What are the limits on the area affected by an Alarm spell?

Question

I am fairly new to D&D, and I am trying to figure out the line between using spells creatively and abusing the spell. With regard to the alarm spell, the text reads:

Choose door, a window, or an area within range that is no larger than a 20-foot cube.

How I read this is that you can designate any contiguous area within the spell's range with a total volume less than that of a cube with a side length of 20 ft., so 20 ^ 3 = 8,000 ft.

It so happens that the volume of a 30ft hollow hemisphere with 1ft thick sides is 8,060.83 cubic feet (I used this "Volume of a partial sphere Calculator" to find the area of a partial sphere with a radius of 29ft and a height of 28ft and subtracted that from the volume of a 30 ft radius sphere). That can be easily made to equal 8,000 cubic feet with only minor adjustments.

Is this use of alarm allowed according to a strict interpretation of the rules and do you think would it be acceptable to use in game play?

So basically, you are proposing to ward an arbitrarily large area by only warding an arbitrarily thin line along its perimeter? — Ryan C. Thompson, Dec 20 '19 at 04:12
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score 7 · Accepted Answer · edited Jun 16 '20 at 10:23

As you state, the text of alarm says:

You set an alarm against unwanted intrusion. Choose a door, a window, or an area within range that is no larger than a 20-foot cube.

The spell allows you a 20 foot cube of space, or smaller. "Cube" is a spell area descriptor which states:

You select a cube's point of origin, which lies anywhere on a face of the cubic effect. The cube's size is expressed as the length of each side.

A cube's point of origin is not included in the cube's area of effect, unless you decide otherwise.

Now the debate of whether or not your cylinder is "larger" than a 20-foot cube. I would argue it is; despite the fact you could make the volume less than that of the cube, your area does not fit inside the designated space. The other reason I argue in favour of this interpretation is that because of how math works, under your interpretation you could theoretically make alarm cover a half-mile radius and be notified whenever something enters that area. That is clearly out of the scope of a first-level spell.

What are the limits on the area affected by an Alarm spell?

1 Answers1