How do I calculate a monster's unarmed attack, specifically for slam?

Question

I'm making a custom monster, a huge flesh golem. I'm trying to determine its unarmed strike as a much larger monster for slam, and need to calculate its damage to determine its offensive challenge rating so I can calculate its actual challenge rating.

So far I know that the medium sized Flesh Golem has these stats for slam:

Slam. Melee Weapon Attack: +7 hit, reach 5ft., one target. Hit: 13 (2d8+4) bludgeoning damage. (MM p.169)

I get that hit dice use constitution to calculate hit points, but when I tried to apply the same equation with strength to determine damage I was off, which makes me believe there is a formula I just don't know.

Answer 1

Here's how we can get the (existing) Flesh Golem's stats:

+7 to hit:

+3 proficiency, +4 STR. You know the flesh golem's proficiency bonus is +3 because it's a CR5 monster; you look this up on the table "Proficiency Bonus by Challenge Rating" on MM p.8.

2d8+4 bludgeoning damage:

+4 STR, the 2d8 is the golem's natural "weapon", its fists. That die type/number is not tabulated anywhere--it's one dial you can tweak for damage when trying to create your monster's offense.

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You can find much excellent advice on creating custom monsters in Angry GM's Monster Building 101/201/202 articles. Be warned that his writing, while full of good advice and wisdom, is liberally sprinkled with rude and vulgar language.

You may have noticed that CR -> proficiency bonus -> damage -> offensive CR -> ... CR? Yes, there is a feedback loop in there. Angry GM's walkthroughs on creating monsters do a very good job of explaining things like this, pointing us to where the actual dials are that one can tweak.