This is a very good article on how PPI (pixels in an image) to DPI (dots per inch(printer)) works.
Also remember that pixels are not all equal, pixels of original resolution are better than enlarged resized and manipulated pixels, and JPG is always a distortion problem if you later resize, use transparency or in other ways choose to manipulate. Never use jpg to save original work if you can avoid it.
Basic rule is 150 to 300 PPI image to print from good to photo quality which equals 600 to 2400 DPI.
When scanning reverse the process, scan DPI / 4 (apx)= PPI.
toby snelgrove...
Thanks for the article
When uploading to redbubble, using lightroom, they ask you to set dpi …. I am shooting raw with a 10mp d100 nikon … what should I set the dpi to?
Dominic Melfi
Link to article 2"::http://www.tildefrugal.net/photo/dpi.php
Unfortunately everyone dosnt use these terms to mean the same. In my sense of it DPI is used only when talking about displaying to printer. These devices will have certain usable settings 300 , 600, 1200.
Dominic Melfi:
There is also a DPI reference for display screens normally about 96 DPI.