Readability. It plays an important role on how easy it is to consume a website’s content. Along with font size, color and style, the number of characters per line (cpl) of text is a major contributor here.

When we read, our eyes travel across a line of text in a pattern of rapid movements and stops (saccades and fixations). Each saccade spans an average 7 – 9 characters and each fixation lasts for around ¼ of a second. During fixation we read and process text around the fixation point then jump to the next fixation point and do the same. When we’re done reading a line of text, we hop to the next line at which point our mind is energized and the saccade – fixation pattern starts again.

The more times this pattern occurs on a single line, the more we lose focus. As our focus lessens, we spend more time processing information at each fixation point and the character span of each saccade decreases. It also makes it more challenging to find the start of the next line of text.

If there aren’t enough characters per line our saccade rhythm gets interrupted too frequently which energizes our mind and in turn makes us feel rushed. When rushed we tend to skip or skim content.

There general cpl rule of thumb is 55 – 75. This allows a reader to develop a cadence conducive to the enjoyment of content and makes engagement more likely.

There are three major contributors to cpl:

Content Width

Font Style

Font Size

At first it will take a little experimentation to get the combination of these three things right. You should know what kind of content you’re working with before you start. This will give you an idea on your content width and limit your font style selection. Then it just comes down to putting elements together and a little trial and error.

There are other factors that contribute to a website’s readability, however, getting the cpl right is crucial.