Pixels and inches are both units of measurement, but they’re used on different mediums. Websites and other digital mediums measure dimensions in pixels, so if you are designing for a digital platform, the dimensions of your design will need to be in pixels. On the other hand, if you want to design something that will be printed and used in the physical world, you’ll need to know what the dimensions of your design are in inches.
To convert inches to pixels we simply take the DPI (dots per inch) or PPI (pixels per inch) and multiply that by the dimensions of the layout. So if we have an 8.5" x 11" piece of paper that was printed at a DPI of 72, then the pixel equivalent is 612px x 792px.