Encumbrance: Weight and Size

The rules in D&D 5E for encumbrance, weight, and size are very generalized.  I've seen the rule generalized as:  "It is absolutely 100% true that almost no character will ever, in their class' native gear, need to worry about carrying too much. And the heaviest armors have STR prereqs anyway, which makes basic gear even less of an issue." (Deliverator) That's a fundamental feature of the most basic encumbrance rule:

Carrying Capacity. Your carrying capacity is your Strength score multiplied by 15. This is the weight (in pounds) that you can carry, which is high enough that most characters don't usually have to worry about it.

Push, Drag, or Lift. You can push, drag, or lift a weight in pounds up to twice your carrying capacity (or 30 times your Strength score). While pushing or dragging weight in excess of your carrying capacity, your speed drops to 5 feet.

Size and Strength. Larger creatures can bear more weight, whereas Tiny creatures can carry less. For each size category above Medium, double the creature's carrying capacity and the amount it can push, drag, or lift. For a Tiny creature, halve these weights.

Variant Encumbrance

That rule is super simple and so unsatisfying that the very next section of the PHB offers another approach in Variant: Encumbrance:

If you carry weight in excess of 5 times your Strength score, you are encumbered, which means your speed drops by 10 feet.

If you carry weight in excess of 10 times your Strength score, up to your maximum carrying capacity, you are instead heavily encumbered, which means your speed drops by 20 feet and you have disadvantage on ability checks, attack rolls, and saving throws that use Strength, Dexterity, or Constitution.

That variant adds three tiers to encumbrance and makes it a very real consideration for many characters. So much so that a lot of characters will be trekking around encumbered which typically makes a lot of players (and DMs) just grumpy.

Effect of those Rules

The rules make encumbrance something that is rarely a worry.  That is a good thing.  It is an important thing A couple of observations:

• Size category doubles/halves capacity, sort of.  Medium and Small are normal.  Large is 2X, Huge is 4X, Tiny is 0.5X.  That makes things scale a bit, but not much.
• Flying is unaffected by carrying capacity until 15xSTR rule is triggered.
• Gear of various sizes doesn't change weight.

I mostly don't want to care about weight/encumbrance.  It just isn't fun. There are three cases where I think it becomes interesting/important:

1. How much loot can be carried? (satisfied by standard rules)
2. How much weight can be carried while flying?
3. Will that rickety bridge collapse when the Halfling carrying gear collapse? 

I'll dive deeper into the last two in following sections.

How much weight can be carried while flying?

I'm mostly interested in a situation that comes up a lot in my campaign based on the Princes of the Apocalypse module: Creature carrying some things hops on a Giant Vulture and flies away.

The Giant Vulture is a large beast and has an anatomy that can accommodate a rider (last part perhaps more by DM fiat than biology) so it can carry a medium or smaller rider per RAW.  It has a strength of 15 and is large so it can carry 15 x 10lbs x 2 (large) = 300lbs.  That gets us to a capacity of one normal-ish character with some gear. The bird can't carry two full sized people unless they are bith rather lightly built. That leaves me pretty well satisfied.  Yea!

Now, consider a gnome passenger on the same bird.  The gnome is size small, 3 to 4 feet tall, and might weigh 40-50 pounds.  Our little gnome might be a fighter with gear easily in excess of 120 pounds.  Our gnome might come in at 150 pounds, so two can be carried, but it is a close thing.  I'd expect at least two gnomes on the vulture, so things seem ok; although that gear weight the same as medium sized is a bit annoying.

Next up, we have a 200lb human under the effects of the Enlarge/Reduce spell.  If she is reduced, she is now a small creature who weighs 25lbs though her gear seemingly retains its old weight, say 100lbs.  My Giant Vulture can carry 6 miniaturized naked humans or two with gear.

Heads start to spin if you consider that a suit of tiny plate armor weighs the same as large plate armor. On the other hand, generalizing this is a lot more work than it is typically worth.

The rules pretty much do what I'd want as long as we stay pretty much in the medium range.  The weight of very big or small gear is likely best left to DM discretion on a case by case basis.

Physics Note: Mass of an object changes by a factor of 8 when it's dimensions change be a factor of 2.

Will that bridge collapse?  

This type of situation puts a laser focus on the weight of gear.  When we consider sizes RAW can be infuriating here.

Consider a human (15STR = 225lb carrying capacity) fighter (240lbs + 120lbs of gear = 240lbs) with a gnome mage (30lbs + 30 lbs of gear = 60 lbs), they also have 150lbs of loot they have been lugging around.  They come to a rickety bridge.  They are concerned the bridge will collapse, so the mage casts reduce on the fighter, dropping his weight by a factor of 8. The fighter now weighs in at 150 lbs (30lbs of fighter + 120 lbs of gear) which is perhaps too much.  The fighter strips down tying his gear into a bundle using his now 25 foot long rope (it was 50 before the reduce).  The mage also strips down, adding his gear to the pile and attaches his rope.  The two of them, now nekid and pushing the scales at 30lbs each proceed to walk across the bridge.  They then pull their 300lbs of gear and loot onto the bridge which collapses.

Oh no, you think, the rope is now holding 300lbs being held onto by only our fighter who weighs 30lbs.  No problem.  Our small sized fighter with a strength of 15 can still carry 255 pounds.  But that pack dangling from a rope is 300 pounds, surely he must let go or tumble after the gear into the chasm.  Nope, no problem, lift capacity is 2X carrying capacity.  Our mighty mite can lift 500 pounds so can easily pulls up the package.


This highlights two problems with RAW.

1. Changing carrying capacity by 2x other than small/medium,
2. Gear weight unaffected by size.

I'm inclined to let the first issue pass, other than pointing out DM discretion if it violates a narrative.

The second one may merit some more thought.

Weight Change with Size

If a solid block of matter, say a 1x1x1 foot cube is doubled in all directions and its desity doesn't change it becomes a 2x2x2 foot cube and weighs 8 times what it used to.  This is exactly what the Enlarge/Reduce spell is getting to with it's weight change.

Now consider a 1 foot radius sphere that is hollow.  It is only a thin skin filled with air.  This might be thought of as some oddly shapped armor.  We'll say it's thickness is negligible in comparison to its other dimensions.  Now, double its radius and we have a 2 foot radius sphere with still negligible thickness.  Assuming density remains the same, the weight of the new object is driven by the relationship in surface areas. The formula for area of a sphere is four x pie x radius squared. Our second sphere has a surface are 4 times larger (2x2) and therefore weighs about 4 times as much.

If we go back to a 1x1x1 cube and imagine it doubling in only one dimension, that is become twice as high, perhaps.  It becomes a 2x1x1 object with twice the volume and 1 2/3 the surface area.  So it's weight increased by 2 if solid and 1.67 if hollow with negligible, unchanging thickness.

This gets me to a wildly approximate rule of thumb:

1. Doubling an object size in one dimension doubles its weight.
2. Doubling an object in three dimensions changes its weight by factor of 8 (solid) to 4 (hollow).

Saying weight changes by a factor of 4 with size might be generally useful as the heaviest object (armor) is hollow, though a lot of weight is in solid mass of heavy things that hit things.

The problem with changing weight with size might be the change in carrying capacity that varies by a factor of two. On the small side, its not too bad.  A tiny "human" can carry 1/2 of what a small "human" can tote.  His tiny plate armor would way 1/4 of the small plate armor so he can carry twice as much, some ant man feeling.  The problem is a large "human" who has a carrying capacity increased by two while his gear would go up by four.  That 65lb plate armor that needs a STR 5 to be hefted at medium size, would become 260lbs and need STR 9 at large.  If weight changes with a factor of 4 per size, while capacity only changes by 2, we can easily pin a large creature to the ground by casting enlarge on it.  That's no good at all.

So, while I believe weight should vary by 4 to 8 times with category, having it change by 2 at least causes things to vary in the correct direction and eliminates the need to deal with changes to carry capacity if a creature changes size with gear on them.

The basic weight of gear varies in the same manner as carrying capacity.  Example: Tiny Plate weighs half of Small/Medium Plate armor.   The DM is expected to make adjustments as appropriate in weight critical situations.