Measure text density in characters per line.
Text density combines two typographic variables that jointly determine how tiring a page is to read. The first is measure — characters per line — where the long-established comfortable range is 45 to 90. Wider than that and the eye struggles to find the start of the next line; narrower and constant line returns break the reading rhythm. The second is leading, the line spacing relative to font size, where anything below about 1.25 makes dense text hard to track because adjacent lines visually interfere. The density index divides line fill by leading, so a full line with generous spacing scores lower than a full line packed tightly.
Measure
Characters per line = total characters / number of lines
Density index
Density = (characters per line / column width x 100) / leading ratio
It balances two costs. A long line makes the return sweep to the next line error-prone, so readers re-read lines without noticing. A short line forces so many returns that the rhythm never settles. Typographers have converged on roughly 66 characters as the ideal single-column measure.
Longer lines need more leading. With a wide measure the return sweep is long, so extra vertical space helps the eye land on the correct line. A wide column with tight leading is the worst combination and is a common problem in dense documents.