How much line-height is too much before paragraphs lose cohesion?

Comfortable body line-height sits around one and a half times the font size, and it becomes too much when the lines drift so far apart that a paragraph reads as a stack of separate rows instead of a single block. That figure of roughly 1.5 is the widely cited working value for body text, the same multiple accessibility guidance treats as the threshold users should be able to apply, so it makes a sound default. The cohesion limit is not a second number you memorize but a perceptual line: the moment the gap between lines grows large enough that the eye stops grouping them into one paragraph, you have gone too far. Tie the value to your text size and line length rather than picking it in the abstract.

The reason cohesion can break is that line-height is doing two opposed jobs at once. It has to open the lines enough that the eye can track from the end of one to the start of the next without catching on the line above, which is why cramped line-height feels suffocating and slow. But the lines also have to stay close enough that they read as a unit, because a paragraph is perceived as a block precisely when its lines sit nearer to each other than the paragraph sits to whatever surrounds it. Proximity signals belonging. Push the internal line spacing too wide and you erase that signal: each line floats with as much air around it as the space between paragraphs, so the grouping cue vanishes and the reader sees rows, not a paragraph.

Concretely, take sixteen-pixel body text. At a line-height around 1.5 the lines breathe and the paragraph still reads as one solid shape, which is the comfortable zone. Now a designer, believing more space always means more readable, bumps it toward 2.2 to make the text feel airy and premium. Each line is now so far from its neighbors that the eye has to jump a real distance to continue, the rhythm of reading stutters, and worst of all the paragraph no longer looks like a paragraph; it looks like a list of unrelated single lines because the within-paragraph gaps rival the between-paragraph gaps. The airiness that was supposed to help has dissolved the block. Pulling it back toward 1.5 restores the paragraph as a recognizable unit and the reading flow returns.

The nuance worth naming is that 1.5 is a center, not a fixed point, and it should move with measure and size. Long lines, the seventy-plus-character measures, benefit from slightly more line-height, nudging toward 1.6 or so, because the eye needs more vertical separation to find the next line’s start after a long return sweep. Short, narrow columns can sit comfortably lower, around 1.3 to 1.4, since the return trip is short and tight lines still read as a block. Large display headings usually want less than body, often well under 1.5, because big type at generous spacing pulls apart fast. The rule adapts; the cohesion test does not.

So set body line-height near 1.5 as your starting point, then read an actual paragraph and check the one thing that matters: does it still look and read like a single block, or has it broken into separate rows? Adjust for your measure, a touch more for long lines, a touch less for short ones and for headings, and let the paragraph staying whole, rather than a bare instinct that more air is always better, be the thing that tells you when you have added enough.