Margins are the decision people make last and least, on the theory that they are empty and therefore do not matter. They are the decision that sets the line length, and line length is the single number that decides whether a page is comfortable to read. Everything below is in service of that one number.
The measure, and why 55 to 75
The measure is the number of characters on a full line of body text, spaces included. Reading works by the eye jumping along a line in short saccades, then sweeping back to the start of the next one. If the line is too long, the sweep back overshoots or undershoots and the reader re-reads a line or skips one. If the line is too short, the rhythm breaks every few words and the page feels choppy.
The range that centuries of book design settled on is 55 to 75 characters, with about 66 as the point most typographers would name if pressed. Below 45 is a newspaper column; above 85 is a web page nobody set. The range is wide enough that you do not need to hit a specific number, and narrow enough that a word processor's defaults miss it.
Why A4 overshoots
An A4 page is 595 points wide. A word processor's default margin is around 2.5cm, or 72 points, a side — which sounds generous and leaves 451 points of text. At 11pt in an average serif, a character is about 5.5 points wide on average, so that line holds roughly 82 characters. Already past the range, and that is with margins most people consider large.
Drop the margins to 1.5cm, as people do to save paper, and the line reaches 95. That is the page everyone recognises as "a Word document" without being able to say why. The why is the measure.
There are only two levers: make the type bigger, or make the margins wider. Bigger type has a ceiling around 12pt before the book starts to feel like a large-print edition. So most of the correction has to come from margins, and the margins a book needs are larger than the margins a memo needs.
Margins in points, with worked numbers
Working in points rather than centimetres keeps the arithmetic in one unit with the type size. A4 is 595 × 842 points. The table below assumes an average serif; a face with a small x-height like Garamond runs a little narrower per character and a wide face like Literata a little wider, so treat the counts as ±5.
| Margin each side | Text width | At 10.5pt | At 11pt | At 12pt |
|---|---|---|---|---|
| 40pt | 515pt | ~98 | ~94 | ~86 |
| 60pt | 475pt | ~90 | ~86 | ~79 |
| 72pt | 451pt | ~86 | ~82 | ~75 |
| 85pt | 425pt | ~81 | ~77 | ~71 |
| 100pt | 395pt | ~75 | ~72 | ~66 |
| 115pt | 365pt | ~70 | ~66 | ~61 |
Read across and the pattern is clear: on A4, getting an 11pt body into the range needs margins around 100 points a side, which is 3.5cm. That looks alarming on screen and looks like a book on paper. Every well-set hardback on your shelf has proportionally more white space than that.
If 100-point margins feel like too much page given away, the alternative is 12pt type with 85-point margins, which lands in the same place with a slightly larger face. Both are correct. What is not correct is 11pt with 40-point margins, however efficient it looks.
Top and bottom margins are freer, because they do not affect the measure. A classical page has a head margin smaller than the foot — the text block sits slightly above centre, which reads as balanced even though it is not. Something like 70 points at the head and 90 at the foot, with room for a page number inside the foot, is a reasonable start.
Inner and outer margins, if it will be bound
A PDF read on screen has four edges that are all the same. A printed, bound book does not: the inner edge of every page curves into the spine, and some of the margin there disappears into the binding. Print designers call this the gutter and compensate by making the inner margin larger than the outer one.
The classical proportions, if you want them, are roughly inner : top : outer : bottom as 2 : 3 : 4 : 6 — the outer margin twice the inner, the foot twice the head. Few modern books go that far, but the direction is right: more room at the outside, where a thumb goes, and more at the foot than the head.
For a book you are printing yourself or through a print-on-demand service, add 10 to 15 points to the inner margin on top of whatever symmetric margin you chose, and check the service's own minimums — what Amazon KDP actually wants from your file has theirs. For a PDF that will only ever be read on a screen, keep the margins symmetric. Mirrored margins on a screen look like a mistake, because there is no spine for them to be mirroring around.
Line height is a function of measure
The two numbers are linked, and this is the part most guides leave out. A longer line needs more space beneath it, because the eye has further to travel back and needs a clearer landing. A shorter line can be set tighter without the lines crowding.
| Measure | Line height |
|---|---|
| 55–60 characters | 1.3–1.4 |
| 60–70 characters | 1.4–1.45 |
| 70–75 characters | 1.45–1.5 |
| Over 75 | Fix the measure first |
That last row is not a joke. Opening up the line height does make an over-long line easier to follow, but it is treating the symptom. If you have found yourself at 1.6 line height to make a page readable, the measure is wrong, and the margins are where to fix it.
The other constraint on line height is paragraph density: below about 1.25, lines of a serif face start to touch at ascenders and descenders, and above about 1.6 a paragraph stops reading as a block and starts reading as a stack of separate lines. The ranges in the table sit inside those limits on purpose.
Where the page number goes
In the foot margin, which is why the foot margin is larger. Centred is the safe choice for a single-column book and is what most tools produce by default. Outer-edge numbers, which alternate sides on facing pages, are a convention of bound books and look odd in a PDF read as single pages.
Leave the number at least a line's height clear of the text block, and set it a point or two smaller than the body. A page number is a signpost, not a sentence. How to add page numbers to a PDF from Markdown covers getting them in there in the first place.
Putting it together on A4
- Body at 11pt. Margins of 100 points left and right, 70 at the head, 90 at the foot.
- Export a page. Count three lines. You should be at 66 to 74 characters.
- If the face runs narrow and you are under 60, drop the side margins to 90. If it runs wide and you are over 78, raise them to 110 or the type to 11.5.
- Line height 1.45.
- Page number centred in the foot, at 9 or 10pt.
- If it will be bound, add 12 points to the inner margin and check the printer's minimums.
In bukpress the controls are margins in points and line height, with page numbers in the footer on or off, on an A4 page. That is the whole of what the above needs. The wider question of what else the page has to get right — type size, the pairing, the heading scale — is in book typography for people who are not typographers, and the sample book shows the numbers above applied to eight real pages.