Many people get confused when it comes image resolution and image size, and how they relate to the amount of pixels versus pixels per inches (ppi, also known as dots per inch or dpi). In my two-decade career as a graphic design professional, I have even known many professional designers who didn’t understand the difference.
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 72ppi 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.
To figure out the measurement of a picture in inches, you will need to divide the number of pixels by the resolution. A 3000 × 4000 pixel image at 72ppi will be roughly 41.6″ x 55.5″. When the image resolution is changed to 300ppi, it will be 10″ x 13.3″.
When it comes to image size, 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 image at 72ppi can be changed to 300ppi, 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.
And remember…not all images need to be printed at 300ppi. 300ppi is the standard resolution for print, but it really depends on the project and the printer. Posters and similar items not meant to be seen up close can get away with as low as 150ppi. Also, anything larger than around 350ppi will increase your digital file size with no improvement in print quality.
This article is an updated version of an article originally published on Artistic Imposter Design.