The PCs ask: "Can I lift the life-sized gold statue?" "How much food will fit in the castle's cellars?" "How many books in the library?" "Can I carry the boat on my back?" "How many coins fit in my backpack?" Dammit, now you have to do calculations.
Here are six numbers that you can learn to make a ballpark estimate on nearly any weight/volume question. These are useful for quick plausibility calculations when the players try something you didn't expect. We want memorable numbers, accurate within 20% or so.
The First Number: Density of water: The first fact you need to know is that a cubic foot of water weighs around 60 pounds. Water is good because the density of the human body is virtually identical to that of water, and we all have a good intuition about how big, say, a 180# person is compared to a 120# person.
Numbers two through six: Density of Everything Else: There are basically six materials that are useful for D&D. Their weights are given in multiples of water.
|60#||cubic foot of water|
|2/3x||Things that float|
|20x||Gold and platinum|
Now the details:
Water: Lots of things are around the same density as water, especially things that are mostly water: wine, ice, people, orcs, dragons, and raw meat. Also leather (a meat byproduct), vellum (a leather byproduct) and paper (a vellum competitor) are around the same weight as water.
Everything that floats: If it floats, and you can use it in D&D, it's probably around 2/3 the density of water: 40# per cubic foot. Oil, wood, cloth, and common medieval foods like wheat, beans, vegetables, and dried meat are all around this weight. The few lighter substances (sawdust, snow, feathers) don't come up that much in D&D.
Dirt: Dirt includes clay (and its byproduct bricks), sand (and its byproduct glass) and soil.
Stone: If you're stoned by a medusa, your weight is multiplied by 3.
Most metal: Metal weight varies: steel is around 8x the density of water; copper and silver are around 9x; lead is 11x. 10x is a convenient and memorable average.
Gold and platinum: The really valuable metals are around 20x the density of water (gold 19x; platinum 21x).
Loose packing: Keep in mind that these densities are for solid, or relatively solid, materials. If something is crated, barreled, shelved, packed with straw, loosely piled, or stacked, multiply its volume by x2. If you need aisles, such as in a storeroom or library, multiply by another x2.
Examples: From these six numbers, and the loose-packing estimates, you can easily calculate the following: