A gutter margin is extra space between the text on the pages of a book and the inside edges of the pages to ensure that the text is still visible and appears centred as the pages bend towards the spine.
Gutter margins are only relevant for books with certain bindings. If a book is spiral bound, the book can be opened and laid flat, with the entirety of each page visible. However, if a book is perfect bound or case bound (which are the only options if you’re publishing through KDP), the book cannot be opened and laid perfectly flat – the pages will bend towards the spine in the middle. This means that the edge of each page closest to the spine is not visible, and anything printed there won’t be visible. It also means that anything that is printed an equal distance from the left and right sides of each page won’t appear to be exactly centred – it will appear slightly closer to the inner edge of each page.
A gutter margin is extra space on the inside edge of each page to compensate for this problem. It ensures that all of the text is visible, and that the text appears properly centred on the page.
The size of the gutter margin is dependent on the length of the book. Longer books require larger gutter margins – because more of each page is hidden by the bend of the pages. It is also affected by the thickness of the paper used.
To determine what size of gutter margin you need for your book, look it up on the publishing platform’s website – it will tell you. If you have a very short book – under about 100 pages – the book likely won’t need any gutter margin at all.
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