The machines are stated as 31-1/8" depth. Additional space is required behind both of them (washer for the water supply and drain hoses, dryer for the exhaust ducting).

The installation instructions for the washer advise minimum space required space behind the machine is 5.9" ... which is when it's standing alone without a dryer stacked on top of it.

The specification sheet for the dryer shows a diagram for a closet installation. It indicates that the dryer is set back a couple inches atop the washer when stacked (presumably with a required stacked mounting kit) which is common to allow access to the dispenser drawer. The dryer calls for 5-15/16" (read that as 6") rear space ... set back for stacking atop the washer calls for 7-1/8" space behind the washer. Plus 1" space in front for a door on the closet (which you can ignore if you take the door off. From front of the washer to the rear wall the indicated required depth is is 38-1/4" (31-1/8" + 7-1/8").

Perhaps less if the dryer is side-vented and/or if your water supply taps/drain are on the sidewall.