So as a couple of answers have already stated, there is nothing in the existing RAW to guide a DM on this.

If you’re anticipating that it will be a major element of a campaign and that your players are likely to think of trying it, it may be worth building a guide into the campaign or simply homebrewing a way to resolve it.

Doing this well requires three things: plausibility, playability and reasonability.

#Plausibility# Is it plausible that the way you rule is how it would work in the real world (or at least in a fantasy world with magic but which still adheres to most of the same physical rules)?

I feel there are two factors which determine how long a person can hold their breath; their lung capacity and the rate at which they consume oxygen (each a part of what a character’s constitution score represents.)

It would be implausible to say a barbarian with con 20 could empty his lungs into the relatively tiny lungs of a rogue with con 10, or to say the air in his lungs after 3 minutes is just as useful to him (or the rogue) as freshly breathed air.

#Playability# Science is often complicated. DnD 5e is certainly less complicated than 4.5, but it’s still complicated enough without trying to be entirely faithful to every element of physics etc. When devising a system that deals with a version of real world mechanics, it still has to be simple enough to avoid holding up play with oodles of rule calculations.

If we go with exact percentages and ratios it starts getting too complex. Something like lung capacity of 3 + con modifier litres might work, and having useable oxygen in a lungful of air decend by quarters based on how much of total time has been consumed might work. (Rounded down, of course.)

#Reasonability# Obviously we don’t want to break the game, and set precedents for really dumb unbalanced things. With a rule for something as incredibly specific as this I don’t immediately see how it could be a problem, but after making your best guess, you really just have to play test to see how it goes.

In the end, all three considerations may need to be revised on the fly as you play test and find the bugs in it.

#Example situation# Let’s assume a spherical cow... I mean, let’s assume a Barbarian of CON 20 and a Rogue of CON 10.

+——————————+——————————+——————————+
|          |Barbarian |Rogue     |
+——————————+——————————+——————————+
|minutes   |         6|         1|
+——————————+——————————+——————————+
|capacity  |        8L|        3L|
+——————————+——————————+——————————+

After 1 minute, the rogue is about to start choking. If the barbarian gives him some of his breath, by the above guidelines the barbarian gives 3 of his 8 L to the rogue, which rounded down leaves him with half his capacity. The 5 minutes he had left becomes 2.5.

He has already used some of the oxygen in that air (1 minute's worth out of 6), so rounded down to the quarter, the 3 L the rogue gets (a lung full for him) is only 3/4 full of oxygen, good for 45 seconds.

That’s far from trivial, but doing it again would take 3 of the barbarian’s 5 L - he’s not gonna be able to sustain that.

So as a couple of answers have already stated, there is nothing in the existing RAW to guide a DM on this.

If you’re anticipating that it will be a major element of a campaign and that your players are likely to think of trying it, it may be worth building a guide into the campaign or simply homebrewing a way to resolve it.

Doing this well requires three things: plausibility, playability and reasonability.

Plausibility

Is it plausible that the way you rule is how it would work in the real world (or at least in a fantasy world with magic but which still adheres to most of the same physical rules)?

I feel there are two factors which determine how long a person can hold their breath; their lung capacity and the rate at which they consume oxygen (each a part of what a character’s constitution score represents.)

It would be implausible to say a barbarian with con 20 could empty his lungs into the relatively tiny lungs of a rogue with con 10, or to say the air in his lungs after 3 minutes is just as useful to him (or the rogue) as freshly breathed air.

Playability

Science is often complicated. DnD 5e is certainly less complicated than 4.5, but it’s still complicated enough without trying to be entirely faithful to every element of physics etc. When devising a system that deals with a version of real world mechanics, it still has to be simple enough to avoid holding up play with oodles of rule calculations.

If we go with exact percentages and ratios it starts getting too complex. Something like lung capacity of 3 + con modifier litres might work, and having useable oxygen in a lungful of air decend by quarters based on how much of total time has been consumed might work. (Rounded down, of course.)

Reasonability

Obviously we don’t want to break the game, and set precedents for really dumb unbalanced things. With a rule for something as incredibly specific as this I don’t immediately see how it could be a problem, but after making your best guess, you really just have to play test to see how it goes.

In the end, all three considerations may need to be revised on the fly as you play test and find the bugs in it.

Example situation

Let’s assume a spherical cow... I mean, let’s assume a Barbarian of CON 20 and a Rogue of CON 10.

+——————————+——————————+——————————+
|          |Barbarian |Rogue     |
+——————————+——————————+——————————+
|minutes   |         6|         1|
+——————————+——————————+——————————+
|capacity  |        8L|        3L|
+——————————+——————————+——————————+

After 1 minute, the rogue is about to start choking. If the barbarian gives him some of his breath, by the above guidelines the barbarian gives 3 of his 8 L to the rogue, which rounded down leaves him with half his capacity. The 5 minutes he had left becomes 2.5.

He has already used some of the oxygen in that air (1 minute's worth out of 6), so rounded down to the quarter, the 3 L the rogue gets (a lung full for him) is only 3/4 full of oxygen, good for 45 seconds.

That’s far from trivial, but doing it again would take 3 of the barbarian’s 5 L - he’s not gonna be able to sustain that.

So as a couple of answers have already stated, there is nothing in the existing RAW to guide a DM on this.

If you’re anticipating that it will be a major element of a campaign and that your players are likely to think of trying it, it may be worth building a guide into the campaign or simply homebrewing a way to resolve it.

Doing this well requires three things: plausibility, playability and reasonability.

#Plausibility# Is it plausible that the way you rule is how it would work in the real world (or at least in a fantasy world with magic but which still adheres to most of the same physical rules)?

I feel there are two factors which determine how long a person can hold their breath; their lung capacity and the rate at which they consume oxygen (each a part of what a character’s constitution score represents.)

It would be implausible to say a barbarian with con 20 could empty his lungs into the relatively tiny lungs of a rogue with con 10, or to say the air in his lungs after 3 minutes is just as useful to him (or the rogue) as freshly breathed air.

#Playability# Science is often complicated. DnD 5e is certainly less complicated than 4.5, but it’s still complicated enough without trying to be entirely faithful to every element of physics etc. When devising a system that deals with a version of real world mechanics, it still has to be simple enough to avoid holding up play with oodles of rule calculations.

If we go with exact percentages and ratios it starts getting too complex. Something like lung capacity of 3 + con modifier litres might work, and having useable oxygen in a lungful of air decend by quarters based on how much of total time has been consumed might work. (Rounded down, of course.)

#Reasonability# Obviously we don’t want to break the game, and set precedents for really dumb unbalanced things. With a rule for something as incredibly specific as this I don’t immediately see how it could be a problem, but after making your best guess, you really just have to play test to see how it goes.

In the end, all three considerations may need to be revised on the fly as you play test and find the bugs in it.

#Example situation# Let’s assume a spherical cow... I mean, let’s assume a Barbarian of CON 20 and a Rogue of CON 10.

+——————————+——————————+——————————+
|          |Barbarian |Rogue     |
+——————————+——————————+——————————+
|minutes   |         6|         1|
+——————————+——————————+——————————+
|capacity  |        8L|        3L|
+——————————+——————————+——————————+

After 1 minute, the rogue is about to start choking. If the barbarian gives him some of his breath, by the above guidelines the barbarian gives 3 of his 8 L to the rogue, which rounded down leaves him with half his capacity. The 5 minutes he had left becomes 2.5.

He has already used some of the oxygen in that air, so rounded down to the quarter, the 3 L the rogue gets (a lung full for him) is only 3/4 full of oxygen, good for 45 seconds.

That’s far from trivial, but doing it again would take 3 of the barbarian’s 5 L - he’s not gonna be able to sustain that.

So as a couple of answers have already stated, there is nothing in the existing RAW to guide a DM on this.

If you’re anticipating that it will be a major element of a campaign and that your players are likely to think of trying it, it may be worth building a guide into the campaign or simply homebrewing a way to resolve it.

Doing this well requires three things: plausibility, playability and reasonability.

#Plausibility# Is it plausible that the way you rule is how it would work in the real world (or at least in a fantasy world with magic but which still adheres to most of the same physical rules)?

I feel there are two factors which determine how long a person can hold their breath; their lung capacity and the rate at which they consume oxygen (each a part of what a character’s constitution score represents.)

It would be implausible to say a barbarian with con 20 could empty his lungs into the relatively tiny lungs of a rogue with con 10, or to say the air in his lungs after 3 minutes is just as useful to him (or the rogue) as freshly breathed air.

#Playability# Science is often complicated. DnD 5e is certainly less complicated than 4.5, but it’s still complicated enough without trying to be entirely faithful to every element of physics etc. When devising a system that deals with a version of real world mechanics, it still has to be simple enough to avoid holding up play with oodles of rule calculations.

If we go with exact percentages and ratios it starts getting too complex. Something like lung capacity of 3 + con modifier litres might work, and having useable oxygen in a lungful of air decend by quarters based on how much of total time has been consumed might work. (Rounded down, of course.)

#Reasonability# Obviously we don’t want to break the game, and set precedents for really dumb unbalanced things. With a rule for something as incredibly specific as this I don’t immediately see how it could be a problem, but after making your best guess, you really just have to play test to see how it goes.

In the end, all three considerations may need to be revised on the fly as you play test and find the bugs in it.

#Example situation# Let’s assume a spherical cow... I mean, let’s assume a Barbarian of CON 20 and a Rogue of CON 10.

+——————————+——————————+——————————+
|          |Barbarian |Rogue     |
+——————————+——————————+——————————+
|minutes   |         6|         1|
+——————————+——————————+——————————+
|capacity  |        8L|        3L|
+——————————+——————————+——————————+

After 1 minute, the rogue is about to start choking. If the barbarian gives him some of his breath, by the above guidelines the barbarian gives 3 of his 8 L to the rogue, which rounded down leaves him with half his capacity. The 5 minutes he had left becomes 2.5.

He has already used some of the oxygen in that air (1 minute's worth out of 6), so rounded down to the quarter, the 3 L the rogue gets (a lung full for him) is only 3/4 full of oxygen, good for 45 seconds.

That’s far from trivial, but doing it again would take 3 of the barbarian’s 5 L - he’s not gonna be able to sustain that.

So as a couple of answers have already stated, there is nothing in the existing RAW to guide a DM on this.

If you’re anticipating that it will be a major element of a campaign and that your players are likely to think of trying it, it may be worth building a guide into the campaign or simply homebrewing a way to resolve it.

Doing this well requires three things: plausibility, playability and reasonability.

#Plausibility# Is it plausible that the way you rule is how it would work in the real world (or at least in a fantasy world with magic but which still adheres to most of the same physical rules)?

I feel there are two factors which determine how long a person can hold their breath; their lung capacity and the rate at which they consume oxygen (each a part of what a character’s constitution score represents.)

It would be implausible to say a barbarian with con 20 could empty his lungs into the relatively tiny lungs of a rogue with con 10, or to say the air in his lungs after 3 minutes is just as useful to him (or the rogue) as freshly breathed air.

#Playability# Science is often compliated. DnD 5e is certainly less complicated than 4.5, but it’s still complicated enough without trying to be entirely faithful to every element of physics etc. When devising a system that deals with a version of real world mechanics, it still has to be simple enough to avoid holding up play with oodles of rule calculations.

If we go with exact percentages and ratios it starts getting too complex. Something like lung capacity of 3 + con modifier litres might work, and having useable oxygen in a lungful of air decend by quarters based on how much of total time has been consumed might work. (Rounded down, of course.)

#Reasonability# Obviously we don’t want to break the game, and set precedents for really dumb unbalanced things. With a rule for something as incredibly specific as this I don’t immediately see how it could be a problem, but after making your best guess, you really just have to play test to se how it goes.

In the end, all three considerations may need to be revised on the fly as you play test and find the bugs in it.

#Example situation# Let’s assume a spherical cow... I mean, let’s assume a Barbarian of CON 20 and a Rogue of CON 10.

+——————————+——————————+——————————+
|          |Barbarian |Rogue     |
+——————————+——————————+——————————+
|minutes   |         6|         1|
+——————————+——————————+——————————+
|capacity  |        8L|        3L|
+——————————+——————————+——————————+

After 1 minute, the rogue is about to start choking. If the barbarian gives him some of his breath, by the above guidelines the barbarian gives 3 of his 8 L to the rogue, which rounded down leaves him with half his capacity. The 5 minutes he had left becomes 2.5.

He has already used some of the oxygen in that air, so rounded down to the quarter, the 3 L the rogue gets is only 3/4 full of oxygen, good for 45 seconds.

That’s far from trivial, but doing it again would take 3 of the barbarian’s 5 L - he’s not gonna be able to sustain that.

So as a couple of answers have already stated, there is nothing in the existing RAW to guide a DM on this.

If you’re anticipating that it will be a major element of a campaign and that your players are likely to think of trying it, it may be worth building a guide into the campaign or simply homebrewing a way to resolve it.

Doing this well requires three things: plausibility, playability and reasonability.

#Plausibility# Is it plausible that the way you rule is how it would work in the real world (or at least in a fantasy world with magic but which still adheres to most of the same physical rules)?

I feel there are two factors which determine how long a person can hold their breath; their lung capacity and the rate at which they consume oxygen (each a part of what a character’s constitution score represents.)

It would be implausible to say a barbarian with con 20 could empty his lungs into the relatively tiny lungs of a rogue with con 10, or to say the air in his lungs after 3 minutes is just as useful to him (or the rogue) as freshly breathed air.

#Playability# Science is often complicated. DnD 5e is certainly less complicated than 4.5, but it’s still complicated enough without trying to be entirely faithful to every element of physics etc. When devising a system that deals with a version of real world mechanics, it still has to be simple enough to avoid holding up play with oodles of rule calculations.

If we go with exact percentages and ratios it starts getting too complex. Something like lung capacity of 3 + con modifier litres might work, and having useable oxygen in a lungful of air decend by quarters based on how much of total time has been consumed might work. (Rounded down, of course.)

#Reasonability# Obviously we don’t want to break the game, and set precedents for really dumb unbalanced things. With a rule for something as incredibly specific as this I don’t immediately see how it could be a problem, but after making your best guess, you really just have to play test to see how it goes.

In the end, all three considerations may need to be revised on the fly as you play test and find the bugs in it.

#Example situation# Let’s assume a spherical cow... I mean, let’s assume a Barbarian of CON 20 and a Rogue of CON 10.

+——————————+——————————+——————————+
|          |Barbarian |Rogue     |
+——————————+——————————+——————————+
|minutes   |         6|         1|
+——————————+——————————+——————————+
|capacity  |        8L|        3L|
+——————————+——————————+——————————+

After 1 minute, the rogue is about to start choking. If the barbarian gives him some of his breath, by the above guidelines the barbarian gives 3 of his 8 L to the rogue, which rounded down leaves him with half his capacity. The 5 minutes he had left becomes 2.5.

He has already used some of the oxygen in that air, so rounded down to the quarter, the 3 L the rogue gets (a lung full for him) is only 3/4 full of oxygen, good for 45 seconds.

That’s far from trivial, but doing it again would take 3 of the barbarian’s 5 L - he’s not gonna be able to sustain that.