysis paralysis out of when to use GWM and make the feat more fun to play with. If it does, awesome. If not, no worries just try a different guideline. We're lucky in the D&D community because there are a lot of really knowledgable people out there, who are very kind and open to helping other people out. Happy Playing. Cheers.

Quote from scatterbraind >>

Quote from Ajexton >> 

Because their "Character" IS experienced in combat. Their character would know this stuff and be able to make a judgement on whether they want to take that risk in-game. The above equation, at the game table, is a way to compensate for the "Characters Skill and Experience" vs the "Players lack-there-of".

 This is well-said and really strikes to the heart of a lot of meta-gaming accusations that I take issue with. I once advised our fighter, who was new to 5e, to use Dodge when we were fighting something that was shooting us from beyond the range of anything we had. I was told that my Reaction was used up by doing that. But in that moment I wasn't the voice of my character, I was telling the player what the character absolutely knew.

I see the immersion-breaking problem with it, and in that game I have just learned to keep my mouth shut and give advice later outside of the game out of respect for the DM. But in the games I run I will always take great pains to make sure the players always know the stuff the characters would know, because it breaks my immersion when a character makes a mistake it wouldn't ever reasonably make due to the player's lack of information.

 Not sure why that would have used a reaction when it wouldn't have used an action on your turn. 

You can communicate however you are able, through brief utterances and gestures, as you take your turn.

I suppose you could say that the difference is that "as you take your turn" part, but I feel that's just silly.

 

• And back to the topic of this thread, nnowing every monster's exact AC so that they can use computations like the ones provided in this thread, in particular at the beginning of the fight is also clearly not what a player should know.

 

I love the passion of your comments, never lose that. The sharing of experience and difference of opinion are great ways to help each other learn and grow.  I will point out to you that you don't need to know every monster's exact AC. The formulas I gave do not require omniscience. You only need to know if the monster you are attacking AC is Below a certain number. And that is fairly easy for most people to figure out after turn or two in combat. Players do this for all their normal attacks already.

Although I feel this had devolved away from the point of the post, so let me reign it in. Don't get me wrong, I find the talking points fascinating but they are probably better off in a different forum or done through a private message so this post doesn't get saturated with a bunch of different topics. The post is about giving newer or inexperienced players a way to figure out if using the Great Weapon Master Feat is a good idea in their current situation. It's just a tool to use (or not). It's to prevent the analysis paralysis and overthinking that a lot of players can get trapped in. It's to help speed up gameplay and to remove player frustration from not knowing if they are using their feat effectively.

A good rule of thumb to remember is that: the "Characters" know what they are doing. They have been trained in combat, they have experience, and they live in the world of D&D where knowing proper tactics and effective weapon use as an adventurer is very common. Very few "Players" have that same training or experience. Many newer players struggle with just the basic rules of the game. It's up to the players and DM's to make a judgement call about what is reasonable based on that, but that judgement should still lean in favour of what is going to be most fun and enjoyable for the player and game group as a whole. 

Cheers and love. 

Without worrying too much about exact numbers, my character uses GWM on opportunity attacks, when he has advantage, or against low-armored creatures. Sometimes he just "swings for the fences" when a big hit or a big miss would improve the story he tells later on around a fireplace at the tavern.

Quote from Lyxen >>

Thanks for the equation, I would just like to point out that this probably involves a bit of metagaming as, in general, you should not know your target's AC. You might guess a range for the AC based on the physical characteristics and, after a few rounds, you might narrow it down, but...

 If everyone is attacking the thing and you know that a 19 hit but an 18 missed.... Can you guess that target’s AC? Sure, I knew you could.