The term PPI is not used much. Many people confuse PPI with DPI or have never heard the term PPI. PPI is a measurement of the quality of a print and understanding this term and what the number means can help you determine the right sizes and resolutions for your next digital photo print.
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 cameras sensor supports (also known as megapixels) and the size of the photo. To calculate a photos 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