I saw this post by a poster named 77IM on ENWorld and thought I’d repost it here because it’s the kind of thing I like to consider in my games:
Do you wonder what size coins are? I sure do! Keeps me up at night.
So let’s math this out a bit.
50 coins weigh 1 lb. so that means 1 coin is 0.32 oz., or 9 g if we are being metric, which we are, because that is how Wikipedia lists coin weights. So, looking to see if any US coins weigh 9 g, it looks like the closest is the dollar coin, at 8.1 g.
The dollar coin is comprised primarily of copper, so BAM, that’s about how big a copper coin is. Actually it will be slightly bigger, about 12% bigger by volume, which is pretty negligible. For those of you unfamiliar with the dollar coin, it is is about an inch across (26.5 mm) and 2 mm thick, so it’s not that much bigger than a quarter.
Now let’s look up metal density. Copper is about 9 g/cm^3, and silver is 10.5 g/cm^3, so pretty close. This means a silver coin is slightly smaller than a copper coin. Since the copper coin is slightly bigger than the dollar coin, this means that the silver coin will be about the size of a dollar coin, or maybe a bit smaller. I’m to lazy to bust out pi*r^2 and figure out exact sizes for these coins so I’m satisfied saying “both silver and copper coins are about the size of a US dollar coin.”
Also, this assumes the metal is pure, and it’s usually not. Silver usually has some copper in it, and copper coins (in the US) often have substantial nickel in them, and once you start putting zinc in there things lighten up considerably. So all these sizes are going to be approximate anyway.
Looking at the density of gold (19.3) and platinum (21.5) it looks like they are pretty similar and both nearly double the density of copper and silver! Well that’s handy. It means that gold and platinum coins should be half the volume of the copper/silver coins.
Since most US coins are copper, a gold or platinum coin of the same dimensions would weigh twice as much. So to get the dimensions of a 9 g coin (50 coins weigh 1 lb. in D&D), we need to find a 4.5 g coin. Crawling around Wikipedia, it looks like the closest is the familar US nickel coin, at 5 g. That’s actually OK, because our D&D fantasy coins are probably not pure gold or platinum, which means they will be less dense and therefore slightly larger than a pure coin.
So there you have it:
A copper or silver piece is about the size of a US dollar coin (or a UK 20p piece).
A gold or platinum piece is about the size of a US nickel (or a UK 10p piece).
I’d add that electrum pieces would roughly be the size of a US quarter.