I recently read an article on iStockphoto about image resolution. Many people get confused when it comes to amount of pixels versus pixels per inches (ppi, also known as dots per inch or dpi). As a graphic design professional, I have known many professional designers who didn’t understand the difference. iStockphoto’s article gives a very good overview of the difference and explains how to calculate the total inches (or centimeters) of an image based on the ppi and total pixels.
Photoshop users have it easy. The Image Size dialog box automatically calculates the inches for you based on the resolution you need. The most important thing to remember when changing an image’s resolution from 72 ppi to a printable resolution is to uncheck the Resample Image checkbox at the bottom of the dialog box. If the box is checked, you will pixelate your image and it will be unusable.
The most important thing to remember about image size is the resolution is not important – the total pixels are. The resolution can be changed, but the total pixels need to stay the same to avoid pixelation. A 3000×4000 pixel 72 ppi image can be changed to 300 ppi, but the 3000×4000 pixels must NOT be changed. There are techniques to get around this in a pinch, but changing the amount of pixels should be avoided unless absolutely necessary.
To figure out the measurement of the picture in inches, you will need to divide the number of pixels by the resolution. A 3000×4000 pixel image at 72 ppi will be roughly 41.6″x55.5″. When the image is changed to 300 ppi, it will be 10″x13.3″.
And remember…not all images need to be printed at 300 ppi. It really depends on the project and the printer.