Pick a trim size every printer offers (before your cover is designed)
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R. S. Breed Founder
The trim size is the size of the finished book. It's worth choosing before anything else, because the cover, the interior page size and the margins all hang off it.
If you might print through more than one platform (KDP for Amazon and IngramSpark for bookshops is a common pair), pick a size they all offer. Then your interior works everywhere and only the spine changes.
Here's every paperback and hardcover size KDP, IngramSpark, Barnes & Noble Press, Lulu, Bookvault and Draft2Digital Print offer, side by side, with the most widely offered at the top: https://bervet.com/tools/book-trim-sizes
A few things to watch:
- Offered isn't the same as offered on every paper. Some sizes only come in white paper, or only in colour. The spine width calculator tells you for your paper and page count.
- Metric and inch sizes line up. Bookvault lists sizes in millimetres; 152 x 229 mm is the 6 x 9 in everyone else offers.
- Hardcovers come in fewer sizes than paperbacks, so if you want both, check the hardcover column first.
The page also shows where each platform prints and sells, which matters if most of your readers are outside the US. No account needed.