Pixels, inches and DPI
A digital image has a fixed number of pixels. How big it prints depends on how many of those pixels go into each inch: the resolution, usually called DPI (dots per inch) and more precisely PPI (pixels per inch). Print size in inches is simply pixels divided by PPI, so a 3000-pixel-wide photo prints 10 inches wide at 300 PPI or 20 inches wide at 150 PPI.
Changing the DPI number stored in a file does not add detail. It only changes the size the file asks to be printed at. To print larger and still sharp, you need more pixels.
Which resolution to aim for
Rules of thumb used by print labs and photographers:
- 300 PPI: photo prints, books and magazines viewed up close.
- 200–240 PPI: good-quality inkjet and lab prints; most people cannot tell the difference from 300 at arm's length.
- 150 PPI: posters and large prints viewed from a distance.
- 72 PPI: an old screen convention; irrelevant for print.
Preparing many photos for print
Print labs usually want JPG or TIFF in sRGB at a specific size. If you are sending a whole shoot, resize and convert the folder in one go rather than file by file. Operimage can resize to exact pixel dimensions and export JPG or TIFF for a whole folder on a Mac.
FAQ
How many pixels do I need for an 8×10 print?
2400 × 3000 pixels at 300 PPI. At 200 PPI, 1600 × 2000 pixels is usually enough.
How big can I print an iPhone photo?
A 12-megapixel iPhone photo (4032 × 3024) prints about 13.4 × 10 inches at 300 PPI and around 20 × 15 inches at 200 PPI.
Is DPI the same as PPI?
Not strictly. PPI is pixels per inch in the image; DPI is ink dots per inch on the printer. In everyday use, including print lab instructions, DPI usually means PPI.
Does changing the DPI make my photo better quality?
No. Only the number of pixels determines detail. Changing DPI without resampling only changes the intended print size.
