When casting a cube spell on a hex grid do you pick a honeycomb for origin or an intersection for origin?

Question

Hypnotic pattern: You create a twisting pattern of colors that weaves through the air inside a 30-foot cube within range.

This question has bugged me for awhile because my DM likes to play with hexes and I am going to pick up this cube spell.

Are the half hexes also counted? I know half squares are counted. Any help at all would be appreciated.

If I use proper squares, the effects become effectively larger than they should be, so that doesn't seem right. For example, the 30-foot square below actually affects a width of 8 hexes (40 feet):

Answer 1

There is advice/rules for this on page 251 of the DMG:

Areas of Effect

[...] 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.

Answer 2

If you are open to a solution that isn't strictly to the rules, consider asking your DM to change it from a cube to a hexagon. It's quite simple if you use just choose a tile as the spell's center and then add one ring around it per 10ft of range. Example: 30ft cube would be 3 rings around the center, or 37 tiles.

It's a little too big when using a 7-tile hexagon for a 10 ft cube, and as you get bigger, it doesn't quite scale fast enough, but in my mind, the simplicity of the method outweighs inaccuracies like that. (Plus I doubt there are very many spells, if any, that are bigger than 30ft.) If you DM feels that the added space from a 10 ft cube is too big, then maybe have it be a 4-tile kite for those spells specifically.

Answer 3

The official rules don't directly answer your question. However, the Dungeon Master's Guide p. 249 illustrates the relationship between squares and hexes when it comes to creature size:

For cube spells, I suggest using the same relationship between squares and hexes for three reasons:

1. There exist no other official guidelines regarding hexes.
2. It is proportional to the size of creatures (which take up smaller areas on hexes than on squares).
3. It is simple and unambiguous to use, i.e.:

Cube size	Hexes	Hint
5-foot cube	1
10-foot cube	3	1 ring around an intersection
15-foot cube	7	1 ring around a tile
20-foot cube	12	2 rings around an intersection
30-foot cube	27	3 rings around an intersection
40-foot cube	48	4 rings around an intersection
x0-foot cube	3 ⋅ x2	x rings around an intersection

Answer 4

I would answer this from the effect it has on the game. i.e. what is the number of squares (effectively creatures) that this spell affects? For squares, it's 36 squares (more for cube, but stay 2-D for now) So, pick a group of hexes, and count out 36 of them, keeping the shape roughly square-shaped. Done and done.

From there, you have two options, define the exact shape of the effect right then and there, and always use it exactly, or, allow the caster to subtly manipulate the 'leftover' hexes as he casts it.

In your specific example, choose either a row of 5+row of 6 +5+6+5+6 and you have a squarish shape of 33 hexes, and you can start a 7th row with the leftover 3 hexes, or you could define the shape as 6 rows of 6 and be more rectangular.