Trim size is the physical dimension of the finished book. Pick a size your genre expects; a thriller at 6×9 looks like a trade paperback, a thriller at 8.5×11 looks like a textbook. This article covers what each size is used for, what margins go with them, and how to get everything right on the first upload.

Common trim sizes

Under Format → Print settings → Trim size, you can also enter custom dimensions if your printer supports them. Bellwether supports any trim between 4×6 and 8.5×11 inches.

Picking a size for fiction

Most trade paperback fiction is 5×8, 5.25×8, 5.5×8.5, or 6×9. What to consider:

• Page count. At 5×8, a 100,000-word manuscript runs around 400 pages. At 6×9, around 320. More pages cost more to print, which affects your royalty margin.
• Genre expectation. Romance and cozy mystery lean smaller (5×8, 5.5×8.5). Thrillers, literary fiction, and fantasy lean larger (5.5×8.5 or 6×9). Pick up three books off the shelf in your genre and measure them.
• Shelf presence. 6×9 books look more substantial in your hand. Some authors pick 6×9 for first-in-series to signal "serious" and 5.5×8.5 for the rest.
• Competitive comp titles. If your book is positioned as a comp to a specific recent release, match their trim. Readers notice.

Mass market

Mass market paperback (4.25×6.87) is the small format you see in airport bookstores. It's cheap to produce and fits in a pocket. Downsides:

• Not all retailers produce mass market through self-publishing (KDP doesn't).
• Margins are tight; typefaces have to be small.
• The shelf impression is "beach read."

If mass market is central to your positioning (genre fiction distributed through non-bookstore channels), use IngramSpark or a specialist printer. For KDP, pick 5×8 instead.

Picking a size for non-fiction

Non-fiction has more variety:

• Memoir, narrative non-fiction. Same as fiction, usually 5.5×8.5 or 6×9.
• Business, self-help. 5.5×8.5 or 6×9. 5.5×8.5 is more common.
• Textbooks, workbooks, reference. 7×10 or 8.5×11. These are "lap books" readers will write in.
• Cookbooks, photography, art. 8×10 or 8.5×11. Bigger is better for photo-heavy books.
• Children's picture books. Custom trims, usually 8.5×8.5, 10×8, or 8×10.

Margins

Bellwether sets sensible margins by default for each trim. The defaults are tuned for readability and for meeting retailer requirements (KDP requires minimum inside margins based on page count).

The defaults, by trim:

Inside margin (gutter)

Also called the gutter. This is the margin on the binding side. Books with 400+ pages need a larger inside margin so the reader doesn't have to break the spine to read. Bellwether adjusts automatically as your page count grows.

Rules of thumb for gutter:

• Under 150 pages: 0.5 in minimum
• 150–300 pages: 0.625 in
• 300–500 pages: 0.75 in
• 500–800 pages: 0.875 in
• 800+ pages: 1 in or more

Going tighter than this on a thick book makes reading physically difficult.

Outside margin

The margin on the side opposite the binding. Most fiction uses 0.5 inches. Smaller outside margins save pages but look cramped.

For design-heavy non-fiction (cookbooks, art books), outside margins of 0.75 to 1 inch give room for pull quotes, callouts, and page ornaments.

Top and bottom margins

Top usually contains the running header (book title on verso, chapter title on recto). Bottom usually contains the page number. Bellwether ensures there's room for both.

If you're turning off running headers and footers, you can go tighter. Under Format → Print settings → Running content → None, Bellwether will let you reduce top and bottom margins down to 0.4 inches.

Overriding defaults

You can override any margin under Format → Print settings → Margins. Switch to Custom and enter values in inches or millimetres.

Two safeguards:

• Minimum inside margin. Bellwether won't let you go below KDP's minimum for your page count. Attempting to save a too-small gutter shows a warning and rejects the save. (Override is possible via Format → Print settings → Expert mode, but you do so knowing the book may get rejected on submission.)
• Bleed compliance. If you have imagery that bleeds to the edge, outside and top/bottom margins below 0.25 inches risk clipping on trim. Bellwether warns about this on pre-flight.

Running headers and footers

Running headers sit in the top margin, footers in the bottom. Typical setup:

• Verso (left page). Book title in the header, page number in the footer.
• Recto (right page). Chapter title (or author name) in the header, page number in the footer.

Bellwether generates these from your metadata and chapter titles. You can override per book under Format → Print settings → Headers and footers.

Page numbers in front matter use lowercase roman numerals by default (i, ii, iii). Body starts at arabic 1.

Suppressing headers on certain pages

• Chapter opening pages. Header suppressed by default. Page number optional.
• Part opener pages. Header and page number suppressed.
• Blank pages. Both suppressed.
• First page of front matter. Both suppressed.

These are defaults that match print convention. Override under Format → Print settings → Suppress on.

Changing trim mid-project

You can change trim size at any time, but:

• Page count will change significantly. A 300-page book at 5×8 may become a 250-page book at 6×9.
• Cover spine width depends on page count, so covers need re-exporting after a trim change.
• Chapter page breaks may land differently. Check for orphans and widows after the change.

Bellwether re-runs pre-flight automatically on trim change and surfaces issues.

Spine width consideration

Before committing to a trim size for a book you plan to sell in bookstores, consider how the spine will look:

• Books under 100 pages have very thin spines, which can't hold any title text. Mass market trims exaggerate this.
• Books with too-thin spines (sub-0.15 inches) aren't really cover-titled; the spine shows only a colour.
• At 6×9 trim with 250 pages, the spine is around 0.6 inches: enough for title, author, and a small logo.

If spine presence matters (bookstore distribution), err toward fewer, longer pages rather than more, shorter ones.

Paper stock

Under Format → Print settings → Paper, three options:

• 55 lb cream. Most fiction. Slightly warmer, friendlier feel.
• 50 lb white. Most non-fiction. Crisper, more contrast.
• 60 lb cream. Heavier, thicker, more premium feel. Adds noticeable page count to thickness.

Paper affects spine width (see cover export) and printing cost. KDP's defaults are reasonable for most books; change only if you have a specific aesthetic reason.

Common edge cases

My book is too long for my chosen trim. Go up one size. A 150,000-word book at 5×8 is too fat; try 6×9, which cuts the page count by ~20%.

Inside margin looks too wide in preview. Pre-flight is showing the correct margin for your page count. Larger books need larger gutters. If you want to shrink it, reduce your page count (trim the manuscript or go to a larger trim size).

My cover is wrong after a trim change. Cover is sized to trim. Re-export the print cover after changing trim size. Bellwether flags this on pre-flight.

The retailer rejected my PDF for margin violations. KDP is the strictest. Use Bellwether's KDP export preset and it will match their requirements. If you exported to the generic print PDF, re-export with the KDP preset.

I want to match a specific published book's trim. Find the book's physical copy. The trim is printed on the back near the barcode (look for "7.5" × 9.25"" or similar), or in the front matter. Enter that trim as a custom size under Format → Print settings → Trim size → Custom.

Mass market on KDP. Not available via KDP. Use IngramSpark or a specialist mass-market printer. Bellwether's mass-market exports target IngramSpark's standards.