Start with the original file
Use the camera original or the largest available download, rather than a screenshot, social-media thumbnail or an image forwarded through a messaging app. Check its width and height in pixels. File size in megabytes alone does not tell you how much usable detail it contains.
Look at faces, lettering and fine edges at 100% image zoom. A large file can still contain missed focus, motion blur or compression blocks. Printing it larger makes those defects larger too.
Calculate effective PPI
PPI means image pixels per printed inch. It is different from the printer’s ink-dot specification (DPI). For an uncropped image: PPI = pixel width ÷ (printed width in cm ÷ 2.54). Repeat for height; if the two differ, check the crop or aspect ratio.
Example: a 3000 × 4500 px portrait printed at 60 × 90 cm gives 127 PPI on both axes. The same file at 30 × 45 cm gives 254 PPI. Changing its metadata from 72 to 300 PPI without adding pixels does not supply more detail.
After cropping, use the pixels remaining inside the crop. A 3000 px wide original cropped to 1500 px and still printed 60 cm wide has about 64 PPI, not 127.
Pixel targets for common poster sizes
These are arithmetic planning targets, rounded up. They are not a guarantee that a print will look sharp. The dimensions assume the image fills the stated rectangle without cropping.
| Finished size | 100 PPI | 150 PPI | 300 PPI |
|---|---|---|---|
| 30 × 45 cm | 1182 × 1772 px | 1772 × 2658 px | 3544 × 5315 px |
| 40 × 60 cm | 1575 × 2363 px | 2363 × 3544 px | 4725 × 7087 px |
| 60 × 90 cm | 2363 × 3544 px | 3544 × 5315 px | 7087 × 10630 px |
Match the test to the way the poster will be used
For close reading, small text, maps or fine line work, start with a sharp higher-resolution original; 300 PPI is a useful planning target for raster artwork. For wall photographs viewed from a few steps away, 100–150 PPI can be a practical range to test. These are starting points, not pass/fail thresholds.
A bold graphic or halftone portrait can tolerate a different level of detail from a classroom chart. Vector letters remain sharp when preserved as vectors; a photograph saved inside a PDF does not become vector artwork.
Our 1024 × 1536 px pet example at 40 × 60 cm is about 65 PPI. It demonstrates a large wall image, not a close-view photographic-quality print. Reduce it to roughly 17 × 26 cm for about 150 PPI, or use a sharper original for a larger print.
Try this in Rasterbator Studio
- Upload the original and keep Original image while checking quality. Set the intended finished dimensions.
- Read effective PPI and inspect the crop. Use the size calculator for a width-based estimate; it does not inspect focus or compression.
- Choose the ruler and adjacent-page test, or print a PDF page containing the most demanding detail. Use 100% / Actual Size.
- Inspect that page from the intended viewing distance. Check eyes, small lettering and dark areas, then join two neighbouring pages.
- If the result is soft, reduce the poster size or replace the source. Use halftone only if you want that graphic style, then test again.
What enlargement and effects can change
Resampling creates additional pixel samples; it does not establish what was present in the original scene. AI enlargement can invent textures or alter letters. Keep an untouched original and compare faces, logos and labels before printing.
Splitting the same poster across more sheets does not add image detail. Making the final poster larger with the same file lowers effective PPI. A sharpening filter may emphasise edges but cannot reliably restore unreadable text.