LPI (Lines Per Inch) is the measure of screen frequency or the number of rows of halftone dots per inch, which directly determines the resolution of printable details and the smoothness of color gradients. This criterion is one of the most important decisions in the Prepress department, which must be specified before setting the input file's PPI.

LPI must be chosen to match the absorption and smoothness of the printing substrate. Smooth, coated papers can support higher LPI (such as 150 to 200) for better detail, while rougher and more absorbent papers (such as packaging board and plain papers) must be printed with lower LPI to prevent excessive Dot Gain and image darkening.