I recently was in a campaign with a Goliath Fighter, who possessed both a ring of jumping and boots of striding and springing. His possible jump height increased dramatically as you would expect. I then ran into a little rules problem:
If your Strength based jumping ability (magic factored in) exceeds your available movement speed, what happens?
Is your jumping ability automatically reduced to your availabile movement speed? (my character’s possible long jump of 162 would be reduced down to 20, 50 if he dashes)
Does your turn end with you still in the air, having traveled a distance equal to your available movement speed, and your trajectory continues until your cumulative movement equals your possible jump? (It would take a total of 6 rounds for my character to land after long-jumping)
Does something else happen?
Furthermore, how does fall damage factor into this? Is my character capable of absorbing the impact force using the same magically-enhanced musculature as he used to jump? Does he take any damage from lift-off due to the reaction force of the ground on his legs? What about other times, when he falls for reasons other than jumping? Could he use the fall damage to hurt enemies (jumps up, arcs, and swings his war-hammer into an enemy on the fall)? Would he take some of the damage if he did this?
FURTHERMORE, he is also a Rune Knight and one of his party-members has Enlarge/Reduce. How would a size change effect my character’s possible jump height?
Both types of jump (long/high) says "each foot you clear on the jump costs a foot of movement." so no jumping further than your speed.
As for the question about fall damage that is a somewhat contested issue but most would agree that if you can jump a certain distance then you can land unharmed at the end too. Of course if the jump turns into a fall at some point then normal rules for jumping apply.
When you run out of movement on your turn you cannot move any further that turn. I know this is obvious and self evident but it is worth noting as it helps understand some of the possible rulings when you attempt to jump further than you can move. I typically see it ruled that once you run out of movement your jump ends and you fall any relevant height. I could see a generous DM ruling that you remain in the position you ended your turn on and then must spend all available actions possible continuing the jump you had started. I don't think I have seen this ruling argued for before though. Almost always I see this ruled as your jump ends and you fall.
Do you take fall damage if you jump more than 10 feet high? This is something that gets debated on occasion on these forums and I don't see the same level as consensus as I do regarding what happens when you run out of movement. I see most people rule that the fall starts at the apex of your jump. There is some suggestion that this is RAW as the rules for High Jumping describe that it only costs movement for each foot you ascend. This implies that on the way down you are falling, which doesn't cost movement. However, I have seen it argued that a jump is not considered a fall and usually this is based off of a tweet from Jeremy Crawford. A quick google search however shows that this is something JC has expressed conflicting views on.
Remember that D&D is a game, not a physics simulation. Many of the rules in D&D do not make sense from a physics perspective. For example, the base rules for falling do not specify a maximum speed, and the Rate of Falling rules from Xanathar's Guide to Everything simply cap it out at 500ft per 6 seconds regardless of what is falling.
With respect to falling on a creature, this was something that was addressed in Tasha's Cauldron of Everything. In short the creature being fallen on must make a DC 15 Dexterity save and on a fail the creature is impaced by the falling creature. When this happens the fall damage is split between the falling and impacted creatures and the impacted creature is knocked prone if it is only one size category bigger than the falling creature or smaller.
There is no mechanical interaction between creature size and jumping in the rules.
As Thezzaruz said each foot you clear on the jump costs a foot of movement. Funny i remember asking exactly this to the Dev on Twitter when 5E released in 2014; ;)
@Plaguescarred Can you jump farther than your movement when using Jump & Boots of Striding and Springing?
@JeremyECrawford To be clear, things like the jump spell don't increase speed. You can jump crazy far, but your speed caps it.
@JeremyECrawford Every foot jumped costs movement, so you can jump farther than your current speed if you take the Dash action.
For example, the base rules for falling do not specify a maximum speed, and the Rate of Falling rules from Xanathar's Guide to Everything simply cap it out at 500ft per 6 seconds regardless of what is falling.
Just as an aside, 500 feet is about how far a creature would fall in 6 seconds if it started its fall with no momentum on Earth gravity. However, on subsequent turns, that would be higher if this were 1:1 physics. Not disputing anything, just pointing out a fun fact (which is likely where they got it from in the first place).
I recently was in a campaign with a Goliath Fighter, who possessed both a ring of jumping and boots of striding and springing. His possible jump height increased dramatically as you would expect. I then ran into a little rules problem:
If your Strength based jumping ability (magic factored in) exceeds your available movement speed, what happens?
Is your jumping ability automatically reduced to your availabile movement speed? (my character’s possible long jump of 162 would be reduced down to 20, 50 if he dashes)
Does your turn end with you still in the air, having traveled a distance equal to your available movement speed, and your trajectory continues until your cumulative movement equals your possible jump? (It would take a total of 6 rounds for my character to land after long-jumping)
Does something else happen?
Furthermore, how does fall damage factor into this? Is my character capable of absorbing the impact force using the same magically-enhanced musculature as he used to jump? Does he take any damage from lift-off due to the reaction force of the ground on his legs? What about other times, when he falls for reasons other than jumping? Could he use the fall damage to hurt enemies (jumps up, arcs, and swings his war-hammer into an enemy on the fall)? Would he take some of the damage if he did this?
FURTHERMORE, he is also a Rune Knight and one of his party-members has Enlarge/Reduce. How would a size change effect my character’s possible jump height?
Both types of jump (long/high) says "each foot you clear on the jump costs a foot of movement." so no jumping further than your speed.
As for the question about fall damage that is a somewhat contested issue but most would agree that if you can jump a certain distance then you can land unharmed at the end too. Of course if the jump turns into a fall at some point then normal rules for jumping apply.
The TLDR for all of this is really going to be just talk to your DM about it.
In general, jumping costs movement as outlined in Chapter 8: https://www.dndbeyond.com/sources/basic-rules/adventuring#Jumping
When you run out of movement on your turn you cannot move any further that turn. I know this is obvious and self evident but it is worth noting as it helps understand some of the possible rulings when you attempt to jump further than you can move. I typically see it ruled that once you run out of movement your jump ends and you fall any relevant height. I could see a generous DM ruling that you remain in the position you ended your turn on and then must spend all available actions possible continuing the jump you had started. I don't think I have seen this ruling argued for before though. Almost always I see this ruled as your jump ends and you fall.
Do you take fall damage if you jump more than 10 feet high? This is something that gets debated on occasion on these forums and I don't see the same level as consensus as I do regarding what happens when you run out of movement. I see most people rule that the fall starts at the apex of your jump. There is some suggestion that this is RAW as the rules for High Jumping describe that it only costs movement for each foot you ascend. This implies that on the way down you are falling, which doesn't cost movement. However, I have seen it argued that a jump is not considered a fall and usually this is based off of a tweet from Jeremy Crawford. A quick google search however shows that this is something JC has expressed conflicting views on.
JC explaining that a high jump can cause fall damage: https://www.sageadvice.eu/jumping-damage/
JC explaining that jumping down from a height mitigates fall damage: https://www.sageadvice.eu/falling-damage-from-jump/
Remember that D&D is a game, not a physics simulation. Many of the rules in D&D do not make sense from a physics perspective. For example, the base rules for falling do not specify a maximum speed, and the Rate of Falling rules from Xanathar's Guide to Everything simply cap it out at 500ft per 6 seconds regardless of what is falling.
With respect to falling on a creature, this was something that was addressed in Tasha's Cauldron of Everything. In short the creature being fallen on must make a DC 15 Dexterity save and on a fail the creature is impaced by the falling creature. When this happens the fall damage is split between the falling and impacted creatures and the impacted creature is knocked prone if it is only one size category bigger than the falling creature or smaller.
There is no mechanical interaction between creature size and jumping in the rules.
As Thezzaruz said each foot you clear on the jump costs a foot of movement. Funny i remember asking exactly this to the Dev on Twitter when 5E released in 2014; ;)
Just as an aside, 500 feet is about how far a creature would fall in 6 seconds if it started its fall with no momentum on Earth gravity. However, on subsequent turns, that would be higher if this were 1:1 physics. Not disputing anything, just pointing out a fun fact (which is likely where they got it from in the first place).
Ooh! Thank you, I didn’t know that! That makes a lot of sense now.