The gutter margin is the inner margin of your book — the space between your text and the spine. It is one of the most important formatting settings in print design because it directly affects readability.

When a book is opened, the pages curve inward toward the binding. If the gutter is too small, text can disappear into the spine, making it uncomfortable to read. This is especially noticeable in thicker books.

The correct gutter size depends on page count. More pages create a thicker spine, which requires a wider gutter to compensate for the curvature.

General gutter margin guidelines for paperbacks:

• Under 150 pages: 0.625"–0.75"
• 150–400 pages: 0.75"–0.875"
• 400–600 pages: 0.875"–1.0"
• 600+ pages: 1.0" or more

These are practical ranges rather than strict rules. The ideal gutter depends on your trim size, paper type, and binding.

Hardcovers require slightly less gutter space. Because the binding is more rigid, pages do not curve as deeply into the spine, but you still need adequate inner margin.

Outer, top, and bottom margins should be balanced with the gutter. A common approach is to set the outer margin at about 60–75% of the gutter width. Top and bottom margins typically fall between 0.75" and 1.0".

Always use mirror margins (facing pages). This ensures the gutter alternates correctly between left and right pages. Using identical margins on every page is a common mistake that results in incorrect spacing on half the book.

KDP and IngramSpark provide minimum margin requirements, but those are baseline thresholds. Slightly larger margins usually produce a more comfortable reading experience.

Sources:

• KDP Help: Margin Requirements
• IngramSpark File Creation Guide
• The Chicago Manual of Style: Book Design Guidelines