How are Constitution-based checks and saves differentiated in these circumstances?

Question

The Constitution ability (PHB 177) lists tasks like the following, in which the DM might ask for Constitution checks:

• Hold your breath

• March or labor for hours without rest

• Go without sleep

• Survive without food or water

• Quaff and entire stein of ale in one go

But the 1st, 2nd, and 4th instances actually have specific rules that ask for saving throws, not ability checks. For example, the second bullet states "March or labor for hours without rest," but there are rules for Forced March (PHB 181). Those rules state that you can march for 8 hours at a slow/normal/fast pace, and then must make a DC 11 Constitution save on the 9th hour of marching, DC 12 on the 10th hour of marching, etc., or take 1 level of exhaustion.

Meanwhile, the 4th bullet point, surviving without food or water, is detailed in PHB 185. A character can go without food for a number of days equal to 3 + their Con modifier and automatically take one level of exhaustion (no save) per day beyond that, and they can go with only half their water requirements if they make a DC 15 Con save per day, or otherwise take 1 level of exhaustion.

So, if I wanted to ask a character who is going to labor for hours without rest to make some kind of roll, should I ask for a Constitution check or a Constitution save? How should the two be separated/chosen from?

Answer 1

I think that part you quoted only tries to highlight what kind of situations would you use Con. In 5e the difference between a save and a check is only what you get the proficiency from, so we can forgive their blurring of the lines here.

Also the sentence before the part you quote reads:

The DM might call for a Constitution check when you try to accomplish tasks like the following: (emphasis mine)

So it is left up to the DM and is not a "strict rule".

On whether to call for a check or a save:

The DM calls for an ability check when a character or monster attempts an action (other than an attack) that has a chance of failure. (PHB 174)

You don't normally decide to make a saving throw; you are forced to make one because your character or monster is at risk of harm. (PHB 179)

The DM should ask for a check when the character is doing something actively, this will also usually take one or more actions to accomplish. Saves on the other hand do not take up an action (not even a reaction) and are the result of external forces acting on the character.

So if you would rule that it takes action(s) to accomplish something, it is a check. If it does not, it is a save.

Example: holding your breath. Even though the character can deliberately try to hold their breath, they are not doing it actively: it would not cost them an action to do it. They could force open an underwater door (which definitely takes an action) while at the same time holding their breath. Also, the external force of the need of air forces the roll.