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 |
| 2x | Dirt |
| 3x | Stone |
| 10x | Most metal |
| 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:
References:
http://www.engineeringtoolbox.com/specific-gravity-solids-metals-d_293.html
http://www.engineeringtoolbox.com/density-materials-d_1652.html
http://go.key.net/rs/key/images/Bulk%20Density%20Averages%20100630.pdf
http://www.loc.gov/today/pr/2002/02-164a.html
We need more posts like this, if we are going to expect “old-school” style GMs to make rulings on the fly about the material world (or new school ones to recognizes when their meticulous simulation rules part company with reality).
This should be included in every DMG. =)
Makes me think of a puzzle where a player in the party has to voluntarily petrify themselves to solve a see-saw balance puzzle. Or maybe bait an angry, large, and dumb monster into the right spot to use them as a counter weight.
Very useful stuff, thanks. I’ve had more complicated measures for weight by density and size, but this is more immediately game useful even if it’s just close rather than exact.
You know what would really rock? A similar system resolving conflicts of force (number of small/big/huge people pushing/bashing/kicking) versus resistance (to bending/breaking/lifting/pushing) (of objects by material and weight/thickness).
This is very nicely put together.
I also agree this should be in every game manual. And love Jeff’s idea about using it for a puzzle!
This is very neat, especially since it can fit in a little tiny card or the like.
Of course, we could know the meaning of density and use a website like SI Metric.
Sure, but I don’t like using websites while I DM.
Regarding statues, very few of them would be solid. Even the wonderful bronzes from Greece to the present day are all hollow, and ring akin to a bell if struck.
Super useful, thanks for sharing.
A DM screen with this and all of the other handy tables and “X on a business card” you’ve made would be the bomb.
I’ve got an idea for just such a thing! It’s in the works!
Do you mind if I re-post this on my website (with full accreditation)? This is a super valuable piece, and I want to be sure I don’t lose it.
Go ahead!
Just a pedantic point – one cubic foot of rain water is 1000oz which is 62½ pounds. In the UK this is exactly 50 pints.
The 1000oz figure is interesting as it first shows up in the 1300’s AFAIK and suggests someone was making an effort at systematising.
I think you got your (‘) and (“) symbols mixed. Your oak table would only be a fooby four inches by one inch.
Sorry. I don’t know where fooby came from (measurement used in Brobdignag?). I meant a foot by four inches by one inch. It’s very dark here in Australia as the bushfire smoke is occluding the light.
One nitpick: for the castle storage, that should be “You could theoretically pack 25k CUBIC feet in there” …
Excellent article!
OMG, your blog was mentioned by Phil and Kaja Foglio. Too bad that they didn’t understand much of it. They wrote that a robot looking like a human should weight 900 kg! Maybe, if it would be like T-1000 from the Terminator 2 movie 😀
Phil, Kaja – reserve some space for internal mechanisms, not everything is made of solid metal!
Anyway, to the authors of this blog. Please use metric units, no one uses retarded feet or pounds anymore, c’mon guys, be serious 😀
Those of us who have been playing since the ’70s, and never fully embraced the metric system, still use Pounds, feet and inches when DMing, or just about anything else.
I’m a little late to the party, but this is great! Big thank you to the authors if you guys are still around. Wonderful post.
just under a decade later…
in the examples, you mention the weight of a table and then a boat.
as a woodworker, i can confirm a 12’x4′ oak table is in fact about 160-180 lbs. the boat… your measurements come from mostly kiln dried wood i think. wood can hold almost its own weight, again, in water. boats hold between 40-60% moisture content, when floating. the 12’x4′ boat likely uses more wood but even if we say it doesn’t thats +250lbs. likely more.
regardless of ANY of what i said!
THANK YOU!!! this page is sooooo helpful! thank you so so much!!