How to Convert JPG Image Pixels to Print Size.  By Don Finkeldei:  I get a lot of questions about this.  It depends on the resolution.  Read on..

 I get a lot of requests about how to determine the print size from the pixel size of a jpg image.  For Instance, "I have an image that's 1024 pixels wide and 768 pixels high.  How big will that image be if I print it".  The pixel width and height will tell you nothing about how big it will print, or how high the print quality would be.  A critical piece of information is missing.... you have to take into account the resolution setting in addition to the pixel dimensions of an image.

All jpg images have pixel dimensions and resolution stored in the image file data.  The only way to get this info is open it in a photo editor like Paint, Photoshop, etc..  Printers read it and print accordingly.  A computer screen does not read the resolution of an image.  A computer renders the image on the screen based solely on pixel width and pixel height and disregards the resolution.  If the image has a resolution of 300 it will appear much larger on your computer screen than it would print.  A printer reads the resolution and prints accordingly.  A print resolution of 300 pixels/inch means that the printer will print 300 of those little dots in every inch both for height and width, but the quality will be higher because the pixel density per print inch is high.

You can read the pixel dimensions (height and width) and Resolution by opening a jpg image in a photo editor like Paint, or Photoshop.  The editor will usually also tell you the print dimensions.... BUT, a knowledge of how that is determined is very useful.  Here's how:

Let's say you have a jpg image that's 1024 pixels wide and 768 pixels high.  You don't have enough information to determine print size.  Open the image in a photo editor and see what resolution it is.  You can also change these parameters as long as you don't exceed the 1024 x 768 pixel width and height.  You will loose clarity (resolution if you change the pixel height and width to more than the original had.   now, say the full information is 300 pixels/inch resolution and the pixel dimension is 1024 wide x 760 high.  How big will the print be:  If it's going to print 300 pixels for every inch, then it's simply 1024/300 = 3.41 inches wide and 768/300 = 2.56 inches high.  A resolution of at least 300 pixel/inch is required for a good high resolution print.  

If the resolution of the above jpg image is 72 pixels/inch, then the print size would be 1024/72 =  14.2 inches wide and 768/72 = 10.66 inches tall.  A much bigger print size -- but the quality will be very low and will look fuzzy.

My 10 megapixel camera will print an image12.2 inches wide and 9.1 inches high at a resolution of 300 pixels/inch.  (3648 pixels wide and 2736 pixels high at a resolution of 300 pixels/inch.  If you have a camera that is capable of only 6 megapixels you can easily see that the print size for a 300 pixel/inch resolution would would be less than the 10 megapixel camera.

If print size is important to you, make sure you are taking pictures in the largest size, usually that setting on you camera is called "Super Fine" or "L." (Large)  Settings like M1, M2 or S (small) use less pixel dimensions than the camera is capable of.   You can take more pictures at a lower density (M1, M2, S) but they won't print out very big or may be fuzzy.   The Super Fine setting will make sure you are using the full pixel capability of your camera for each picture.  I always use the L setting (Largest) because I have a 32 gigabyle memory card (actually I have several).   Enough memory to take thousands of pictures at the highest setting.