PPI: stands for Pixels per Inch. PPI describes the number of pixels per inch in a photo. PPI is a function of the number of pixels the camera’s sensor supports (also known as megapixels) and the size of the photo. To calculate a photo’s PPI simply multiply the page length by its width in inches. The result is the number of square inches on the page. Now divide the number of pixels the sensor supports by the number of square inches. The result is the number of pixels per square inch. All that is left to do is to find the square root of this number. Following is a table that shows the PPI for various page sizes for a 5 megapixel camera.
page size 4X6 - 456 PPI
page size 5X7 - 377 PPI
page size 8X10 - 250 PPI
page size 11X14 - 180 PPI
page size 16X20 - 125 PPI
page size 20X30 - 91 PPI
DPI: stands for Dots per Inch. DPI is really a physical characteristic of a printer. Every printer prints dots that when put next to each other comprise a photo. Each dot has a physical size. DPI is also known as the maximum resolution that a printer is capable of. Low-end printers have lower DPI while high-end printers have higher DPI. DPI is defined as the number of dots a printer can print per inch. For example if a printer supports 1200 DPI it means that the printer can print 1200 dots per inch (on both X or Y axis). When printing it is important to make sure that the DPI is higher or equal to the PPI. If the DPI is lower than the PPI the printer will not be able to fully display the high resolution of the photo. When printing a photo that has a lower PPI than DPI the printer will use multiple dots to represent one pixel. As opposed to PPI, DPI is not relative to the page size. DPI is a fixed number for a given printer.
