How much do coins weigh? How big are they? How many fit in that box?
Early D&D has bad answers for these, and if you go looking in Dragon magazine you're going to get a uselessly high level of detail.
So here's my system:
Coins are mostly pure alloys -- pure enough that we will pretend they are for determining weights and values, but still we remember a gold coin is not as soft and easily melted as pure gold.
The rate of exchange between metals is 1:10 as usual.
There are 50 coins to a pound (or 0.02 lbs/coin). This is definitional. The real-life densities of gold and platinum are roughly equal, and both roughly double that of silver and copper. Thus the coins of the lighter metals is about the size of a US quarter (or 1 euro, or 20c euro), while the heavier metals are a little bigger than a US penny (or a 0.02 euro coin.)
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| US $0.25 vs €1 |
These sizes of coins seem intuitive and 50 coins per pound is an easy number to do math with, so this seems like a good baseline. These are a little too big to be medieval coins -- but kill the D&D medievalist in your brain, and contemplate that these coins are roughly the weight/value of the coins of Croesus.
An easy-to-calculate standard amount for trade would be a 20 lb ingot, or one thousand coins of the metal.
How many coins fit in that chest?
When dealing with piles of coins, sometimes we don't just want to know their weight, but we want to know their volume.
For simplicity, we will say that silver and copper have the same density, while gold and platinum have double that density. The density of gold is set at 1,000 lbs per cubic foot and the lighter metals at 500 lbs per cubic foot. (These are both a little less than in reality, but this allows for calculations that assume that we are working with coins which do not fill the volume exactly.) (And while we're at it, stone is about 150-200 pounds per cubic foot, and wood, or water, or fleshly creatures, is about 50 lbs/cuft.)
Because gold is double the density of silver, you can count a gold coin as half the volume of a silver coin: a horde of 3,000 silver and 700 gold has the same volume as 3,350 silver coins; that many coins is 67 pounds, which is a little more than 10% of the 500 lbs of coins a cubic foot of silver coins is; call it an eighth, which is a 6" x 6" x 6" bag.