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Per the Wild Magic sorcerer's Wild Magic Surge table (PHB, p. 104)

07-08: You cast fireball as a 3rd-level spell centered on yourself.

There's other questionable area spells in that list (grease is a 10 foot square, so your square and which three?), but I'll focus on spheres like fireball (PHB p. 241):

(...) Each creature in a 20-foot-radius sphere centered on the point must (...)

My problem is, the sources I found so far - including the other answers to similar questions here - talk about measuring spheres from grid square intersections, whereas this clearly talks about it being centered on the unlucky Wild Magic sorcerer. Plus, since it's a wild magic effect, it doesn't exactly have a caster that can pick a point of origin. Is there an accepted way to measure an effect like this?

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This is a DM call.

As observed in another answer from Thomas Markov, there are two ways of reading this.

The first reading is literal: the Fireball is centered on yourself, hence the point of origin is the center of the square in which the characters is. The situation is depicted in the below figure, where the green circle is the Sorcerer, the red circles represent other creatures and the small circle the point of origin of Fireball.

enter image description here

The second reading applies the rules for using a tactical map from the DMG and the rules from XGTE:

Choose an intersection of squares as the point of origin of an area of effect, then follow the rules for that kind of area as normal (see the “Areas of Effect” section in chapter 10 of the Player’s Handbook). If an area of effect is circular and covers at least half a square, it affects that square.

In this case, there are 4 intersections around the square in which the Sorcerer is: then it has to be decided from which intersection the Fireball originates. The 4 situation are depicted in the figures below: pay attention that this choice affects in different ways the creatures around the Sorcerer.

North - east corner

enter image description here

South - east corner

enter image description here

South - west corner

enter image description here

North - west corner

enter image description here

It is up to the DM deciding who picks the point of origin: the player or themselves.

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The sphere originates from one of the intersections around the square(s) your character occupies; you select which intersection the effect originates from

From the DMG, page 251:

Choose an intersection of squares or hexes as the point of origin of an area of effect, then follow its rules as normal

Therefore, when playing on a grid, you select an intersection of squares as the point of origin. The rules then follow as if the spell had been cast on that intersection.

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First, a ruling from the DM is necessary.

The effect in question is:

You cast fireball as a 3rd-level spell centered on yourself.

The DM must determine if this creates an exception to or if it is subject to, the rules for adjudicating areas of effect on a grid:

Choose an intersection of squares or hexes as the point of origin of an area of effect, then follow its rules as normal.

It simply is not clear which is the "most correct" rules-as-written ruling here.

Either way, you determine the area in the same way.

However, in either case, the method of determining the area of effect is the same:

If an area of effect is circular and covers at least half a square, it affects that square.

So, for each ruling, we have the following areas of effect:

enter image description here

Centered on an intersection, total area: 52 squares

Then:

enter image description here

Centered on the center of a square, total area: 45 squares

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  • \$\begingroup\$ Since the wild magic surge specifically designates a non-standard point of origin, it actually makes sense that it have a non-standard area of effect as well. \$\endgroup\$
    – Valkor
    Dec 23, 2021 at 9:02
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I've always used something pretty much identical to this:

a visual grid of pixellated circles of various sizes

For circles whose diameters are an even number of squares, I just center the circle on the origin square (i.e. so it's offset from the actual map grid by half) and then roll an attack on any square that's at least 50% covered by the pixelated circle.

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  • \$\begingroup\$ How do you determine if a square is more than 50% included in the circle? \$\endgroup\$
    – Mark Wells
    Oct 13, 2018 at 21:35
  • \$\begingroup\$ Because you're working with identical grids, and the offset is always either 0 or 1/2-square, ambiguous cases simply don't occur. It pretty much boils down to the fact that if a square of the circle overlaps a square of the map, that square counts. I'd say that's half the value of using these predefined pixelated circles. \$\endgroup\$
    – Tom
    Oct 13, 2018 at 23:21
  • \$\begingroup\$ Still doesn't really help me, since the even diameter circles have a 2x2 square for a middle. \$\endgroup\$ Oct 14, 2018 at 6:46
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    \$\begingroup\$ This guidance from DMG p. 251 might be relevant: "The area of effect of a spell, monster ability, or other feature must be translated onto squares or hexes to determine which potential targets are in the area and which aren’t. Choose an intersection of squares or hexes as the point of origin of an area of effect, then follow its rules as normal. If an area of effect is circular and covers at least half a square, it affects that square." Basically, just pick one of the corners of your square (or roll a d4 if you want it to be random), then measure from there. \$\endgroup\$
    – V2Blast
    Oct 14, 2018 at 19:52
  • \$\begingroup\$ @Tom, is your answer intended to be a house rule suggestion, or you basing yourself on the rules of the game? If the later, could you specify which rule(s) you base yourself on? \$\endgroup\$ Mar 2, 2020 at 17:01

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