Type size, leading and the line that reads well
A page that is tiring to read is almost never tiring for one reason. It is three numbers set independently of each other, when they should have been set together.
Those numbers are size, leading and measure. Get their relationship right and the page disappears, which is the goal. Get it wrong and readers will tell you the book was hard to read without ever being able to say why.
Size: 10 to 12 points, and the number lies
A trade book is normally set somewhere between about 10 and 12 points. Below 10 you are asking a lot of a reader in a paperback; above 12 the book starts to look like it is for children, unless it is deliberately large print.
The trap is that the point size is not a measurement of how big the letters look. It describes the body the type is set on, and every face uses that body differently. Two faces at the same 11 points can differ noticeably in apparent size, because their x-heights differ — the height of the lowercase against the capitals is what your eye registers.
So 11 points is a starting position, not an answer. Set a page in your chosen face before you decide whether 11 is right for that face.
Leading: the distance between baselines
Leading is the distance from one baseline to the next — not the gap between lines, but the full step down the page. It is the single most effective lever you have over how a page feels.
A common starting point is about 120% of the type size: 11 point type on roughly 13 points of leading. Tighter and the lines crowd, with ascenders and descenders from neighboring lines nearly touching. Much looser and they stop reading as a block of text and float apart.
That 120% is a floor, not a rule. The longer the line, the more leading it needs. A long line asks the eye to travel further back to the left margin, and the only thing helping it land on the right line is vertical separation.
Measure: the one most books get wrong
The measure is the length of the line — the width of your text block, not the width of the page.
The old rule is 55 to 75 characters per line, spaces included, or roughly 9 to 12 words. It has survived because it describes something physical: how far the eye can travel before the return sweep becomes unreliable.
This is the measurement self-published books most often get wrong, and they get it wrong in one specific way — a wide trim with narrow margins, producing lines of 90 or 100 characters. The page looks efficient and reads badly, and it is a large part of why a book can feel like a manuscript. Margins are what set the measure. See Margins and gutters.
In the Interior Formatter. The interior styles — Literary, Modern, Genre, Compact, Classic and Large Print — each set a face, size, leading and measure together, so the combination is chosen rather than assembled by accident. Start a book → · about the Interior Formatter
The three pull against each other
Change one and you have changed the other two, whether you meant to or not.
A longer line needs more leading. Extra vertical space is what lets the eye find the start of the next line after a long journey back. A long measure set tight is the most reliably unpleasant page you can make.
A smaller size on the same measure means more characters per line. Drop from 11 points to 10 and a measure that held 68 characters now holds 75 or more. You did not touch the text block width, but you lengthened the effective line and made the page denser at once.
A bigger size is not automatically more readable. Going to 12 points on a fixed text block shortens the line to perhaps 55 characters, which is workable — but keep going and the lines get so short the eye jumps constantly and word spacing goes ragged. Every bump also adds pages.
The order that works: choose the trim, set the margins so the measure lands in range at your intended size, then set the leading to suit the measure you ended up with.
Check a real page, not the numbers
Theory gets you to a starting point. A printed spread tells you whether you are there.
Read a full spread, properly. Two or three hundred words, at normal speed. Notice whether your eye returns cleanly to the left or whether you ever re-read a line or skip one. That stumble is a leading problem or a measure problem, and it is the only test that matters.
Hold it at arm's length and stop reading. The page should look evenly gray, with no light channels running down it and no dark clots. Uneven texture at that distance is a word-spacing problem, and it usually traces back to a measure that is too short for the type size. See Widows, orphans and rivers.
Look at the last line of every paragraph. A single short word stranded on its own line is a break you can usually fix by adjusting the settings rather than the prose.
Every one of these changes the page count
This is the consequence that catches people out.
Size, leading and measure decide how many lines fit on a page and how many characters fit on a line. Change any of them and the book gets longer or shorter — often by twenty or thirty pages on a novel. A longer book costs more to print per copy, and that comes out of your royalty on every sale.
It also changes the spine, which is calculated from the final page count. A type decision made in the interior is a cover decision too, so settle the interior first. See Spine width.
In the Interior Formatter. Every setting recomposes the book and gives you the real page count, and that final count flows to the Cover Maker so the spine follows the interior you actually chose. Start a book → · about the Interior Formatter
Make yours
Everything here is built into the tools, and they are free to use: name your book, bring your manuscript or your artwork, and work as long as you like. You only pay when you want the files without the preview watermark.
Next: Large Print edition · Back to Choosing a typeface for the body of a book · Also: Choosing a trim size