There is a four-step process that can help determine the best line length. Keep in mind that these formulas work only when using the selected font. Changing the font and type size alters the results of the line length formulas. Follow through on all four steps when determining the ideal line length to get a range that best works with your layout and font size.
View the supporting illustration to see the line length formulas in action.
"Right and wrong do not exist in graphic design. There is only effective and non-effective communication." Peter Bilak - Illegibility
If the desired size of type and the line length used in the page layout are incompatible, one of them needs to change. Typically it is the font size that gets adjusted but don't be afraid to re-evaluate your page layout to accommodate wider or narrower text columns.
The Bottom Line: The line length rule applies primarily to body copy. Headlines, subheadings, and other small bits of text have more flexibility. Almost any reasonable line length will work in a design if combined with the right size font. The longer the line, the larger the font can be. The shorter the line, the smaller the font can be.