Part of an ongoing series of 5e Rules notes.

This one is pretty straightforward, but also gets into complexities from how earlier versions of D&D have done it, and how some other systems do it, too.

Cones are not 90° angles

So that’s the main thing to remember. It was a mistake we made in our first game, and an easy one to because that’s how some other systems do it (like 3.5e and Pathfinder). But not 5e.

When playing on a battle map (if you are doing Theater of the Mind, then just do what the GM says), cones are defined as:

1. Coming from one of the corners of your square. (If you are running on a hex map, then cones are one of the few things that are easier that way, and please look it up yourself.) They don’t come from the middle of the square. They come from the corner. This is true for most spellcasting in 5e (DMG 251), and missile weapons, and line of sight — though the difference does not stand out in most cases.
2. The width of the cone equals the range from that corner. So the first five feet (square) the cone is one square wide. At two squares away, the cone is two squares wide. At three squares (the classic 15-foot cone), the cone is three squares wide. Etc.

Or, to quote rules:

Starting point, as the rules put it (DMG 251):

Choose an intersection of squares or hexes as the point of origin or 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.

Width,  as the rules put it (PHB 204):

A cone’s width at a given point along its length is equal to that point’s distance from the point of origin.

That comes out, I am told, as a 53° angled cone, not 90°.

Which all seems simple until you try to map it out on squares, because squares suck.

If you want to be really technical, you could use an actual cone template that is X feet wide when it is X feet out, and then pick squares that have a majority of their space included in the cone. But I find it easier to just say “Pick a single square, now pick two beyond that, now pick three along the same angle beyond that,” and let the player figure it out.

(I also have some square templates that can be dragged onto the VTT map which can sometimes help. But most cones are short enough that it’s not necessary.)

So, for example …

What does that look like, practically? Here is a simple drawing, which can be rotated in 90° increments:

So a straight cone on a square grid map. The question marks indicate a choice — pick one or the other to be in the cone (arguably, based on whichever corner you are casting from). As noted, at 5 feet the effect is 5 feet across. At 10 feet, the effect is 10 feet across (two squares). At 15 feet, the effect is 15 feet across.

Here’s another:

This one’s at an angle, and is serving double duty.

The red mage is doing a cone at an angle downward (remember this can be rotated in 90° increments, or rotated). At 5 feet, it’s 5 feet wide. At 10 feet it’s 10 feet. At 15 feet, it’s 15 feet wide.

(While cones emanate from a corner, they don’t necessarily target a corner.)

The yellow mage is shooting at a straight 45° angle down and right. This gets a bit more complex because of 5 foot increments and how you calculate diagonals on a square grid in D&D, but again, 1=1, 2=2,. 3=3

Player’s choice. As long as you are starting from a corner, following a line of some sort, and are X squares wide for an X square length of the spell cone, you’re golden.

If that’s still confusing … maybe go for non-cone spells. 🙂 (Though, to be honest, cubical spells have their own weirdities …)