[RPG] Champion Fighter’s Remarkable Athlete and Hit Point Calculation

You Regain 21 Hit Points

That at least seems to be the final word from Mike Mearls: http://www.sageadvice.eu/2014/09/01/durable-feat/

He asserts that a character can use durable to get back more than max possible hit points. For your barbarian, when you roll the hit die, the result is 14 no matter what you roll, and then you add your Con modifier to it.

Your current hit points may never exceed your hit point maximum. Unless a rule specifically states it also affects hit points (not just hit point maximum), only the hit point maximum is lowered.

If your hit point maximum drops to below your current hit points, your current hit points will be reduced to the new hit point maximum.

Keep in mind that certain effects cause both your hit points and hit point maximum to be reduced. An example is the attack of a Wraith which causes damage and then lowers the characters hit point maximum if the save is failed.

Example:

A level 3 Barbarian with 14 Constitution and 39 hit point maximum with 31 current hit points loses 1 constitution due to disease. His hit point maximum drops to 36, but his current hit points stay at 31.

The disease is horrible, and even before healing he loses another 4 constitution (total of 9 now). His hit point maximum drops to 30, and, since his hit point maximum is now lower than his current hit points, his current hit points drop to 30 as well.