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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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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 '18 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 '18 at 23:21
  • \$\begingroup\$ Still doesn't really help me, since the even diameter circles have a 2x2 square for a middle. \$\endgroup\$ – vonBoomslang Oct 14 '18 at 6:46
  • \$\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 '18 at 19:52

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