Why the gutter changes
A bound page disappears partly into the spine, so its inside margin must protect the text block from the curve and glue. Thicker books need a larger gutter because more paper resists opening flat. A margin measured on a loose sheet therefore does not predict the reading experience of a 600-page paperback.
KDP's current minimums rise by page-count band: 0.375 inches 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. These are acceptance thresholds. Add room when the trim is small, the book is thick, the binding is stiff, or the design intentionally uses a narrow text block.
Mirror the page instead of copying it
A left page and right page are reflections: each has an inside and an outside edge. Use mirrored margins rather than fixed left and right margins so the gutter always follows the binding. Judge the complete spread; two individually centered pages can look pulled apart when the inside space is too generous or swallowed when it is too narrow.
Top and bottom margins also participate in balance. Running heads occupy the upper zone, folios occupy the lower zone, and chapter openings often need a different vertical start. Keep the body rectangle stable enough that page turns feel calm even when display pages temporarily remove headers or move the title downward.
Bleed and safe area are different
Bleed extends artwork past the trim so cutting variance does not leave a white edge. It does not make text safer. KDP currently states a 0.25-inch outside minimum for non-bleed interiors and 0.375 inches for bleed interiors. Keep important content comfortably inside the trim and allow images that intentionally bleed to extend to the required enlarged page boundary.
Do not add bleed to an ordinary prose book merely because the export dialog offers it. It changes the submitted page dimensions and introduces another production check. Use it only for pages whose background or image is meant to reach an outer edge.
Test the physical object
Measure the PDF, but proof the book. Paper, page count, trim, and binding affect how the gutter feels. Read several pages near the front, middle, and back without forcing the book flat. If words disappear into the curve or the outer margin feels crowded under a thumb, revise the page frame.
A margin change alters line length, page count, and the cover spine. Make the correction in the formatting source, regenerate the PDF, update the cover, and order a new proof when the change is material.
Sources: Amazon KDP: Paperback submission guidelines · Amazon KDP: Trim size, bleed, and margins. Reviewed ; recheck current platform requirements before production.