Often the term PPI (Pixels per Inch) is confused with DPI (Dots per Inch). PPI is the measurement of the quality of a print. It is the number of pixels per inch in a photo. PPI To calculate a photo’s PPI, multiply the page length by it’s width in inches. Then divide the number of pixels of the sensor by this number. Then find the square root of the result.
For example, for a 5 megapixel camera:
page size 4 x 6
4 x 6= 24
5000000 / 24 = 208333.32
The square root of 208333.32 is 456.435
Therefore the PPI for 4 x 6 photo from a camera of 5 Megapixel is 456
page size 5 x 7 - 377 PPI
DPI - Dots Per Inch, is defined as the number of dots a printer can print per inch. DPI is not relative to the page size. DPI is a fixed number for a given printer. 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 number of pixels per inch in the source photo. If the DPI is lower the printer will not be able to fully display the high resolution photo. When the DPI is higher the printer will use multiple dots to represent one photo pixel.
If you plan to use your photos for enlargements then a rule of thumb is to be able to print at least 300DPI resolution. The following is a table for different print sizes and the needed mega pixels for such print quality:
page 4X6 2MP
page 5X7 3MP
page 8X10 7MP
page 11X14 14MP
page 16X20 28MP
page 20X30 54MP