Falling damage based on creature size

Question

The party is on the second story of the Great Fane of Lolth and is running low on spells. But they do have a wand of polymorph and a great number of open windows. For whatever reason they typically choose to polymorph enemies into slugs, and I can imagine them pitching slugs out the windows for carry-over fall damage. However, I have a difficult time with slugs taking much in the way of fall damage because of real world surface area to volume ratios.

While this specific situation in this week's game session is the impetus, I have long wondered about having fall damage scale with size, as it does in real life. Perhaps because I come from a 1e background, where standard monster HD were d8's, but I only just now realized that since creature size determines HD in 5e, there is a simple and ready-made way to do this. If we take fall damage to be d6 per ten feet (so as to change as little else of the falling rules as possible) and decide that damage should be unchanged for Size Medium (whose HD is d8), then the way I am considering implementing falling damage is the following house rule:

Falling creatures take damage based on their creature size. Falling damage is rolled on the next die lower than the one rolled for the creature's HD.

For creatures that have class-based HD, falling damage nevertheless scales with their size-based HD, as given in the HD by Size table.

This would result in the following:

Creature Size	HD	Falling Damage (per 10 feet fallen)
Arbitrarily smaller than Tiny	none/d1	no damage
Tiny	d4	d2
Small	d6	d4
Medium	d8	d6
Large	d10	d8
Huge	d12	d10
Gargantuan	d20	d12

My goal is have a large [edit: targeted] increase in verisimilitude at the cost of a small increase in complexity and administrative burden, while still tethering the rule to other game features so that it integrates well rather than just being an independent system.

My question is whether / how well you think this house rules proposal meets this goal.

If you have additional thoughts on unexpected balance changes I would be interested in those as well. For example, it is immediately obvious that this would reduce the danger from falling for size-small PCs, but for the small difference it would make I am ok with that.

Related: Does the rule on falling damage only apply to Small and larger creatures?

Is there any damage that changes with the size of a creature?

Answer 1

This is too complicated for little gain

With the D&D falling damage rules, verisimilitude went out of the window a loong time ago. I heard stories that Gygax originally intended a cumulative ladder (1d6 for 10 feet, 2d6 more for the next ten, 3d6 more for the third ten and so on). We instead got linearly increasing damage and 9-feet falls that have no chance to even sprain your ankle, in exchange for a simple formula we all can easily remember. Probably for the better.

If your rule's goal is to have an increase in verisimilitude, at a small increase in complexity, I think it unfortunately fails on both counts:

• This has some increase in verisimilitude in that it deals less damage to smaller creatures, which matches our daily experience, especially for extremely small and light ones like insects. But it does not address the issue that you survive ridiculously high falls without even being bloodied as a medium creature with enough class levels, in fact it makes that worse for, say, a small fighter that uses d10 for hit dice, decreasing verisimilitude there.

• This is not just a rule, it's a whole table with three columns you've got to memorize. Or look up. You as an experienced DM might know the HD by size by heart, but many new DMs won't and will have to memorize this table for medium and small PCs, or learn it from the MM by inference or the monster design rule in the DMG. The game also next to never uses d2 (only in the variant:demon summoning rule), while a lot of magic items use d3, so I would have expected d3 to be the next smaller die after d4, not d2. That also needs to be learned if you provide a table, or is unclear in the rule. I think this is too complicated for this little gain.

You had to edit your question to clarify that you intended to always base the damage on size, not on actual hit dice of the creature, so this is so complicated that you originally did not manage to express the rule unambiguously.

If you yourself find this is simple to use, then go ahead and use it, it's not going to break the game. But for a general rule, this extra baggage is not worth it.

If you want a simple rule, just rule that sub-tiny creatures (snails, cockroaches and the like) never take more than d6 damage from the fall.

Answer 2

I agree entirely with @Nobody's answer (that this is excessively complex without adding sufficient verisimilitude).

If you do decide to go with a similar system to this, however, I would strongly suggest that Small and Medium creatures both take the standard level of fall damage. This is in line with other strongly player-facing rules treating them the same in 5E, and means that your rule change is irrelevant to the vast majority of cases that the players need to consider. The players are then incentivised to knock giants off cliffs and so on without needing to memorise how fall damage affects their PC.

Answer 3

The rule seems reasonable

Personally, I don't have a problem with the proposed house-rule. It's a legitimate oversight that the D&D falling rules were calibrated to humans, and don't take into account the large real differences in threat from falling for differently-sized creatures. There's also a tradition of looking to tweak rules like falling for greater realism, even if variant rules never "stuck" in mainline D&D. See in particular Dragon Magazine #88 (August 1984), which is largely devoted to dueling variant falling rules, and the motivating physical theories to support each.

The one added tweak I'd make is to just waive any damage at the Tiny level or smaller; which conveniently limits any of the damage levels to existing Platonic dice. The biologist J.B.S. Haldane has written (quote from Wikipedia):

To the mouse and any smaller animal [gravity] presents practically no dangers. You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away. A rat is killed, a man is broken, a horse splashes. For the resistance presented to movement by the air is proportional to the surface of the moving object. Divide an animal's length, breadth, and height each by ten; its weight is reduced to a thousandth, but its surface only to a hundredth. So the resistance to falling in the case of the small animal is relatively ten times greater than the driving force.

See more on my old-school D&D blog.

Answer 4

This is an underestimate of the difference in damage you would expect with larger creatures.

In the 1926 essay "On Being the Right Size" by J. B. S. Haldane, he recounts how a mouse that falls down a mineshaft would survive despite a mouse being so small the slightest injury would end it but a horse that fell the same distance wouldn't just die but "splash" (his words, it was the 1920s).

Because the larger a creature is the more mass is above the point of impact so at any given velocity the impact with the ground will impart greater crush forces even though a larger creature is more sturdily built its weight works against it. A larger creature will also be slowed less by air resistance.

So I'd expect damage to be so much greater that even small falls would be very dangerous to large-size or larger creatures. Maybe instead of increasing the damage per 10ft, instead narrow the increment.

A medium creature like a big dog should be able to safely drop 5ft but a 5ft drop for a horse is not safe, it stands a serious risk of injury. For a huge creature like an elephant even a 2 foot drop risks injury and 4ft is a much greater risk.

Without this you may have strange phenomena where a horse can safely drop 9ft but 10ft suddenly takes huge damage.

Another reason this helps is that it's easy to get large numbers of d6 dice, so if you do need to calculate a lot of fall damage you can pour a load of d6 onto the table.

[EDIT]: To be clear, it would not significantly add to the realism or verisimilitude to have such a small change in fall damage such as going from 1d6 to 1d8 or 1d4 damage per 10ft with Small or Large size. The fall damage is only a 28% difference on average when on impact a large creature hit with double the mass per surface area, or half for small creatures. Damage should double or halve respectively.

The main gameplay effect would be to annoy the players that a different set of dice than usual would be used, particularly a type that people are less likely to have multiples of. It's a lot easier to get a great number of d6 dice than any other type of dice.

A design ethos for 5e is to avoid table lookups as much as possible but just memorize iterations on the rules. Yes, this is broken with the exhaustion table, and it's considered a flaw in 5th Edition's design that's a typical topic of discussion of how to correct it.

Answer 5

I don't think this is a good idea

From my experience, any house rules need to be carefully thought out in terms of the benefit they provide vs. extra complexity and things to memorise. In this case I don't think it's worth it.

You say your goal is

a large increase in verisimilitude at the cost of a small increase in complexity and administrative burden

but it doesn't really increase verisimilitude in a meaningful way and gives you a whole table that you will have to look up any time falling damage comes up so it's kinda the reverse of what you want - a small (if any) increase in verisimilitude at the cost of a large administrative overhead.

D&D is not supposed to be a real world physics simulator. Falling rules (and a ton of others) are simplified on purpose. Real world falling damage doesn't scale linearly in neat 10ft increments to begin with, so if your goal is to reflect real physics here, that goes out of the window before you even begin.

And then you have this thing with both creature size AND hit dice, which is needlessly complicated and ambiguous. What if a Small creature has a d12 HD, which row do you pick then?

If you really have such a big problem with slugs taking fall damage that it ruins the game for you, just make a rule that any creature smaller than Small is has resistance to falling damage and only takes half. I'd still say it's not the best idea but at least better than having a whole table for it.