Three measurements with different jobs
Trim is the cut edge. Bleed carries artwork beyond that edge so small cutting shifts do not leave a white strip. Margins keep text and important details inside the page. The gutter is the inside margin beside the binding.
A background may extend through the bleed, but a child’s face, a caption or a page number should sit safely away from the cut. Making the artwork larger is not a substitute for leaving safe space.
A KDP paperback example
For a 6 × 9 inch KDP paperback interior with bleed, the page file is 6.125 × 9.25 inches. The extra 0.125 inch is on the outside edge, top and bottom, not the binding edge. A no-bleed interior uses the trim dimensions.
KDP’s minimum gutter increases with page count: 0.375 inch for 24–150 pages, 0.5 for 151–300, 0.625 for 301–500, 0.75 for 501–700 and 0.875 for 701–828. Its outside minimum is 0.25 inch without bleed or 0.375 with bleed. These are minimums, not a design recommendation for every book.
Leave room to read comfortably
Open a thick paperback near the middle. Some of the page curves into the binding. Text that technically clears a printer’s minimum may still feel cramped. Review a physical proof with the book held normally.
For a workbook, allow for hands and writing. For an illustrated spread, keep a face or a key word away from the center fold. Two adjacent pages can form one visual scene without putting the most important detail into the gutter.
Check the exported file
Inspect the PDF page dimensions after export. The document settings alone do not prove that the exported file is correct. Look at several left and right pages, including chapter openings and any full-page images.
Use the selected printer’s current specification for the final check. Hardcover covers and other binding styles have different construction requirements. If the platform reports a margin error, send the exact message with the relevant page instead of guessing at a repair.
Official references
Check the current requirements for your chosen edition before final export.