We summarize a large set of published values of forest floor mass and develop large-scale estimates of carbon mass according to region and forest type. Published values of forest floor carbon mass or information relevant to compiling such summaries are scarce. We present a simulation model based on observations obtained from literature surveys for use in the 2002 version of FORCARB, a carbon budget model for U.S. forests. The forest floor is the distinct layer of dead and decaying plant material that accumulates on the soil surface, which lies above the mineral soil and includes small woody debris. Estimates of average forest floor carbon mass per hectare of forest applied to a 1997 summary forest inventory, sum to 4.5 Gt carbon stored in forests of the 48 contiguous United States.