The density of amorphous Si has been measured. Multiple Si implants, at energies up to 8.0 MeV, were made through a contact mask to produce alternating amorphous/crystalline Si stripes with amorphous thicknesses up to ~5.0 mm. For layers up to 3.4 mm (5 MeV), the amorphous Si is constrained laterally and deforms plastically. Above 5 MeV, plastic deformation of the surrounding crystal matrix is observed. Height differences between the amorphous and crystalline regions were measured for as-implanted, thermally relaxed, and partially recrystallized samples using a surface profilometer. Combined with ion channeling measurements of the layer thickness, amorphous Si was determined to be 1.8±0.1% less dense than crystalline Si (4.90 x 1022 atom/cm3 at 300 K). Both relaxed and unrelaxed amorphous Si show identical densities within experimental error (<0.1% density difference).