My House Rules on D&D Coinage
A Short Treatise on my House Rules for D&D Coinage
When I'm playing a game, it never fails that someone will ask a question related to coinage size and weight. Usually, it's when we're trying to figure out how many coins are in that chest we just found in the dragon's hoard, or how much does 1500 silver pieces weigh and can Brother Oen carry them all in his backpack.
Normally, I try not to sweat the small stuff when handling a game. If it's not critical to the plot, then I tend to make a common-sense ruling and then carry on so as to not get mired into tedious detail. However, sometimes it is necessary to make a simple calculation so that, even in the fantasy realm, a modicum of logical consistency can be achieved. The D&D Rulebooks have a habit of setting forth a regulation for standard coinage that is superficially simple, but not self-consistent. And this lack of self-consistency makes further reconciliation of the size with the mass of such coins actually quite problematic. Thus, this is one of the many areas where, when I manage a game, I institute my own house rules.
I will hand it to the rulebooks: the latest incarnation of standard coinage in the official rules is better that previous efforts. It still suffers in a number of ways, though, that a house rule can address.
First, the official rules offer a standard coin size that is usually completely impractical for a medieval economy, fantasy-based though it may be. The standard size, as shown on page 146 of the latest Player's Handbook, measures 1 and 3/16 inches (3 cm) wide. Now, this size of coin may well appeal to fantasy gamers, and in that sense, it's a great idea. But it creates a huge inconsistency in a variety of other areas that confound simplicity. Namely, that amount of precious metal is incredibly heavy in any significant bulk. And furthermore, it's not even self-consistent with the 50-coins-per-pound guideline set forth by the rules.
A slug of (mostly pure) gold that size, for example, would weigh nearly a full ounce avoirdupois (26.86 grams). If you're trying to figure out how many coins are in a chest of a given volume, and how much those coins weigh, it's easy to see how the game rules actually give you completely incompatible (and insolvable) mass vs volume calculations.
Second, a chunk of precious metal (silver, for example) that large, while flashy, is hardly likely to be encountered in common trade. It's entirely too large to be practical and one would expect that coins of much smaller size would be in use. Notwithstanding that the fantasy realm can have widely divergent standards, it is usually the case that such simple tropes as the manner of currency trade and precious metals economics are fairly consistent.
Third, the 50-coin-per-pound rule completely ignores the variable density (a significant difference) for different precious metals.
It would be helpful to have a standard coin size that was reasonably attested to in history and which could yield reasonable coin masses for various metal compositions. All the better if it is a coin size that is in common circulation, so folks had a tangible example they could see and feel.
In our own world, for example, Athenian silver "owl" tetradrachms were in common usage for centuries and their size was copied throughout later centuries even after that coin was discontinued. While coins were larger or smaller, they are the average size of coins in common usage. And, in fact, the US 5-cent piece (the "Jefferson nickel") is about exactly the same size: they are struck on blanks 21mm (0.83 inches) in diameter and 1.95 mm (0.08 inches) thick.
(This is rather thinner than the Athenian owls, but as it turns out, it makes for a mass about that of the Athenian silver stater, the next smaller sized coin in Athenian currency. Overall, this makes for very good standard size of coin.)
By using a house rule that makes the US nickel the standard coin size in my campaigns, it enables me to have a simple and self-consistent mass and volume for coinage for those times when it's needed.
Based on the above dimensions, the volume for a given coin is approximately 675 cubic mm. That converts to 0.675 cubic cm or 0.04 cubic inches. In one cubic foot, therefore, the maximum number of coins (assuming perfect packing with no gaps) is 1728 cu.in / 0.04 cu.in = 43,200 coins. Because coins do not pack perfectly, assume a conservative one-half volume efficiency for loose coins. Therefore, approximately 21,600 (or more) loose coins will fit in one cubic foot (regardless of the metal) and for our metric friends, 763 coins per liter.
Because of the density of metals varies, weights for the coins will also vary. Based on nearly 100% purity (not perfectly achieved, but a close enough approximation), coins would weigh approximately:
i. Platinum: (d=21.4 g/cm3)(0.675cm3) = 14.45g = ~32 per pound (or 71 per kg).
ii. Gold: (d=19.3 g/cm3)(0.675cm3) = 13.03g = ~35 per pound (or 77 per kg).
iii. Silver: (d=10.5 g/cm3)(0.675cm3) = 7.09g = ~64 per pound (or 141 per kg).
iv. Copper: (d=9.0 g/cm3)(0.675cm3) = 6.08g = ~75 per pound (or 165 per kg).
Thus, the house rule standardizing coinage on the Athenian tetradrachm model allows a ready sample of the coin size (the US nickel), a reliable volume calculation for bulk coinage and reliable mass calculations for bulk coins.
These figures aren't generally necessary to deal with for most of a campaign. But every now and then I'll get a question like "how many silver pieces can I fit in my bag of holding and how much would it then weigh?" With this reliable means of calculating coinage mass and volume, you can do a 30-second calculation on a scratch pad and give an answer that is entirely self-consistent and won't destroy the local economy.
What your players do with those coins once they get them back to town, well, that's their business...