How KDP Gutter Margins Work
Gutter margins are the inner margins on a printed book page. They need to scale with your page count, or text disappears into the spine. Here is how KDP calculates them and how to get them right.
What are gutter margins?
Gutter margins are the inner margins on a printed book page. The gutter is the side of the page that faces the spine. The outer margin is the side that faces the edge of the book.
Gutter margins exist because text near the spine disappears into the binding. The thicker the book, the more text disappears. This is why gutter margins must scale with page count: a 100-page book needs less gutter than a 400-page book.
How KDP calculates gutter margins
KDP provides a table that maps page count ranges to minimum gutter widths. For example, a book with 24 to 150 pages needs a minimum gutter of 0.375 inches. A book with 151 to 300 pages needs 0.5 inches. A book with 301 to 500 pages needs 0.625 inches. And so on.
These are minimums. You can use larger gutters if you want more white space near the spine. You should never use smaller gutters than the minimum, or text will disappear into the binding.
The two-pass render approach
The challenge with gutter margins is that you do not know your page count until you render the book, but you need the gutter margin to render the book. This is a chicken-and-egg problem.
The solution is a two-pass render. In the first pass, render the book with default margins and count the pages. In the second pass, look up the correct gutter margin for that page count and render again with the correct margins. The second pass may produce a slightly different page count, but the gutter will be correct for the first pass count, which is close enough.
This is what a good formatting system does automatically. You do not have to look up the KDP table and calculate margins by hand.
Other margins to consider
In addition to the gutter, you have outside, top, and bottom margins. KDP has minimums for these too, but they do not scale with page count. Typical values are 0.25 inches minimum for outside, top, and bottom.
If you choose double-sided printing (mirrored margins), the gutter alternates between left and right pages. Chapters should open on right-hand pages (odd-numbered) in double-sided books. A good formatting system handles this automatically.