Xanathar’s Guide to Everything (p. 77) has this to say about the rate of falling:

The rule for falling assumes that a creature immediately drops the entire distance when it falls. But what if a creature is at a high altitude when it falls, perhaps on the back of a griffon or on board an airship? Realistically, a fall from such a height can take more than a few seconds, extending past the end of the turn when the fall occurred. If you'd like high-altitude falls to be properly time-consuming, use the following optional rule.

When you fall from a great height, you instantly descend up to 500 feet. If you’re still falling at the start of your next turn, you descend up to 500 feet at the end of that turn. This process continues until the fall ends, either because you hit the ground or the fall is otherwise halted.

Now, imagine the following “cartoonish” situation : Bob the Unlucky Hiker is climbing a very high mountain and almost reached the top, when an evil Warlock surprises him and releases a Readied Repelling Blast that pushes Bob off the ledge, and he falls 400 feet down to the next “plateau” of the mountain, where he painfully lands (and stops moving). But today is not his day, and another evil Warlock surprises him right after he lands and releases another Readied Repelling Blast that pushes him off that new ledge and he falls for another 400 feet before painfully landing on another ledge. The process repeats itself again and again, because damn, Bob is an Unlucky Hiker.

Does the rate of falling optional rule apply per fall (making Bob suffer several crushing falls in a single turn), or per turn (making the second fall of the above scenario “pause” at 100 feet to continue at the end of Bob’s next turn)?

To each fall, because D&D doesn't do physics well

There is no provision in the rule for multiple falls per turn, so the rule is applied the same to each. The scenario given will (using that optional rule) go as follows:

After the first Warlock initiates the chain, it will follow these looping steps:

• Bob starts a (new) fall from one platform.




• As per that rule, when he starts falling he descends up to 500 feet.




• Bob lands on the platform 400 feet below. The fall is over.




• Next Warlock casts eldritch blast and pushes Bob off the platform

This repeats until we run out of Warlocks/Platforms, one of the Warlocks miss, or Bob dies and is a corpse rather than a creature (Repelling Blast requires a creature). At no point does the turn end. This is dumb. It is important to mention that D&D is not a physics simulator and edge cases (hah!) like this are where the DM is expected to use their common sense. A more rigorous rule might be:¹

At the end of each turn or when a creature starts falling, a creature that would fall descends until it lands or has fallen 500 feet since the start of its last turn.

However this rule is also likely to cause some problem in a specifically constructed scenario, though I haven't figured that one out yet.

It's worth emphasizing that this is an optional rule provided as a DM tool (as is my proposed one) and it might be more useful to think of it as how falling could be approximated inside of the initiative system and not a replacement of a DM's judgement.

1: If you think this rule is a bit convoluted, you may have found the reason the rule in XGtE doesn't try to be rigorous.

It's an optional rule to begin with. It's already DM's call.

Taken RAW, as soon as you start falling, you instantly drop 500 feet, and then hang motionless in midair until the end of your next turn. This has a number of bizarre and entertaining implications, but for our case, it means that, using this rule, RAW, Bob's going to hit every ledge on the way down, in that same turn. Each fall is interrupted before he hits that magical 500 foot pause marker, and each fall is a new fall.

This clearly fails the stated intent of the rule, however. ("If you'd like high-altitude falls to be properly time-consuming...") and pretty much any DM can see it. If they've already decided that in their campaign they want the falling rule, then it's probably because they do want falls to be properly time-consuming, which means that they are highly likely to Rule 0 this one (and, in this DM's assessment, would be correct to do so).

On the flip side, any DM who'd set up a repeating pattern of highly territorial warlocks just to keep knocking Bob off of cliffs is also one who's likely to be running off of Rule of Funny, at least to some degree. The real question, then, is whether it is more entertaining for Bob to make his ledge-assisted way down the mountain all in one go, or for him to be left haplessly hanging in mid-air for a turn between each impact, able to see his doom coming but unable to prevent it (possibly while frantically drinking healing potions).

It should probably be per-turn.

Since we're using the optional rule, in this case we're clearly concerned with how long it takes to fall. That being the case, the optional rule is talking about how much time it takes to fall a distance, and it shouldn't matter that there's a hard stop in the middle. The 500 foot limit should apply to all the falling that occurs during this turn, regardless of what else was happening during the turn.

There are good physical reasons why it needs to be per round, not per fall.

A little to the left of Bob is Carl the Slightly Luckier Hiker. Bob falls onto this ridiculous Mario Brothers sequence of ledges and warlocks. At the same time*, Carl falls from the same height, but hits nothing, and falls 500 feet in the first round.

Suppose we allow the 500-foot limit to reset for each fall, and Bob covers a total of 5000 feet in one round via successively getting blasted off ledges. What does this look like to Carl? They both jump, and suddenly Bob rockets toward the ground like Iron Man? What's propelling him?

Let's go full Galileo here and rope them together. Since Bob is separating from Carl at 4500 feet per round, he's going to break the rope almost instantly, long before reaching the first ledge.

Now Bob gets to the first ledge (while Carl is still just barely down the cliff!) and Walter the Warlock fires off an Eldritch Blast, and... misses. Well, now it's incorrect for Bob to have been going at 5000 feet per round in the first place, which means he should still be roped to Carl. So whether the rope breaks (within a fraction of a second of their jump) depends on whether the Eldritch Blast is going to hit (several seconds later).

It gets even sillier if they both hit the ledge and then Walter gets to decide which of them to blast, and whoever it is retroactively gets their falling speed increased tenfold.

Even worse than that, Walter can say "I ready an Eldritch Blast to hit the second person to land on this ledge" and now it's a true paradox, unless they both accelerate to Ludicrous Speed during the fall, which I'll admit makes as much sense as any of the rest of this scenario.

On the other hand, if we read the rule as "500 feet per round" then it's really clear narratively what's going on: Bob and Carl fell off together and they're falling together at a constant speed. If there's any interaction between them (say one of them wants to cast feather fall), the DM can simply assume they're at a fixed distance from each other for the entire fall. There's no crazy reverse causality due to invisible warlocks or hidden trampolines or anything else they might hit on the way down.

*If we're applying combat turns, then one of them is "first" in initiative order, but the difference in time is negligible. If this is hard to accept, then assume they get hit with thunderwave while on the edge of the cliff.

D&D "isn't a physics simulator" but it ain't a Roadrunner cartoon either.

This rule isn't supposed to be highly accurate (falling objects don't really have constant speed, of course). It's supposed to produce intuitively plausible results while also not requiring anyone to get out a calculator. For that purpose it's good enough.**

But allowing a creature to fall an unlimited distance in one round because it's divided into "multiple falls" isn't even intuitively plausible. Basic kinematics is intuitive for those of us playing the game--we're descended from tree-climbing apes and we instinctively know how falling works. We were throwing spears at antelope for a long time before Isaac Newton came along and explained it with math.

My point is that if you're applying this rule in a way that produces nonsensical results, then it's not achieving its purpose and you really shouldn't bother with it.

** The usual quoted values for terminal velocity of a human are 50-70 m/s, which is 1000-1400 feet per round. 500 feet per round is not a bad estimate for the first round, though, and honestly you probably don't care much beyond the first round. The question in most cases is "do I have time to cast a spell / deploy my parachute / transform into a hawk, or not?"

Optional Rule as Written, per turn. Actually, Optional Rule as Written doesn't apply here, so we're only using the intent of it as a guide.

The intent of the optional rule is clearly stated: "If you'd like high-altitude falls to be properly time-consuming" while discussing a fall distance that may span multiple turns. Furthermore, it justifies the optional rule by saying "Realistically". The optional rule is thereby intended to guide fall distance based on a sense of realism. In a "realistic" sense, multiple 400' falls are the same cumulatively as a long high altitude fall.

The unit of time in DnD 5e combat is the round. See How does time pass in combat?
This question explains that all actions are simultaneous-ish, with rounds and initiative being management tools for actions in game space. The optional rule is to guide how far to fall per combat round.

QED, it has to be per PC turn.