There is often confusion over the use of terms regarding image and print resolution. You might often see DPI and PPI interchanged and wonder what is the difference.

Technically the measurements of dpi and ppi convert to the same number. 300dpi = 300ppi, 72dpi = 72ppi, and so forth. The terms themselves mean different things.

 

PPI or "Pixels Per Inch": Refers to digital devices, typically computer monitors and screens, smartphone screens, image scanners, digital cameras, etc. PPI is the number of square pixels per inch in your image. PPI affects the size of the photo and the quality of printing. If there are too few pixels per inch, then the pixels will be larger creating a more jagged edged look. The more pixels per inch the smaller the pixel will be and the sharper and better the image will look.

 

DPI or "Dots Per Inch": Refers to printing and is the number of individual dots that are printed within the span of 1 inch. A printer with a maximum DPI of 720 can print up to 720 dots of ink every inch. A printer with a maximum DPI of 1200 can print up to 1200 dots of ink every inch. By just picturing this you can see how the final print quality is affected. A photo printed at 300dpi versus one printed at 72dpi will be much sharper, smoother, have better color, and an overall better picture. Images printed at a low resolution will look fuzzy and have poor quality. The higher the DPI the better. The printer can only print the resolution of the printed image. If the image is 72ppi the maximum DPI the printer can print that image at is 72dpi. A 72ppi image can't be printed at 300dpi.

 

DPI = Output Resolution / Print Resolution

PPI = Input Resolution / Image Resolution

 

Computer screens generally are optimized to view images at 72ppi. That is why images that are 72ppi on the internet look crisp on your computer monitor but print poorly.

Here is an image that might help you visual these terms.

PPI vs DPI


The Main Thing To Know:
Images with larger PPI printed at a large DPI will look better than those images with a low PPI printed at a low DPI.

Images printed on small documents that can be viewed close up like flyers, posters, letterheads, business cards should be printed at a high DPI, generally 300 dpi or higher. Images that are printed on larger signs that are not often viewed close up like billboards or banners can be printed at a lower dpi like 72dpi. From far away our eyes can't distinguish the poorer quality of the larger printed signs.