DPI and Image Resolution Explained: Screen vs Print
What DPI and PPI really mean, how many pixels you need for printing, why 72 DPI is a myth for screens, and which DPI to choose when scanning or converting PDF to images.
“Please send a 300 DPI image.” Few requests cause more confusion. Some people change a number in an image editor and send the same blurry photo; others send a 50 MB file for a website thumbnail. Understanding DPI takes five minutes and saves hours of back and forth.
Pixels are what count
A digital image is a grid of pixels, for example 3000 × 2000. That is its real resolution. DPI (dots per inch) or, more precisely for images, PPI (pixels per inch), is only an instruction for how large to print those pixels. The same 3000 × 2000 image can be:
- printed at 300 PPI: 10 × 6.7 inches (25 × 17 cm), very sharp;
- printed at 150 PPI: 20 × 13.3 inches (51 × 34 cm), fine from a distance.
Changing the DPI number in a file without changing the pixels changes nothing about the quality. It only changes the default print size.
The formula
Pixels needed = print size in inches × PPI. One inch is 2.54 cm. For a 15 × 10 cm photo print at 300 PPI: 15 / 2.54 × 300 ≈ 1772 pixels by 10 / 2.54 × 300 ≈ 1181 pixels.
How much resolution do you need?
- Photo prints and brochures viewed up close: 300 PPI.
- Posters viewed from a metre or more: 150 PPI is usually enough.
- Billboards: as low as 20 to 50 PPI, because they are seen from far away.
- Screens: PPI does not matter at all. Only the pixel dimensions do. A 1200-pixel-wide image fills 1200 screen pixels whatever DPI value is stored in the file.
The old idea that web images must be “72 DPI” comes from early Mac screens and is irrelevant today. Size web images in pixels, not DPI.
Scanning: which DPI to choose
- Text documents for reading and archiving: 200 to 300 DPI, greyscale or black and white.
- Documents for OCR: 300 DPI. Lower values reduce recognition accuracy; higher values rarely improve it. See our OCR guide.
- Photos you want to reprint at the same size: 300 DPI; to enlarge, 600 DPI.
- Slides and negatives: 2400 DPI or more, because the originals are tiny.
PDF to image: picking the DPI
When you convert PDF pages to pictures with PDF to JPG, the DPI decides the pixel size of each page. An A4 page is 8.27 × 11.69 inches, so:
- 96 DPI gives about 794 × 1123 pixels: fine for a quick preview or a chat message.
- 150 DPI gives about 1240 × 1754 pixels: sharp on any screen, readable small text.
- 300 DPI gives about 2480 × 3508 pixels: for printing or zooming into fine detail.
Why upscaling does not help
Enlarging a small image to reach a required pixel count invents pixels by blending neighbours. The file meets the number, but the picture gets softer, not sharper. AI upscalers can guess plausible detail, which is impressive for photos but risky for documents, where invented detail can change numbers or letters. Always go back to the original, a higher-resolution export, or a new scan.
Our take
Think in pixels. When someone asks for 300 DPI, work out the print size they need and check you have enough pixels for it. When an image is for a screen, ignore DPI entirely and use Resize Image to set the pixel width the page actually displays.