What is a gutter margin?

The gutter margin is the inside margin of each page — the side that runs into the spine. In a perfect-bound paperback the pages curve into the binding, so text placed too close to the spine disappears into the fold and readers have to crack the book open to read it. A wider inside margin keeps every line readable.

Because the binding is on the left of right-hand pages and on the right of left-hand pages, the gutter alternates sides. Book layout software handles this with "mirror margins", so the wide margin is always on the binding side.

Thicker books need a wider gutter, because more pages curve deeper into the spine. Amazon KDP, for example, publishes a minimum inside margin that grows with page count — currently 0.375" for books up to 150 pages, rising in steps to 0.875" for the thickest books, with outside margins of at least 0.25". Other printers publish their own tables, so check the platform's current rules for your page count. Many designers go a little beyond the minimum for comfort.

If you change your page count significantly after layout — adding chapters, changing font size or trim size — check that your gutter still meets the minimum for the new page count before you upload.