How long should a line of text be before readability starts to drop?

Readability holds when a line of body text runs roughly forty-five to seventy-five characters, counting letters, spaces, and punctuation, with the mid-sixties often cited as the comfortable target. Stray much past seventy-five and reading gets harder; drop much below forty-five and it gets choppy. This is the long-standing measure that readability research keeps landing on, and it is worth treating as a working guide rather than a hard law, because the right number shifts a little with audience and type size. The important shift in mindset is that you control line length in characters, not in pixels or percentages of the container.

The reason the upper bound bites is the return sweep. When you finish a line, your eye has to travel back across the full width to find the start of the next one. On a long line that journey is far, and the eye loses its place, sometimes re-reading a line or skipping one entirely. On very short lines the opposite failure shows up: the eye barely settles into a rhythm before it has to jump again, so the constant return motion fragments the flow and the prose feels jittery. The comfortable measure is the band where the eye can sweep, land, and move on without effort, which is why both edges of the range, not just the long one, matter.

Here is where designers get bitten in practice. You build a content layout, the body text is set to fill its parent, and on a laptop the column reads fine at maybe seventy characters. Then the same page loads on a wide desktop monitor and the paragraph stretches edge to edge across thirteen hundred pixels, producing lines of a hundred and twenty characters or more. Nothing in the CSS broke; the text simply inherited the container width. The fix is not to constrain the whole layout but to cap the text column. A common approach is to give the body a max-width expressed in the ch unit, since one ch is roughly the width of a character in that font, so max-width: 66ch keeps the measure honest no matter how wide the viewport grows, letting it track readability instead of geometry.

The wrinkle worth naming is that the character count is not uniform across all type. Wider fonts and larger sizes fill the measure faster, so a heavy display face at a large size may hit the comfortable upper limit at fewer characters than a condensed text face. Short UI text, captions, single-line labels, and pull quotes also live outside this rule, since they are not sustained reading and the return-sweep problem never arises. The range is a guide for paragraphs people actually read top to bottom, not for every string on the screen, and pushing slightly past seventy-five for an experienced-reader audience or pulling toward the low end for a narrow mobile column are both reasonable judgment calls.

So the action is concrete. Stop letting the body text inherit the container width and instead constrain the text column directly to a comfortable measure, around forty-five to seventy-five characters with the mid-sixties as a good default, then read a real paragraph at your widest and narrowest breakpoints to confirm the lines feel scannable rather than sprawling or clipped. When the measure is right, the eye stops working and just reads.