Specific printing requirements will vary according to the publishing solution you are using, but if you have opted to use CreateSpace then you will need to have a margin of at least quarter of an inch (6.35mm) on every side of the page.
In reality, however, you should probably opt for a larger margin than this, so that your readers have room to hold the book without obscuring the text. We'd suggest at least half an inch, or about 12–15mm as a minimum.
However, one of your margins will need to be bigger than the others, and that is the inside margin, otherwise known as the gutter.
The reason for this is that books do not lay flat when they are opened: the pages have to bend out from the spine. This creates a "gutter" where the two pages join. As the pages disappear towards the binding the edge of the page is obscured, so the margin has to be wider on this edge to allow for this. The more pages in a book, the more pronounced this effect, and so the wider the gutter has to be.
In order to have the option of setting an inside margin you'll first have to make sure that you've set your document to have mirror pages / mirror margins. This is because the gutter will appear on the left on rectos (right-hand pages) and on the right on versos (left-hand pages).
Once you've set your document to have mirrored pages / margins you should be able to set either an inside margin or a gutter. These both mean the same thing, so you only need to set one. The minimum size of the gutter varies according to how many pages there are in your book. For CreateSpace, you need to use the following measurements:
|Page Extent||Minimum Inside Margin (Inches)||Minimum Inside Margin (mm)|
This is the total minimum width of your inside margin. Remember that if you're using a wider margin for your other margins then you should treat that as your minimum for your inside margin, too.
Don't forget that your first page will be a recto (right-hand page), and will have the large inside margin on the left.
For the average book that includes normal pages of text, this should be all you need to know for setting your margins. However, if your interior includes printed content (be it illustrations, shaded areas, or any other printed material) that extends to the edge of the page, you will also need to include a bleed. If you think this might apply to you, click here for more information.