D&D 5E Creatures partially in an area of effect. How are they affected.

Kor

First Post
I have not been able to find if the 5e rules details what happens when a creature is partially in an area of effect. On this same accord, I have not been able to find rules on whether this can be used to the caster's advantage when using a map grid.

In past editions, if you wanted to cast a 20-foot radius fireball, you had to pick a grid intersection as the point of origin. I don't think the same requirement exists in 5e? So in theory, you could pick the center of a square as the spell's point of origin. This now leads me to the next question -- if a medium size character is in a square where half of it is subjected to an effect (such as a fireball) is the character affected by it?

I am just looking to see if there are any rules reference regarding this. Since such "favored positioning" is essentially extended the radius of effect of the spell, I'm going to guess that most DM's would just tell players to pick a grid intersection. Regardless though, I'm just curious if there are any rules which govern either of these concepts, as I haven't been able to find such rules references.
 

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Voort

Explorer
Check the DMG page 251. Short version: you pick a grid intersection as the origin. If at least half of a square is in the area of effect, creatures in that square are affected.
 

Or to put [MENTION=6785649]Voort[/MENTION] 's answer another way, if a creature is in any square that is affected then they are targeted. So to hit a large or bigger creature, you only have to be able to hit one of the creatures occupied squares.
 

ccs

41st lv DM
It's a very simple calculation.
Is the models base in a square being affected by an AoE?
If NO: model not affected.
If YES: model affected, make saves as appropriate, etc.
 

jgsugden

Legend
It does resut in some logical oddities. A gigantic monster with a toe in the area of a fireball is treated no differently than one that has the fireball blow up front and center and cover their whole body... but that is RAW an RAI. However, RAF (Rules as Fun), I like to give creatures advantage on saves against area spells if they are on the fringes, and disadvantage if someone intentionally targets somthing like a fireball front and center on one specific target (sometimes asking the attacker to roll an attack roll to force the disadvantage). I'm not too worried about it as damage spells are not overwhelming in 5E as much as they have been in prior editions.
 

Kor

First Post
Check the DMG page 251. Short version: you pick a grid intersection as the origin. If at least half of a square is in the area of effect, creatures in that square are affected.

Thank you, the Areas Effect rules on this page address this very well.

I was also thinking about spells like Spirit Guardians as to how this would apply, but it appears the rules cover this off very well. Even though the range is given as: Self (15-foot radius), it essentially means 15-radius around the square that you occupy. Or as the spell states, "They flit around you to a distance of 15-feet..."

(I'm still trying to get my head out of 3.5e where as far as I know, all areas of effect always originated from a grid intersection.)
 

Harzel

Adventurer
Thank you, the Areas Effect rules on this page address this very well.

I was also thinking about spells like Spirit Guardians as to how this would apply, but it appears the rules cover this off very well. Even though the range is given as: Self (15-foot radius), it essentially means 15-radius around the square that you occupy. Or as the spell states, "They flit around you to a distance of 15-feet..."

I guess you could see the spell description wording as ambiguous and treat it as you say, but I don't see how you would get that interpretation from the AoE rules in the DMG.

Personally, I ignore the 'point of origin must be grid intersection' restriction in general, and in particular I treat the area of Spirit Guardians as a 15ft diameter circle centered on the (center of the) caster.
 

D&D 5e rules were mostly written not having a grid in mind anyway.

And yeah that it must be a grid intersection is a bad rule for self-centered spells. For example Thunderwave should hit the 8 grid cells next to you and not farther (if you place the 15ft square on a grid intersection then the AoE is essentially double as big because you cover exactly half of the next grid cells).
 

Kor

First Post
Personally, I ignore the 'point of origin must be grid intersection' restriction in general, and in particular I treat the area of Spirit Guardians as a 15ft diameter circle centered on the (center of the) caster.

For medium sized creatures, this is wouldn't matter either way. 15-feet from the center of the square you are in, will reach squares 15' away from you and the coverage area in those squares would be 50%, so effectively the area of effect is 15.5-feet. For a large sized creature (or larger) casting this, then the area of effect would greatly diminish -- this is why I think they went with the wording "around you to a distance of 15-feet". Accordingly, I don't think you count the square you are in when determining the distance -- otherwise they would not have described it in these specific words.

However, I do see your point -- and while talking about "points", I suspect the point of origin rules could likely favor your interpretation. I think all area affect spells all seem to originate from a "point" and not location -- such a creature?

While I'm on the fence on the proper way to apply this, if we take this to the extreme and have a huge creature cast Spirit Guardians, the area of affect is much larger now if we apply my reasoning of "15-feet away from the area you occupy".

I would like to hear what others think about this.
 
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D&D 5e rules were mostly written not having a grid in mind anyway.

And yeah that it must be a grid intersection is a bad rule for self-centered spells. For example Thunderwave should hit the 8 grid cells next to you and not farther (if you place the 15ft square on a grid intersection then the AoE is essentially double as big because you cover exactly half of the next grid cells).

I think you're playing Thunderwave wrong. It's a 15 ft cube from the caster. The caster is on one edge of the cube (as shown below on left), not in the center (below, right). This is stated in the rules for the point of origin of spells.

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