Coinage

A standard [d20] coin weighs about a third of an ounce, so fifty coins weigh a pound.

Coin	cp	sp	ep	gp	pp
Copper (cp)	1	‌1⁄10	‌1⁄50	‌1⁄100	‌1⁄1000
Silver (sp)	10	1	‌1⁄5	‌1⁄10	‌1⁄100
Electrum (ep)	50	5	1	‌1⁄2	‌1⁄20
Gold (gp)	100	10	2	1	‌1⁄10
Platinum (pp)	1000	100	20	10	1

Background

Original D&D—“Basic” through Rules Cyclopedia—coins have an encumbrance of ∼‌¹⁄₁₀ pound each.

More specifically, encumbrance (enc.) is a combination of the weight and clumsiness of gear measured in coin-weight equivalents (cn), where 1 𝖼𝗇 equals approximately ‌¹⁄₁₀ of a pound in weight and awkwardness. For example, a ‌¹⁄₂ lb wineskin filled with a quart of water/wine (∼ 2 lb), would only weigh ∼ 25 𝖼𝗇, but has a total enc. of 30 𝖼𝗇.

Other units

A d20/D&D mile (𝗆𝗂^𝖣) is 6,000 feet, and thus apparently a fantasy equivalent to the ≈ 6,076-foot nautical mile rather than the 5,280-foot land mile.

A nautical mile (𝖭𝗆𝗂) is one minute of latitude (‌¹⁄₆₀1°), making Earth’s meridional circumference 21,600 𝖭𝗆𝗂. Since Earth is not a perfect sphere, the actual length of a minute of latitude varies between 1,813 𝗆 and 1,861 𝗆 with a de jure average of 1,852 𝗆 (≈ 6076.12-ft).

The original intent for the meter was to be one ten-millionth of 90° of latitude, making Earth’s m.circ. 40,000 ㎞. The best measurements at the time were slightly off and Earth’s actual m.circ. is ≈ 40,003 ㎞.

From this one can extrapolate that a standard D&D fantasy world could be:

1. a 21,600 𝖭𝗆𝗂 ≈ 40,003 ㎞ m.circ. spheroid like Earth,
2. a 21,600 𝗆𝗂^𝖣 ≈ 39,502 ㎞ m.circ. spheroid with 2½% less surface area than Earth, or
3. a 21,600 𝗆𝗂^𝖣 = 40,000 ㎞ m.circ. sphere with D&D feet/inches etc. 1¼% longer than standard feet/inches.