We conducted a number of case studies in an attempt to identify common design solutions and practices. So far, we have analyzed Web forms, blogs, typography and portfolios; and more case studies will be published next month. We have found some interesting patterns that could serve as guidelines for dealing with Typography, Line Heights, and Line Length.

• Line height (in pixels) ÷ body copy font size (in pixels) = 1.48
  1.5 is commonly recommended in classic typographic books, so our study backs up this rule of thumb. Very few websites use anything less than this. And the number of websites that go over 1.48 decreases as you get further from this value.
• Line length (pixels) ÷ line height (pixels) = 27.8
  The average line length is 538.64 pixels (excluding margins and padding), which is pretty large considering that many websites still have body copy that is 12 to 13 pixels in font size.
• Space between paragraphs (pixels) ÷ line height (pixels) = 0.754
  It turns out that paragraph spacing (i.e. the space between the last line of one paragraph and the first line of the next) rarely equals the leading (which would be the main characteristic of perfect vertical rhythm). More often, paragraph spacing is just 75% of paragraph leading. The reason may be that leading usually includes the space taken up by descenders; and because most characters do not have descenders, additional white space is created under the line.
• Optimal number of characters per line is 55 to 75
  According to classic typographic books, the optimal number of characters per line is between 55 and 75, but between 75 and 85 characters per line is more popular in practice.

Have any thoughts on what we’ve covered, or know of other useful usability findings? Please leave a comment below.