Felicity Huffman Will Only Serve 13 of 14-Day Sentence

Felicity Huffman won't even spend a full two weeks behind bars despite her 14-day prison sentence ... TMZ has learned.

Felicity will only spend 13 days behind bars. That's because when she was initially arrested at her home, taken in and booked, that counts as one day toward completing her sentence ... even though she was only in custody for a few hours.

So, she entered FCI Dublin Tuesday, October 15, and Federal Bureau of Prisons officials tell us she'll be out October 27. Do the math, and use your hands if you must -- 13 days.

This must be especially galling for prosecutors, because as we reported they initially wanted Felicity to serve a year behind bars -- although they never said that publicly. When they did get around to a public proclamation, they said they wanted 4 months, but when they realized the probation dept. wasn't supporting that, they reduced it to 1 month and only got half that.