FRS / BRZ / FT86 Trunk air space volume using packing peanut method. Spare tire, fabric side liners, and floor removed. I used Bubble packaging to approximate even to the very top and got as close as an accurate answer as well get.
13.2 cu. ft.
This does not take into account the air space over top of the rear wheels between the two panels of sheet metal connecting behind the rear seat panels to the trunk. I measured this for those looking to do IB who would seal up the rear deck and that airspace over the rear wheels. I am also considering it but won't make any final decisions until next summer.