Weight is an item property, and is measured in units Γ, also known as urists - one unit urist is equivalent to one real kilogram.
Weight is largely used by the game to determine how fast someone or something should move, due to encumbrance effects and carry weight restrictions; to calculate damage, since object mass factors into impact momentum; and to some extent to make temperature calculations, as mass factors into heat capacity. Note that, for DF, weight and mass are currently indistinguishable because as it stands DF gravity is uniform both across the whole universe and from world to world and buoyancy isn't implemented.
Weight is displayed as a quantity associated with an item. Weight calculation is not entirely straightforward, since each item category - or sometimes even individual items - have their own formula for determining weight.
 Weight Calculation
The general form for calculating weight in DF is to take the material density (as found in the raws) multiplied by the item's volume.
- Weight (in Γ) = Density * Volume / 100,000
Items also have a "bulk" which does not depend on the material and is used to determine clutter as well as how many items can fit into a container.
|Corpse||Special||Special||Based on the size of the corpse and what it's made of|
|Weapon||SIZE / 10||6000|
|Body armor||?||9000||Based on UBSTEP, LBSTEP, COVERAGE, LAYER_SIZE, and the race that crafted it|
|Shoes||?||1800||Based on UPSTEP, COVERAGE, LAYER_SIZE, and the race that crafted it|
|Shields||?||6000||Based on UPSTEP and the race that crafted it|
|Helms||?||3000||Based on COVERAGE, LAYER_SIZE, and the race that crafted it|
|Gloves||?||1200||Based on UPSTEP, COVERAGE, LAYER_SIZE, and the race that crafted it|
|Box||2000 or 100||6000||60000||Bags weigh 100, boxes/chests/coffers weigh 2000|
|Ammo||SIZE / 10||600|
|Body part||Special||Special||Based on the size of the body part and what it's made of|
|Live vermin||Special||Special||Based on creature's adult size|
|Tame vermin||Special||Special||Based on creature's adult size|
|Thread||ceil(Dimension/500)||6000||20 for freshly extracted/gathered thread|
|Cloth||ceil(Dimension/500)||6000||30 for freshly woven cloth|
|Pants||?||1800||Based on LBSTEP, COVERAGE, LAYER_SIZE, and the race that crafted it|
|Ballista arrow head||1000||6000|
|Small rock||200||600||As thrown by adventurers|
|Tool||100*||3000||CAPACITY||Broken - supposed to be SIZE/10Bug:4979|
|Egg||Special||Special||Presumably the caste's EGG_SIZE|