Point size is the wrong number to argue about

Why two 12pt fonts look completely different sizes

Point size measures the body the letters sit on, a leftover from metal type, not the letters themselves. What the eye actually responds to is x-height: the height of a lowercase x, and therefore of the bulk of any lowercase text.

Two faces at the same point size can differ in x-height by a third. This is why a 12pt setting in one typeface is comfortable and a 12pt setting in another is squinting, and why “make it 14pt” is not a specification. The specification is: set a paragraph, print or view it at the real size and distance, and look.

A high x-height helps at small sizes and on screens. It costs something at large sizes, where the more varied proportions of a low-x-height face give a page more texture.

The measure: 45 to 75 characters

Line length is the setting that most reliably separates readable body text from unreadable body text, and almost nobody adjusts it.

The working range is 45 to 75 characters per line, including spaces, with about 66 as a comfortable middle for continuous prose, and 35 to 45 for a narrow column.

The reason is the return sweep. At the end of each line the eye jumps back and down to the start of the next. That jump is not precise; it lands approximately and corrects. The longer the line, the further the jump and the more often it lands on the wrong line, so a reader loses their place, re-reads, and gradually gives up without knowing why. Very short lines fail from the other side: too many sweeps, and the phrase-sized chunks reading depends on get broken across lines.

You can count it. Type x sixty-six times at your body size and set the column to that width.

Line height, and the relationship it has with measure

Body text usually wants line height of about 1.4 to 1.6 times the font size. Below 1.3 the lines interfere, and the return sweep has nothing to aim at. Above about 1.8 the lines stop reading as a block and start reading as separate items.

The two settings are coupled: a longer measure needs more line height, because a longer return sweep needs a larger target. This is why pasting a comfortable setting into a wider column breaks it.

All-capitals costs reading speed, and here is why

Setting a paragraph in capitals slows reading measurably: the figures in the literature vary, commonly landing in the region of ten to twenty per cent for continuous text.

The mechanism is word shape. Lowercase words have distinctive outlines produced by ascenders and descenders: yellow has a low start, two tall letters and a descender. Capitals remove all of it and make every word a rectangle of the same height, so the reader loses a cue they were using and has to work letter by letter.

This does not make capitals wrong. It makes them expensive, so they are for things that are short and want to be conspicuous rather than read fluently: a two-word label, a heading of four words. A paragraph of capitals is a paragraph most people will skip.

Floors worth knowing

• 16px is the practical minimum for web body text, and it is also the threshold below which mobile Safari zooms a form field automatically when it is tapped: a real, visible bug caused by a type size.
• 9 to 11pt is normal body text in print. A flyer read at arm’s length and a poster read from three metres are different problems entirely; type size follows viewing distance, not habit.
• Justified text on a narrow measure produces rivers of white space, because the software has too few word spaces to distribute. Either widen the measure, enable hyphenation, or set it ragged-right. Ragged-right is not a compromise; most books set ragged-right would be fine.

Contrast is part of readability

Light grey text on white is a style that has cost more reading than any typeface choice. Type carries information in thin strokes, and thin strokes lose contrast first, which is why a light weight at low contrast disappears in bright daylight on a phone, a condition a large share of your audience is in.

The next lesson deals with contrast ratios properly. The point here is that it belongs to typography, not to colour: choosing a hairline weight is choosing a contrast problem.

The limitation

Nearly all of the legibility research behind these numbers was done on Latin script, on screens or paper in controlled light, with young adults who had normal vision. The findings are sound within that frame and should be held loosely outside it.

Other writing systems have different determinants (Devanagari hangs from a headline stroke, Arabic is cursive and connected with contextual letterforms, Chinese characters are read by a different process at a different density) and rules derived from the Latin alphabet transfer unevenly at best. Older eyes, low-vision readers, poor phone screens in sunlight and cheap print all shift the thresholds upward. When in doubt, go bigger and higher-contrast than the numbers require, because the cost of overshooting is aesthetic and the cost of undershooting is that nobody reads it.