A Cloak of Arachnida allows the wearer to cast the web spell, with the additional property:

the web created by the spell fills twice its normal area.

The area of an ordinary web spell is a 20-foot cube. What would be the shape and size of the area covered by this special enhanced web spell? Would it be a 40-foot cube? A cube with exactly twice the volume of a 20-foot cube? Two adjacent 20-foot cubes? Or something else entirely?

  • 17
    \$\begingroup\$ This question is deceptively difficult to answer. \$\endgroup\$
    – goodguy5
    Nov 9, 2018 at 19:44
  • 2
    \$\begingroup\$ The ambiguity of this wording seems like it'd make a good candidate for something to tweet Jeremy Crawford and ask. I imagine it might not get fixed in the errata coming soon with the Core Rules Gift Set release, given that it doesn't seem to have been asked (to him or elsewhere) before, but it's definitely unclear enough that it warrants better wording. \$\endgroup\$
    – V2Blast
    Nov 10, 2018 at 0:33
  • \$\begingroup\$ @XAQT78 See this FAQ for why your comment was removed. Thanks! \$\endgroup\$ Nov 10, 2018 at 8:12

3 Answers 3


Twice the area of a 20-foot cube is two 20-foot cubes

I'm afraid this is one of those "There is no raw answer". So, we rely on a handful of "hints" from the devs until we get an official answer. The most relevant hint here to me is "plain text interpretation".

The book doesn't give any guidance I could find about doubling areas, at least not in this way. Normally, I'd pull up some definitions, but entries for words like "size" or "twice" aren't super helpful here.

Size (for example)


the relative extent of something; a thing's overall dimensions or magnitude; how big something is

The only course of action left is to decide upon the simplest and easiest to implement option.
Question: How many is one 20 foot cube?
Answer: One
Question: How many is twice of one?
Answer: Two

Two twenty-foot cubes... or a 20ft long, 20ft high, 40ft wide rectangular prism. Whichever.

I also realize that the spell says: (emphasis mine)

The web created by the spell fills twice its normal area.

An argument can be made that they're telling us to double the ground area, which supports my answer.

The possibilities (from most to least likely in my view):

Twice the volume, but keep the height at 20
This is the most literal and simple to understand reading. This gives you either a shape that is 20x20x40, or if it is still a square, about 30 feet to a side (technically 28.28). This is also the same as doubling the area.

Twice the side length
We go from a 20 foot cube to a 40 foot cube. This is the easiest to envision, in my opinion. That gives us 64000 cubic feet of volume. You and I both know that 40 foot cube is much more than twice the volume of a 20 foot cube. But D&D is not a great reality simulator and it's an even worse math lesson.

Twice the volume (from the center)
A 20 foot cube is 8000 cubic feet. Twice that is 16000 cubic feet. The cube-root of 16000 gives us about 25 feet to a side.

Twice the side length, but keep the height at 20
20x40x40 gives us 32000 cubic feet.

  • 2
    \$\begingroup\$ +1 This is actually sort of implied in the AD&D version of the rule ("Once per day the wearer of this cloak can cast a double-sized web" source). It would have been a lot nicer if they'd just used the word "volume" instead in any particular ruling. I wonder if we'll find an official errata or answer. Double-sized is so much easier to understand. \$\endgroup\$
    – phyrfox
    Nov 10, 2018 at 0:09
  • \$\begingroup\$ The area is defined as space not dimensions. So twice the normal space which is a 20 ft cube, is only 2x 20 ft cubes or 1 25 ft cube. It doesn't refer to volume or cubic feet either, but in order to "twice the normal space" we double the volume. \$\endgroup\$
    – XAQT78
    Nov 10, 2018 at 5:00
  • 2
    \$\begingroup\$ @phyrfox How is "double-sized" easier to understand? If anything, it seems even more ambiguous than "fills twice its normal area". \$\endgroup\$ Nov 10, 2018 at 9:02
  • \$\begingroup\$ Ultimately, I think "no RAW answer" is correct unless an official ruling happens, so I'm going to mark this as the accepted answer. \$\endgroup\$ Nov 18, 2018 at 5:06

Probably a 30-foot cube

(but possibly 25-foot cube depending on how we explicate the RAW)

In 5e, "area" normally means two-dimensional map area, whereas a cube's dimensions define a three-dimensional volume of space, of course. So in the (poorly worded) description in RAW, to speak of the "area" of a "cube" is rather awkward, and has to be interpreted one way or the other (map area vs. cubic volume).

If it means "map area", then it's a cube that takes up double the map area of the usual cube, and that's about a 28.3-foot cube (square root of double the map area of a twenty-foot cube), which I would think a DM would round up to "30-foot square" map area.

But if you interpret "area" here as "cubic volume", then the answer is 25.2 (cube root of double the volume of a twenty-foot cube), which I would think the DM would round to "25-foot cube".

As DM I would incline toward the former rather than the latter interpretation, though both are defensible due to the ambiguity in RAW.

As for having two adjacent 20-foot cubes, I find that departs from RAW more than necessary, because it alters the default shape of web to something else besides a cube (it becomes more rectangular), and this is entirely unnecessary.

  • 2
    \$\begingroup\$ There is another, more literal possibility that it means surface area of the cube. Luckily, since the surface area is proportional to the area of a single face, so this ends up being equivalent to map area. \$\endgroup\$
    – Vaelus
    Nov 10, 2018 at 0:55
  • 1
    \$\begingroup\$ The area is defined as space not dimensions. So twice the normal space which is a 20 ft cube, is only 2x 20 ft cubes or 1 25 ft cube. It doesn't refer to volume or cubic feet either, but in order to "twice the normal space" we double the volume. \$\endgroup\$
    – XAQT78
    Nov 10, 2018 at 5:17

The Web's text states "fill a 20-foot cube" meaning, if you have a 20 ft cube (20 ft x 20 ft x 20 ft as per Cube PHB pg. 204) it will fill the whole area (area defined as a space, not a dimension).

If you cast Web that doesn't "fill a 20-foot cube", i.e. not anchored, or layered across a floor, wall or ceiling, then it will collapse into a dense 5-foot depth (20 ft x 20 ft x 5 ft). Subsequently, at the start of the next turn the spell ends with this result.

Cloak of Arachnida states "...twice its normal area..." (area defined as a space, not a dimension).

If the initial result fills a 20 ft cube "anchored" then twice the normal area (area defined as a space, not a dimension) the modified result would be 2 x 20 ft cube.

Since the area is space, then twice a 20 ft cube (in volume) is a 25 ft cube.

Unlike 3/3.5 walls, there is no option for changing the dimensions with Web. You only have a 20 ft cube to work with.

  • \$\begingroup\$ The discussion of anchored vs unanchored webs is irrelevant. I'm only asking what space the webs fill when the spell is cast, not the space they fill after they fall from being unanchored. (The latter is trivial to work out once the former is known.) \$\endgroup\$ Nov 9, 2018 at 21:13
  • \$\begingroup\$ @RyanThompson it is relevant you either cast it that fills the area the same as the web or it will spread out and condense to 5ft depth. \$\endgroup\$
    – XAQT78
    Nov 10, 2018 at 4:07
  • 1
    \$\begingroup\$ @XAQT78 The collapse of an unanchored web only occurs after the spell is already cast, so it has nothing to do with how much space the webs can fill when you're casting the spell. \$\endgroup\$ Nov 10, 2018 at 5:30
  • \$\begingroup\$ @RyanThompson the collapse happens if when you cast the spell, it doesn’t fill the 20 ft cube .. i.e. if you have a cube 20x25 .. it will collapse .. if you cast it out in the open or against a wall .. it will collapse. It’s only anchored when it fills up to a 20 ft cube, when you cast it \$\endgroup\$
    – XAQT78
    Nov 11, 2018 at 0:20

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .