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I posted this question in the comments for the Prestidigitation cantrip on DnD Beyond, but I'm thinking that's not trafficked frequently.

I'm curious how DMs have interpreted "1 cubic foot" for this cantrip. My take is this is different than, for example, a 5-foot cube (Minor Illusion) in that it only specifies volume, not dimensions. This means I could (conceivably) clean the blood off a (idealized) longsword that is 36" x 1/2" x 3" (at the hilt). Volume of a rectangular-based pyramid is 1/3 * a * b * h (a and b being the sides at the base) so in this case 1/3 * 1/2 * 3 * 36 = 108/6 = 18 cu inches (well within the 1728 cu inches in a cubic foot).

How wrong am I?

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  • \$\begingroup\$ "I could (conceivably) clean the blood off a (idealized) longsword that is 36" x 1/2" x 3"" That's one chonky longsword... \$\endgroup\$
    – nick012000
    Sep 1 at 3:54
  • \$\begingroup\$ @nick012000 – not if you include the guard and pommel. I don't think it would be unusual for the guard and pommel to exceed 1" thick (depending on the pommel style), and the guard would typically be 8-12" across. Taking a typical longsword, you'd be looking at more like 48" length, 9" guard width, and maybe 1" max thickness – though this still gives you a box volume of 0.5 cubic ft, well under Prestidigitation's limit. However, a rapier would be well over the limit, due to the much wider hilt. \$\endgroup\$
    – Dan W
    Sep 1 at 9:02
  • \$\begingroup\$ Welcome to RPG.SE! Take the tour if you haven't already, and check out the help center for more guidance. \$\endgroup\$
    – V2Blast
    Sep 1 at 14:47
  • \$\begingroup\$ @DanW - I like the simplification to a rough polyhedron to get volume, as that's arithmetic not math :). \$\endgroup\$ Sep 1 at 14:54
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As the designer of the world, the Dungeon Master determines volumes and dimensions of objects.

“1 cubic foot” is not ambiguous, it’s just a volume of undefined dimensions. What is ambiguous is the volume of objects the Dungeon Master is describing, and so if it becomes relevant, the Dungeon Master needs to make a ruling on the volume of the object. As a mathematician, I generally try to avoid math when I DM, I do enough of it at my job, and more importantly, calculating volumes while we’re trying to play the game is generally a waste of time.

For something like this, I’d just make a decision and move on, just to keep the game moving, and I highly recommend everyone else do the same. And that’s the beauty of being the DM: you don’t have to do math because you can just make it up and it becomes true in your world.

The way I make rulings like this is rather inelegant: I imagine the object in my head and make a guess. I’ve just never encountered a player that much cared about whether or not their prestidigitation worked based on the size of the object. To be clear, in every campaign I have run I’ve had characters taking prestidigitation, and I’ve had to rule on this before, but every time I’ve said “nah that’s too big”, the player says “alright” and moves on. The stakes are just never high enough to care much. If I encountered a scenario with exceedingly high stakes and the outcome depended on prestidigitation working, and the volume of the object was the deciding factor, then I might consider doing the math.

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  • \$\begingroup\$ I don't want to do the math, either, but I did want validation (which you provided both in this response and your comment below) that there is a distinction between dimensions and volume. Thanks! \$\endgroup\$ Sep 1 at 14:49
  • \$\begingroup\$ Yeah, this is how I handle it -- it almost never really matters, and I figure you can clean/soil virtually any piece of clothing since it'd easily fold up to fit in a one-foot box, so there's no real need to get super technical about it. I think I might've one time had a player try to clean up bloodstains on the floor and rejected the idea, but more based on "the floor is not an independent object" rather than any volumetric measure. \$\endgroup\$ Sep 2 at 13:28
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By RAW, it won't work

A cubic foot is defined as a cubic area with each dimension being exactly one foot in size. A longsword would be larger than one cubic foot so it is not a valid target for the cleaning effect of the prestidigitation spell. A dagger would be however.

Oddly enough, an item of clothing like a shirt would be an invalid target when laid out flat, but would be a valid target when scrunched up into a tight enough ball.

That being said, I'd expect most DMs to allow prestidigitation to be used to clean most small items regardless of size. Trying to use it to clean a house or something would be a definite no though.

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    \$\begingroup\$ Your first sentence is incorrect. When the rules describe an NxNxN cube, they say “N foot cube”. When they describe a volume of arbitrary dimension, they say “N cubic feet”. \$\endgroup\$ Aug 31 at 23:04
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    \$\begingroup\$ A cubic foot is not an area, it's a volume. \$\endgroup\$
    – Dan W
    Sep 1 at 8:54
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    \$\begingroup\$ Just compare Prestidigitation text to cantrip Control Flames, which explicitly specifies "fits within a 5-foot cube". \$\endgroup\$ Sep 1 at 14:48

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