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I have a Wizard who is level 5, currently running in a pirate campaign (homebrew), and I thought about some scams I could pull to earn the ship some extra coinage whenever we touched port to resupply or whatever. I have the School of Transmutation, and I was thinking of turning copper coins into silver ones using Minor Alchemy.
In the Player's Handbook it states that I can take a cubic foot of non-magical material and transform it into a different listed substance (wood, stone, iron, copper, silver).

Now, I talked to the DM and he said that that would be acceptable, as long as I do not abuse it. At later levels, I might (if this works, and assuming the DM approves) take it up a notch, like copper coins to gold coins.

However, we got to talking: just how many copper coins would it take to make a cubic foot? We know that 50 coins equal a pound, but... That is about it.

Is there any official rulings that I am missing or something really obvious I am overlooking?

You can only transmute one coin at a time

Other answers have given you good estimates of the number of coins that will fit in a cubic foot, but that doesn't matter for your purposes, because you're missing an important limitation of the Minor Alchemy feature: you can only transmute one object at a time:

Starting at 2nd level when you select this school, you can temporarily alter the physical properties of one nonmagical object, [...] After 1 hour, or until you lose your concentration (as if you were concentrating on a spell), the material reverts to its original substance.

So you can't transmute a pile of coins all at once. You can spend 10 minutes transmuting a single coin, but as soon as you transmute a second one, you will lose concentration on the first one, causing it to revert.

• Copper is 8.96 g per cm^3. That means a copper ingot with one cubic feet volume is 559 lbs.


• A heap of copper coins is no solid ingot. As a first approximation, imagine there are stacks of cylinders. A cylinder 1" high and 1" in diameter has a volume of 0.78 cubic inches. (A coin is flatter, but think of it as stacks of a dozen or so.) That means the heap is 436 lbs.


• 436 lbs. of coins are 21,800 copper pieces. Call it 20,000 because they won't be stacked and aligned perfectly.

But there is a problem. This scheme will yield silver coins with the image of a copper coin. Everybody would suspect that it is a copper coin coated with a thin silver layer. Much smarter to take a mixed heap of copper goods and to transform them. This could include a few ingots, but also copper kettles and the like. Well, perhaps not copper roof slates, because nobody has silver roof slates.