Line length is the distance between the left and right edges of a text block. Overly long lines are a common problem, but they’re easy to correct. Shorter lines will make a big difference in the legibility and professionalism of your layout.
The most useful way to measure line length is by average characters per line. Measuring in inches or centimeters is less useful because the point size of the font affects the number of characters per inch. Average characters per line works independently of point size.
Shorter lines are more comfortable to read than longer lines. As line length increases, your eye has to travel farther from the end of one line to the beginning of the next, making it harder to track your progress vertically.
Aim for an average line length of 45–90 characters, including spaces. You can check line length using word count.
Alternatively, use the alphabet test to set line length. You should be able to fit between two and three alphabets on your line, like so:
I set the width of a div containing body paragraphs to 65em (give or take) and find that works well as a way of controlling line length.
65em is too wide because M is a very wide character. You’ll easily be able to fit 90+ characters on a 65em line.
One alphabet in most typefaces comes out to around 13em, i.e. 2 characters per em. A quick computation then gives a line width of around 35em — slightly less for narrow typefaces like Times, slightly more for wider ones like Garamond.
Ideally, set your 8.5 x 11 sheet into 3 column width, and then don’t make your copy span wider than 2 of the 3 columns, as a maximum.
A 2 column grid also works well, but the 3 column will fit more copy per page.