The short answer
PPI means pixels per inch. It describes how tightly the pixels in a digital image are placed when that image is printed at a physical size. DPI means dots per inch. It describes the tiny marks a printer can place on paper or another surface.
For image sizing, the number you normally need is effective PPI: image pixels divided by printed inches. A 3000-pixel-wide image printed 10 inches wide has an effective resolution of 300 PPI. The printer may use many ink dots to reproduce each image pixel.
| Measurement | Describes | Controlled by | Typical use |
|---|---|---|---|
| PPI | Image pixels per printed inch | Pixels and print size | Image quality check |
| DPI | Printer dots per physical inch | Printer and print mode | Hardware output setting |
| Pixel dimensions | Total image width and height | Camera, scan or document | Available detail |
How effective PPI changes
The same file can have different effective resolutions because print size is part of the calculation. Print a 2400-pixel image at 8 inches and it supplies 300 PPI. Enlarge it to 16 inches and it supplies 150 PPI.
This is why a file cannot be judged by pixel dimensions or a metadata label alone. You need the intended output size. Cropping also matters because discarded pixels are no longer available for the final print.
- Smaller print size increases effective PPI.
- Larger print size decreases effective PPI.
- Cropping reduces the pixels available for printing.
- Resampling can add pixels, but it cannot recreate detail that was never captured.
Why changing the DPI field is not enough
Many image files contain a resolution metadata value. Editing 72 to 300 may change the default physical size shown by layout software, but it does not add pixels. A 1200 × 1200 pixel image remains 1200 × 1200 pixels.
A metadata change is useful when it tells software how large to place an image. It is not a quality improvement. Check actual pixel dimensions and calculate effective PPI at the final size before deciding whether a file is suitable.
Choosing a realistic resolution target
A target is not a universal pass-or-fail rule. Fine photographs viewed closely usually need more image information than a large display viewed from several metres away. Paper, screening method, sharpening and subject matter also affect the visible result.
Use the printer or publisher specification whenever one is available. In the absence of a specification, 300 PPI is a common planning target for close-viewed work, while lower values may be appropriate as viewing distance increases.
| Effective PPI | Planning use | Important caveat |
|---|---|---|
| 300+ | Close-viewed photos and detailed print | Common target, not a guarantee |
| 240–299 | High-quality general printing | Often visually strong |
| 150–239 | Posters and less critical work | Inspect at expected distance |
| Below 150 | Large or distant displays | Softness may be visible nearby |
These are planning ranges. The printer’s own specification takes priority.