Terms like resolution calculation, image dimensions, screen width, etc. are a great mystery to laypersons, because the terms relate to a very technical branch of desktop publishing. The issue is further confused by the fact that all three terms essentially express the same idea. The names simply sprung from different branches of the same field, though some still prefer to distinguish between them.
First of all, the print resolution is given in DPI, which indicates the number of dots (print points) per inch. In addition, the screen resolution is defined in LPI (lines per inch) to indicate the amount of printable screen lines per inch. Last but not least, the unit PPI (pixels per inch) is used to indicate the scanner or monitor resolution.
In the end, all three terms describe the density of image points per unit of length – the amount per inch. An American inch is 2.54 cm and is often used as a unit in technical fields, e.g. for picture diagonals on screens or memory device diameters.
To clarify things, however, there are just a few simple rules of thumb to follow. In comparison to images displayed on monitors, printed graphics require a significantly higher resolution, because printed products require many more pixels than a screen would for the same area to present the human eye with homogenous colour blocks.
While monitors only operate with a resolution of 72 ppi, print graphics should always be laid out at a minimum of 300 dpi (whether greyscale or colour) to produce a satisfactory printed result. For line art that contains delicate designs or text in a very small font, 1200 dpi is advisable to avoid a squaring effect, which occurs when images feature high contrasts and hard colour edges.
Also keep in mind that pictures should never just be scaled bigger to increase the resolution, as this will only harm the quality.
