This formula means that if you use 8 pt type, then you would have a 20 pica column (3.333 inches or 8.46 cms) which is roughly 10-13 words depending on how you count them. However, there’s no hard and fast rule, and many things can be finessed. margins, typeface characteristics make a difference. In my mind, the list goes on. Check out about.com for their info at Line Length Formulas – Choose an Ideal Line Length For Text.