“How large can I print my photo?” is a question I get from Lightroom users all the time. The ultimate answer of course is - it depends. In the end you will need to do your own tests and find out what you are satisfied with. Below is some information that should help you with that process. It applies whether you do your own printing, or send your photos out to be printed.
Your photo has a limited number of pixels, or squares of information, in it, based on what your camera captured or what you cropped your photo down to. For example, a capture from an old 6 megapixel camera is 3,000 pixels wide x 2,000 pixels high. (There are various ways to display this information for a photo in Lightroom — try typing “I” once or twice in Library or Develop. Type I again to turn off the information display.)
When you decide on a print size, those available pixels get spread out to fit that print size. Here are some examples from that 6 MP camera:
4″x6″ print: 2,000 pixels / 4″ = 3000 / 6″ = 500 pixels per inch (ppi)
8″x12″ print: 2,000 pixels / 8″ = 250 ppi
16″x24″ print: 2,000 pixels /16″ = 125 ppi
This ppi is called the native resolution of your photo, at the given print size. It is what is inherently, or natively, available to you at the size you plan to print.
But guess what — your printer (or Shutterfly’s or Costco’s or anyone’s) doesn’t print at your photo’s native resolution!! It prints at what it likes to print at — i.e. at its own native resolution. Generally this is 360 ppi for Epson printers, and 300 ppi for HP, Canon and other printers.