For a KDP paperback, the inside margin is the blank strip on the side of each page nearest the spine. Amazon sets a minimum width for it, and that minimum goes up as your book gets longer. If you pick a number by eye, or copy the margin from a novel you like, you can end up with words that disappear into the binding or a page that looks cramped. Here is what KDP asks for, why the rule exists, and how to check your own file before you upload.
The inside margin table by page count
KDP's minimums for the inside margin, which is also called the gutter, currently run like this:
- 24 to 150 pages: 0.375 inches (0.95 cm)
- 151 to 300 pages: 0.5 inches (1.27 cm)
- 301 to 500 pages: 0.625 inches (1.59 cm)
- 501 to 700 pages: 0.75 inches (1.91 cm)
- 701 to 828 pages: 0.875 inches (2.22 cm)
These are minimums, not targets. You can always set a wider gutter, and many designers do for longer books. KDP also revises its help pages from time to time, so check the current figures on KDP's own margin guidance before you lock in a layout. Treat this table as a starting point, not a substitute for that page.
Notice that the jumps happen at round page counts. A 150-page book needs 0.375 inches, and a 151-page book needs 0.5 inches. That single page can change your layout, so the page count you measure at the end matters more than the one you estimated at the start.
Why the gutter needs more room
When a book is bound, the pages curl into the spine. Glue or stitching takes up some of the paper near the binding, and the page itself bends as the book opens flat. The more pages you have, the thicker the block and the more the inside edge disappears into the curve. A margin that looks generous on a 100-page proof can look tight on a 400-page one because the text sinks toward the spine when the book lies open.
That is the reason the minimum grows with length. The gutter is not a style choice; it is the space the binding needs so that letters near the fold stay readable and don't get lost in the curl.
The outside margin and the top and bottom
The inside edge gets the most attention, but the other margins matter too. KDP's minimum for the outside margin is 0.25 inches (0.635 cm), and the top and bottom also have minimums around that size. Those figures are the floor for a file that will print without trimming text, not a comfortable layout.
In practice, most readers notice a page that feels cramped at the top or bottom more than one that is a little narrow on the outside. A common approach is a gutter at the KDP minimum, an outside margin somewhat wider than 0.25 inches, and top and bottom margins a bit larger still. Pick numbers that look balanced in your own proof, then confirm they clear the minimums for your page count.
Mirrored margins and running heads
Because the inside and outside edges are different widths, your layout needs mirrored margins: the gutter on the left of odd-numbered pages and on the right of even-numbered pages. If your word processor treats all margins the same, your text will sit off-centre across the spread. Check that your layout is set to mirror margins before you export, and look at a spread, not a single page, when you review it.
Check the margin on the actual page count
Estimating a page count from a word count is useful early on, but the final number comes from the typeset file. Word counts vary with the font, trim size, chapter headings, scene breaks and front matter. A free Words to Pages Estimator can give you a rough figure while you plan, which helps you see roughly which margin tier you will land in. Once the book is formatted, though, trust the page count in the PDF.
Watch for the threshold trap. Suppose your book comes out at 148 pages with a 0.375-inch gutter. You then add a short afterword, a bonus excerpt and a blank page before the back matter, and you reach 158 pages. Now you need at least 0.5 inches on the inside edge. If you do not recheck the margin, your file may pass the layout stage but fail KDP's checks, or you may hit a print issue you only notice on the proof.
Measure the proof, not just the settings
Settings tell you what you asked for. A printed proof tells you what you got. Order a proof copy before you publish. Open a few interior spreads from the middle and the end of the book and measure the gutter with a ruler. Check that text near the fold reads cleanly and that no line runs into the binding. Look at a chapter opening, a page with a footnote or a table, and the last page of the book, since those pages often break the pattern.
How Pendrilo's Formatter handles inside margins
Pendrilo Formatter is a book formatter that shows typeset pages as you work and produces a print PDF with embedded fonts. It supports paperback, hardcover and large print, with trim sizes and KDP-based inside margins built in. So when you pick a trim size for a paperback, the inside margin is set from KDP's published values rather than a generic default you have to remember.
Two things are worth doing anyway. First, check the margin values for your trim size and page count against KDP's current table, especially if your book is near one of the thresholds above. Second, measure the exported PDF the same way you would any other file: open a middle spread and a final page, and compare the gutter against your proof. The Formatter is in beta, so a quick check on the output is a sensible habit for any tool, and it takes only a few minutes compared with a reprint.
If your book is a long one, recheck the margin after every significant change to the front or back matter. Adding a dedication, a list of illustrations or an extra chapter can move you across a threshold without you noticing. Re-export and look at the new page count before you upload.
If you want to try the Formatter on your own manuscript, you can create a free Pendrilo account. The programs are free during the beta.
A quick pre-upload checklist
Before you upload a paperback to KDP, confirm the following:
- Your final page count, including front matter, back matter and any blank pages.
- The inside margin meets the minimum for that page count on KDP's current table.
- Outside, top and bottom margins clear KDP's minimums and look balanced on a proof.
- Margins are mirrored, so the gutter sits on the binding side of every spread.
- A printed proof has been checked at the fold, at a chapter opening and at the last page.
Getting the inside margin right is a small job with a big payoff. Once you know your page count and the matching minimum, the rest of the layout falls into place, and the book you hold in your hands looks the way you intended.
