What font size should a book be?

You're about to set your book and need a type size. KDP's only rule is a minimum of 7 points, which tells you what's allowed, not what reads well. So we set one novel at six sizes and three trim sizes, and measured the pages and the lines.

The short answer

There's no single right size, because it depends on the trim size and the font. For our test novel in EB Garamond, we chose 11.5 points for 5 × 8 and 5.5 × 8.5, and 12 points for 6 × 9. At those sizes a line holds 60 to 70 characters, inside the 45 to 75 that the typographer Robert Bringhurst gives as comfortable, and the 80,000-word novel comes to 292 to 352 pages. In our test, every half point up added pages, and every page adds to the printing cost and the spine.

Pages at each size

The same 80,000 words each time, in EB Garamond. The figures in bold are the sizes our Novel design chooses for each trim size.

Characters on a line

These are full lines of running text, letters and spaces counted, as we measure them on the finished page.

The length of a line matters as much as the size of the type. A very long line makes it harder to find the start of the next one, and a very short one chops the sentences into pieces. Robert Bringhurst gives 45 to 75 characters as a comfortable line for a single column of serif text, in The Elements of Typographic Style. When we set your book, our report points out any line shorter than 55 characters or longer than 75.

Why smaller type doesn't always mean fewer pages

You might expect smaller type to fit more words on every page. It doesn't always, because we set the width of the text to suit the type, aiming for about seventy characters a line, and give what's left of the page to the margins.

• 5 × 8. At 10 points the outside margin grew from 0.54″ to 0.6875″, so the book came to only 16 pages fewer than at 10.5 points.
• 6 × 9. We never make the text narrower than two thirds of the page, so at 10 points the line ran to 78 characters, past the comfortable range.
• Larger sizes. A 5 × 8 page has no room to widen the text. From 10.5 points up, each half point shortened the line and added 18 to 46 pages. On a 6 × 9 page the margins gave the room, and from 11 points up the line stayed at about 70 characters.

What the size changes on KDP

The printing cost. On Amazon.com, a black-ink paperback of more than 110 pages costs $1.00 plus $0.012 a page to print at these three trim sizes. On a 5 × 8 page, going from 11.5 to 12 points added 18 pages, which is $0.22 more for every copy.

The spine and the margins. Every page widens the spine, so measure the cover after you've settled the size. Past 300 pages, KDP's minimum inside margin also grows, from 0.5″ to 0.625″. How KDP gutter margins work explains why.

The planner in How many pages will my book be? works out the pages, the spine, the printing cost and the royalty for your word count.

The sizes we set

We choose the size from the design and the trim size. Our Novel design starts from 11.5 points and our Classic design from 11, both in EB Garamond. Our Manual design starts from 10.5 points, in Source Serif 4.

A wider page gets larger type, so that its lines don't grow too long. We add half a point at 6 × 9 and 6.14 × 9.21, one point at 6.69 × 9.61 and one and a half at 7 × 10.

How we measured

• The text. The novel from How many pages will my book be?, at 80,000 words in 29 chapters, each opening on a right-hand page.
• Front pages. A half title, a title page and a copyright page.
• Type. EB Garamond, at each size in the table.
• Margins. Our own for each size, with the inside margin at least 0.25″ above KDP's minimum for the page count.

Sources

• KDP: Paperback Submission Guidelines: the 7-point minimum.
• KDP: Paperback Printing Cost: the fixed and per-page costs.
• Robert Bringhurst, The Elements of Typographic Style, section 2.1.2: the comfortable length of a line.