The weight of individual coins has varied from edition to edition, as has the relative value of the various coins. To my recollection, no edition has actually pinned down a specific size though. Even if a size were specified somewhere, I would suspect it was a purely arbitrary number. Most folks are fine just knowing that a coin is “coin-size” and leave it at that. But I’m not “most” people. :)

I would have sworn I had already written an article on this but when I went to find it, to use as a link in an upcoming article on gems, I couldn’t locate it. I guess that’s just one of the “perks” of getting older. <sigh>

50 Coins to a Pound

Various weights have been suggested over the years. I prefer to use 1/50th of a pound per coin. If you use a different weight, it shouldn’t be too difficult to adjustment accordingly.

US Coins

I thought it would be useful to start off looking at some actual coins that most people are familiar with. The data in the following table was taken from Wikipedia:

Fifty Cent Piece1.205″0.085″11.340g
Sacagawea Dollar1.040″0.099″8.100g
Silver Dollar1.500″0.102″22.68g

Most people agree that the size of a fantasy coin should be somewhere between a nickel and a fifty cent piece, with a quarter being more or less in the middle of that range. I like the size of the quarter so I’ll start off testing some numbers around that size.

Specific Gravity

Weight (in grams) divided by volume (in cm^3) gives us the specific gravity (in g/cm^3), a representation of how dense a material is. We already know the weight: 50 coins to a pound (0.02 pounds or 8.96g). The following chart gives the specific gravity of the various precious metals.

Copper8.92 g/cm^3
Silver10.50 g/cm^3
Gold19.30 g/cm^3
Platinum21.45 g/cm^3

So all we need to do is work the equation backwards (specific gravity times weight equals volume) and determine what dimensions we want to use that will provide the necessary volume.

However, minted coins (as well as jewelry, statuettes, and other works of art made from precious metals) are not made from pure copper, silver, gold, or platinum. The metal will also contain impurities that will alter the specific gravity of the alloy. This may be due to naturally-occurring impurities that were not extracted, or other metals that were intentionally added to make the metal easier to work with.

Furthermore, I want all coins to be of the same size and of the same weight. To accomplish this, all precious metals must have the same specific density. To that end, I will assume that each of the precious metals has other metals added to them that result in each alloy having the same specific gravity. This isn’t too great a stretch and should still remain within the realm of possibility.

This also allows us some latitude in what specific gravity to use.

Determining an Exact Size

I stated above that I would like to use a quarter as a model. A US quarter has a diameter of 0.955″ (2.4257cm) and a thickness of 0.069″ (0.17526cm). The volume is then 3.14159 * (2.4257/2)^2 * 0.17526 = 0.809928 cm^3. Multiply that by 8.96 grams (the equivalent of 50 coins to a pound) and you get a specific gravity of 11.06271 g/cm^3. That value falls nicely within the middle of the range of specific gravities of the precious metals involved (almost like I planned that in advance).

The specific gravity of an actual quarter is 7 g/cm^3 so a fantasy coin would weigh 60% more than a US quarter.

Note that the dimensions of a quarter (used above) are very nearly (but not exactly) 15/16″ diameter and 1/16″ thick. Using these dimensions instead would change the volume from 0.8 to 0.7 cm^3, and result in a specific gravity of 12.67345 g/cm^3 (which also would be a reasonable value to use). I will stick with the actual dimensions though.

Who Cares

Offhand, I can’t think of a single reason for wanting to know the exact dimensions of a coin. But I did want to know, with at least reasonable accuracy, the rough dimensions so I could gauge how many coins could fit in a sack or chest. Some people envision a gold piece as being the size of a nickel while others maintain that a gold piece is enormously bigger (say the size of a silver dollar). That’s a pretty huge range and I wanted some data to show those folks that would back up my choice of a gold piece being the size of a quarter.

Filling a Chest

Now that we know the size of a coin, it’s time to determine how many coins fit in a chest. I define a small chest as having inner dimensions of 9″ x 6″ x 4″ which composes a volume of 216 cubic inches. A single coin has a volume of 0.049425 cubic inches (0.8 cm^3). If you were to melt down the coins and pour them into the chest, it could hold 4,370 coins (216/0.049425). That sets our upper limit. Since a bunch of coins dumped into a chest will not fully fill the area within the chest, the number of coins the chest can actually hold will be far fewer. Sadly, I don’t know how to accurately determine what that number is. Therefore, I am arbitrarily going to assume that the chest can hold 3,750 coins (roughly 85% of the upper limit). This is a convenient number as it means that the chest will then weigh 75 pounds plus the weight of the chest itself.

Edit: Some preliminary research shows that stacking coins in rows and columns results in 21.46% wasted space (1 – pi*r^2 / d^2). Stacking coins in offset rows results in 13.59% wasted space (higher at the edges). Loose coins will most likely come closer to 30% wasted space, resulting in a maximum of roughly 3,000 coins per chest (60 pounds place the weight of the chest).

Bars of Gold

When vast amounts of gold or silver are involved, it may be more convenient to deal in bars instead of coins (I’ve never been fond of bank notes in a fantasy environment). One bar of gold (or copper, silver, platinum) is equivalent to 1,000 coins and weighs 20 pounds. The dimensions are roughly 10″ x 5″ x 1″ making them easy to stack and carry.

Shaved Coins

If an unscrupulous person were to shave a tiny bit of gold from each coin (or bar), she could melt down those shavings and form them into rings and such for a nice profit. To circumvent this, coins are minted with reeded edges (as seen on the US quarter). Merchants can then easily see if a coin has been shaved and refuse to accept it. For transactions of any significant value, coins and bars are weighed to determine their exact weight.