The optimal reading line length equals 12 to 20 words. Alternatively, ideal readability is achieved by having 65 to 70 characters per line, that is including the spaces. Studies show that moderate line lengths significantly increase the legibility of text.
A good rule of thumb to quickly remember is this:
2 x the alphabet + 2 x the numbers 1 to 9 = 70 characters.
Example of an optimal line length
This line, my good people, has exactly 70 characters including spaces.
While this line is exactly 65 characters long, including the spaces.
It is a good idea to aim for a line length that approaches this number of characters, as it will ensure that your readers do not leave the site prematurely because of reading fatigue.
Line length and attention span
The ideal line length for text layout is based on the physiology of the human eye, At normal reading distance the arc of the visual field is only a few inches, which is about the width of a well-designed column of text or about 12 words per line.
Research shows that reading slows and retention rates fall as line length begins to exceed the ideal width, because the reader then needs to use the muscles of the eye and neck to track from the end of one line to the beginning of the next line. If the eye must traverse great distances on the page, the reader is easily lost and must hunt for the beginning of the next line.
We read faster with longer lines but prefers short ones
People read faster with longer line lengths (100 characters per line), but they prefer a short or medium line length (45 to 72 characters per line).
The research also shows that people can read one single wide column faster than multiple columns, but they actually prefer multiple columns.
So if you ask people which they prefer they will say multiple columns with short line lengths. Interestingly, if you ask them which they read faster, they will insist it is also the multiple columns with short line lengths, even though the data shows otherwise.