line length

Line length (also called measure) is the horizontal width of a block of text, commonly expressed as a character count per line. It is one of the primary controls on readability.

Lines that are too long force the reader’s eye to travel far from the end of one line back to the beginning of the next, increasing the chance of losing place. Lines that are too short break phrases awkwardly and produce a choppy reading rhythm. The conventional guideline for body text is 45 to 75 characters per line, with 66 characters often cited as an ideal. For shorter formats like captions or sidebars, shorter measures are appropriate.

Line length interacts with line height: longer lines need more vertical space between them so the eye can track correctly. It also interacts with margins, since narrower margins produce longer lines within the same container.

In responsive web design, line length is managed through max-width on text containers rather than by setting a fixed width. This allows the text to reflow at narrower viewports while keeping the measure within readable bounds at wider ones.