There were basically never any 5.25" or 3.5" single-density disks on the PC, and the definition of double-density varied depending on disk size. 5.25" disks we

There were basically never any 5.25" or 3.5" single-density disks on the PC, and the definition of double-density varied depending on disk size. 5.25" disks were 40 track, 9 sectors per track with 512 byte sectors for 180KB per side. For 3.5" disks, the definition was 80 track so 360KB per side. High-density increased the number of sectors per track to 15 for 5.25" (600KB per side) and to 18 for 3.5" (720KB per side).