Line Spacing Calculator

Line spacing is the baseline-to-baseline distance in a block of set text. CSS names it line-height, while print typography names the extra space leading. The arithmetic here is deliberately simple: line height equals font size multiplied by a ratio, and leading equals line height minus font size, distributed as equal half-leading above and below each line box. Expressing the value as a unitless ratio rather than a fixed pixel figure matters because a ratio is inherited by child elements and recomputed against their own font size, so nested headings and captions do not inherit a stale absolute value. Practitioner ranges are well established: roughly 1.4 to 1.6 for screen body text, 1.2 to 1.4 for printed body text where higher resolution and paper texture assist the eye, and 1.1 to 1.3 for display headings above about 36px. A 16px base at 1.5 gives the familiar 24px baseline unit. Measure interacts with all of this, with 45 to 75 characters per line the standard comfortable range and about 65 the usual target. The main limitation is that a ratio knows nothing about the typeface. Two fonts set at 16px and 1.5 can look very different because x-height, ascender and descender lengths, stroke weight and counter size all change the apparent density of a line. Faces with tall x-heights such as Verdana or Open Sans read tighter at the same ratio than a small-x-height face such as Garamond. Non-Latin scripts diverge further, since Arabic and Devanagari carry deep descenders and stacked marks that a Latin-tuned ratio will crowd. WCAG 2.1 Success Criterion 1.4.12 sets a floor for user-adjusted spacing at line height 1.5 times font size, paragraph spacing 2 times font size, letter spacing 0.12 times and word spacing 0.16 times, and layouts must survive those values without clipping. Any single readability score, including the one shown here, is a heuristic rather than a measured reading-speed result.

The word leading is literal. From the hand-press era through hot metal, compositors inserted thin strips of lead alloy between lines of cast type to open up the text block; type set with no strips at all was described as set solid. The measurements involved were standardised over the eighteenth and nineteenth centuries, first through the point systems of Pierre Simon Fournier in 1737 and later Francois Ambroise Didot, then through the Anglo-American point adopted in 1886 at approximately 0.013837 inch. Mechanical composition changed the workflow rather than the concept: the Linotype machine, patented by Ottmar Mergenthaler in 1886, cast a whole line as a single slug with the leading built into the body, and Monotype followed with single-character casting. Phototypesetting in the 1950s and 1960s finally separated leading from physical metal, which for the first time permitted negative leading, and PostScript and desktop publishing from 1984 onward carried those controls into software. On the web, line-height appeared in CSS Level 1, published as a W3C Recommendation in December 1996, and the unitless form that inherits as a ratio was clarified in CSS2. The empirical case for generous spacing is older than the web: Miles Tinker and Donald Paterson ran a long series of legibility experiments at the University of Minnesota from the 1920s through the 1940s, measuring how leading and line width jointly affect reading speed, work later consolidated in Tinker's 1963 book Legibility of Print. Accessibility requirements arrived much later, with WCAG 2.1 published in June 2018 adding the Text Spacing criterion that made user-overridable line spacing a formal conformance issue rather than a matter of taste.