when I make custom NPCs for my campaign, I don't use their ability scores to determine the attack and damage bonuses of their weapon attacks, i just think of a number that seems reasonable. I was wondering, though, is that wrong? should i be using their ability scores? when a character makes a weapon attack, they obviously add the necessary modifiers to the attack bonus and the damage, but does it always have to work the same for NPCs?

That's a perfectly fine way of doing it, personally, I find it easier to calculate for the NPC's/monsters CR when doing it this way.

Just make sure the attack bonus + damage are reasonable though.

Quote from Andrea_McConville >>

when I make custom NPCs for my campaign, I don't use their ability scores to determine the attack and damage bonuses of their weapon attacks, i just think of a number that seems reasonable. I was wondering, though, is that wrong? should i be using their ability scores? when a character makes a weapon attack, they obviously add the necessary modifiers to the attack bonus and the damage, but does it always have to work the same for NPCs?

 Not really, they can have all kinds of abilities and effects that could change their scores like fighting styles, magic items, class abilities ect..... 

You can really hand wave anything as some unknown collection of those and realistically no one else sees the ability scores so they wont know to complain any way.  The ability scores, classes and proficiency  are really more for player balance, monsters can do what ever is cool. 

It can be relevant if something happens to modify the creature's ability scores, such as drinking a potion of giant strength, but the main reason is a desire to be simulationist, and that's not required.

I personally use their ability scores and just adjust them when needed to adjust for CR, but you don’t have to. There are plenty of examples of where WotC didn’t (usually for weaker beasts). As long as everyone is having fun you’re doing it right. I just do it for my own verisimilitude. 

The whole process of creating monsters is made more complicated for no real advantage by giving monsters things like hit dice.

They’ve always had Hit Dice, it was the equivalent of CR back in the day. It’s tradition. 🤷‍♂️

Quote from IamSposta >>

They’ve always had Hit Dice, it was the equivalent of CR back in the day. It’s tradition. 🤷‍♂️

  Hit dice let you randomize the monsters health if you want and heal it if you want with a rest.  Good if players run away from a monster or it uses hit and run tactics. 

Personally I wish that instead of average it listed; Max, Minimum and hit dice.  Want a weak one go minimum, want a strong one go maximum. Average is easily calculated from the two and you can randomize if you want. Though not everyone is as big on adjusting monsters as me. 

I may find it disgusting, but creating an npc like that has precedent. Take a look at the mind flayer's tentacle attack.

Quote from Heartofjuyomk2 >>

I may find it disgusting, but creating an npc like that has precedent. Take a look at the mind flayer's tentacle attack.

 
The mind flayer’s Tentacles uses the monster’s Intelligence for the To Hit bonus, Damage bonus and Save DC.

Better examples are the bat and cat.

Quote from Solarsyphon >>

Quote from IamSposta >>

They’ve always had Hit Dice, it was the equivalent of CR back in the day. It’s tradition. 🤷‍♂️

  Hit dice let you randomize the monsters health if you want and heal it if you want with a rest.  Good if players run away from a monster or it uses hit and run tactics. 

Personally I wish that instead of average it listed; Max, Minimum and hit dice.  Want a weak one go minimum, want a strong one go maximum. Average is easily calculated from the two and you can randomize if you want. Though not everyone is as big on adjusting monsters as me. 

True. I personally almost always just use the maximum, if I want the fight to be weak I use the average.

You can randomize hit points without rolling hit dice.

True, but Hit Dice gives a range for the monster. I appreciate it anyway.

Quote from Andrea_McConville >>

when I make custom NPCs for my campaign, I don't use their ability scores to determine the attack and damage bonuses of their weapon attacks, i just think of a number that seems reasonable. I was wondering, though, is that wrong? should i be using their ability scores? when a character makes a weapon attack, they obviously add the necessary modifiers to the attack bonus and the damage, but does it always have to work the same for NPCs?

You're the DM, if you and your table are comfortable playing that way, it's all that matter.

That said, I like to have stats making sense for the sake of consistency, so that players feel more grounded. Bad guys are naturally going to have different abilities than players, but giving consistency makes so that are perceivable patterns that helps players feel that the game has rules.

That's a cornerstone of game design.

Once again, DnD doesn't force you to go that way, it's your choice in the end.

that's really helpful everyone. thanks so much : )