What gutter does KDP need?

It depends on page count: 0.375 in for 24 to 150 pages, rising to 0.875 in for 701 to 828 pages. Thicker books need more room because the pages curve into the spine.

What size should my PDF be with bleed?

Trim width + 0.125 in by trim height + 0.25 in. A 6 x 9 in book with bleed uses 6.125 x 9.25 in pages.

Are these the margins I should use?

They are KDP's minimums. Most novels use 0.75 to 1 in on the outside, top and bottom for comfort, and a little more than the minimum gutter.

KDP interior margin and gutter calculator

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KDP's minimum inside margin (gutter) grows with page count, from 0.375 in for 24 to 150 pages to 0.875 in for 701 to 828. The top, bottom and outside margins must be at least 0.25 in without bleed or 0.375 in with bleed. Enter trim size and page count to get the margins, PDF page size and text block.

Inside margin (gutter), minimum: -
Top, bottom, outside, minimum: -
File page size: -
Largest text block at these margins: -

KDP minimum margins

Page count	Inside (gutter)
24 to 150	0.375 in (9.6 mm)
151 to 300	0.5 in (12.7 mm)
301 to 500	0.625 in (15.9 mm)
501 to 700	0.75 in (19.1 mm)
701 to 828	0.875 in (22.3 mm)

Outside, top and bottom: at least 0.25 in without bleed, 0.375 in with bleed. With bleed the page is trim width + 0.125 in by trim height + 0.25 in.

Example: a 6 x 9 in book with 240 pages and no bleed needs a 0.5 in gutter and 0.25 in elsewhere, leaving a text block of up to 5.25 x 8.5 in.

Questions

What gutter does KDP need?: It depends on page count: 0.375 in for 24 to 150 pages, rising to 0.875 in for 701 to 828 pages. Thicker books need more room because the pages curve into the spine.
What size should my PDF be with bleed?: Trim width + 0.125 in by trim height + 0.25 in. A 6 x 9 in book with bleed uses 6.125 x 9.25 in pages.
Are these the margins I should use?: They are KDP's minimums. Most novels use 0.75 to 1 in on the outside, top and bottom for comfort, and a little more than the minimum gutter.

The rules behind it

Sources

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