How are the values derived in the 1e Ad&d DMG for encumbrance? Encumbrance values are given in gold pieces, e.g. a large metal bound book has an encumbrance of 200 gold pieces.
DMG p.225 states
The encumbrance of most items not on this list may be inferred by comparison with objects similar to them; thus a decanter of endless water will encumber as much as a bottle or flagon. In some cases no equivalent may be found on the table; such instances require the judge to decide.
It then goes on to clarify that encumbrance is not merely weight:
Many people looking at the table will say, "But a scroll doesn't weigh two pounds!" [A leather scroll case has an encumbrance of 25 coins, a coin weighs 1/10 lb in 1e AD&D] The encumbrance figure should not be taken as the weight of the object -- it is the combined weight and relative bulkiness of the item. These factors together will determine how much a figure can carry.
The relative bulkiness mentioned here is defined as volume in the PHB p.101
Whatever you select to carry will have both weight and volume (or bulk)
In the Dragon Magazine #80 article "How Many Coins In a Coffer?", gives the only "official" value for the volume a gold coin takes
A tenth of a pound (about 45.36 grams) of any coin metal, therefore, would have a volume of 2.9 cc or 0.177 cubic inch.
So my theory for calculating an encumbrance is to look at two of the items values: weight and volume (or bulk). Convert each of these values into gold piece equivalents. Encumbrance equals whichever is higher.
But this doesn't seem to match up with by-the-book encumbrance values.
E.g. A backpack by-the-book has an encumbrance of 20gp. That indicates either the backpack weighs 2 lbs and has a volume less than 20 coins, or it has a collapsed volume of approximately equivalent to 20 US silver dollars. OK fair enough, 2 lbs seems reasonable, although it seems like i should have a bigger volume, but maybe the cloth is really thin making it compact when collapsed.
What about the infamous 10' pole? By-the-book it has an encumbrance of 100 gold pieces, which means it either weighs 10 lbs and has a volume of less than 100 gold pieces, or it has a volume of 100 gold pieces and weighs less than 10 lbs. Again, the volume estimate seems to small. Assuming for convenience sake that the pole is 1.5" in diameter (the same as a coin according to Dragon #80) it would have a volume of 1200 coins! (Dragon #80 calculates that each coin is 1/10" thick).
So it seems my theory isn't right. How did Gary get these values?