Cabinet Width is the EXTERNAL width of a carcass, which includes the 2 side walls. Most cabinet walls use 16mm board. Ikea kitchens use 18mm board.
INTERNAL width = EXTERNAL width – 32mm (if 16mm board)
Use the INTERNAL width to determine if a pull out unit can fit.
Most cabinets are made to standards widths in the table below. If your cupboard is not in standard width, you may need to patch out on sides.
Standard Cabinet widths for 16mm sideboard