Swimming / Holding Breath (DnD5E)

Hi there, My question is about swimming and holding breath in DnD5E.  Can someone please clear up how to work out: 1. How long a character can hold his/her breath under water; and 2. How far that character can swim under water while doing so? Apologies if this is in the wrong forum. Many thanks, Scott

From the Suffocating rules on PG 183 of the PHB A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier (minimum of 30 seconds). When a creature runs out of breath or is choking, it can survive for a number of rounds equal to its Constitution modifier (minimum of 1 round). At the start of its next turn, it drops to 0 hit points and is dying, and it can’t regain hit points or be stabilized until it can breathe again. For example, a creature with a Constitution of 15 can hold its breath for 3 minutes. If it starts suffocating, it has 2 rounds to reach air before it drops to 0 hit points. Unless a character has a swim speed, it costs 2 feet of movement for every 1 foot traveled. So so long as they still have breath and are not unconscious they can swim have their movement speed.

Many thanks for that. I'm obviously not picking this up though. How did they calculate that Constitution 15 example?!! Thanks

A character can hold its breath for 1 + X minutes where X is the Constitution modifier of the character. If a character has a negative Constitution modifier (meaning that its Constitution score is less than 10), it can only hold its breath for 30 seconds (5 rounds). So a Constitution score of 15 means a modifier of +2. When you put that into the formula (1+X - > 1+2 -> 3), so this character could hold it's breath for 3 minutes (plus 12 seconds [2 rounds] before the character is unconscious). 

From the Roll20 Community Code of Conduct : The Roll20 Forums exist to discuss topics directly related to the use of the Roll20 program. Anything that more fittingly could be discussed on another website SHOULD be discussed there. Here are some good places to discuss this topic: /r/rpg /r/dnd