From the official Raspberry site:
Video mode
Custom mode
If your monitor requires a mode that is not in one of the tables above, then it's possible to define a custom CVT mode for it instead:
***please note, need to ad a blank space before the bracket, in order to display it here correctly***
hdmi_cvt=< width> < height> < framerate> < aspect> < margins> < interlace> < rb>
Value Default Description
width (required) width in pixels
height (required) height in pixels
framerate (required) framerate in Hz
aspect 3 aspect ratio 1=4:3, 2=14:9, 3=16:9, 4=5:4, 5=16:10, 6=15:9
margins 0 0=margins disabled, 1=margins enabled
interlace 0 0=progressive, 1=interlaced
rb 0 0=normal, 1=reduced blanking
Fields at the end can be omitted to use the default values.
Note that this simply creates the mode (group 2 mode 87). In order to make the Pi use this by default, you must add some additional settings. For example, the following selects an 800 × 480 resolution and enables audio drive:
hdmi_cvt=800 480 60 6
hdmi_group=2
hdmi_mode=87
hdmi_drive=2
This may not work if your monitor does not support standard CVT timings.