Book margin calculator
Set your margins before you format, and you won't have to redo the book when the page count grows. Enter your trim size and page count, pick where you're printing, and you get the page size and the margins each platform asks for, plus one setup that works for all of them.
Why the inside margin grows with page count
The thicker the book, the more of each page disappears into the spine when it's open. That's why KDP's inside minimum goes from 0.375 in for up to 150 pages to 0.875 in for over 700, and why Lulu's recommendation grows the same way. Set it for the page count you'll end up with: if your book grows from 280 to 320 pages after editing, KDP's minimum goes up.
Rules or recommendations?
KDP publishes minimums and flags files that break them. IngramSpark, Lulu and Barnes & Noble Press publish recommended margins. Bookvault and Draft2Digital Print don't publish margin sizes. Each platform card says which kind its numbers are, and the date we checked them.
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