in Design
Line length, and the measure readers never notice
Readers rarely notice the length of a line, but they feel it. Why typographers settle on somewhere between 45 and 75 characters.
In typography, the measure is the width of a line of text. Readers almost never think about it, and when it is right they have no reason to. When it is wrong, reading becomes tiring in a way that is hard to name.
The range typographers use
The most widely cited guidance comes from Robert Bringhurst’s The Elements of Typographic Style, which suggests that anything from around 45 to 75 characters per line is satisfactory for single-column text set in a serifed face, with 66 characters often treated as the ideal. The count includes letters, spaces and punctuation.
These figures describe a comfortable zone, not a precise target. They also describe continuous prose. Multi-column layouts, such as newspapers and some reference books, work with shorter lines, and Bringhurst gives a lower range for those.
It is worth noticing that the guidance is expressed in characters, not in millimetres or inches. The same physical width holds more characters in a narrow face or at a small size, and fewer in a wide face or a large size. Changing the typeface can therefore change the measure without changing the text block at all.
Why long and short lines both tire the eye
When a line is very long, the eye has a long way to travel back to the start of the next line, and it is more likely to miss its target. Readers find themselves re-reading the same line or skipping one. Long lines also make paragraphs look grey and unbroken, which is discouraging before a word has been read.
More leading helps a long line, because it makes each line easier to find. But there is a limit, and past it no amount of leading will rescue the page.
Short lines break reading rhythm by forcing the eye to return too often. In justified text, they cause a more visible problem: there are fewer spaces in each line to absorb the adjustments that justification requires, so word spacing becomes uneven, gaps open up, and hyphenation increases. Rivers of white space running down the page are much more common on a narrow measure.
This is why a measure that works for ragged-right text can be too narrow for justified text, and why narrow columns are often set ragged right.
Measure and the rest of the page
In a printed book, the measure is not decided in isolation. It follows from the trim size, the margins, the typeface and the type size, and changing any one of them affects the others. A small-format paperback with generous margins may have a narrow text block, which pushes the designer towards a slightly smaller or narrower face to keep enough characters on the line. A larger format may need a larger size or wider margins to stop the measure becoming too long.
A quick check is to count the characters in a few typical lines of a test page. If most lines fall somewhere in the middle of the range, the measure is probably fine. If they are regularly above 80 or below 40, it is worth revisiting the page before going further.
On screens
Reflowable ebooks and web pages hand much of this control to the reader, whose device, orientation and text size settings determine the measure. Designers can still set sensible limits where the format allows it, such as a maximum text width on the web. The principle is the same everywhere: the reader should be able to follow the line without thinking about it.
The best measure is the one nobody comments on.