Troubleshooting · Output
Output the Wrong Size? How to Fix DPI in ScanTailor Advanced
Your processed pages look right on screen, but the exported files print at the wrong physical size, produce PDFs with odd page dimensions, or come out at an unexpected resolution. Nine times out of ten this is not an export bug — it is the DPI value attached to your source images.
The Symptom
- Printed output is much larger or smaller than the original page.
- A PDF built from the output has wrong page dimensions even though the pages look fine.
- Pages within one project come out at inconsistent sizes.
- The output resolution is far higher or lower than the scan you started with.
The Cause
DPI (dots per inch) is the bridge between pixels and physical size: it tells every tool downstream how big a page of N pixels is supposed to be on paper. ScanTailor Advanced reads the DPI stored with each source image and uses it when computing page layout and output dimensions.
The trouble is that the stored DPI is often wrong. Camera captures typically carry an arbitrary default DPI that has nothing to do with the real capture resolution. Scanner software can stamp a value that does not match the scan you actually made, and images edited in other tools can lose or change the value entirely. Feed ScanTailor a wrong source DPI and every size it derives — margins, page dimensions, output DPI — is wrong in proportion, even though the pixels themselves are fine.
The Fix, Step by Step
- Check what DPI your source images claim. Look at the DPI reported for the images you imported, and compare it with how you actually captured them. If you scanned at 300 DPI but the files claim 72 or 96, you have found the problem.
- Correct the DPI per image where it is wrong. ScanTailor lets you review and correct the DPI assigned to an image. Fix the value to match the real capture resolution — correcting the number does not resample or damage the image; it only tells the program the truth about its size.
- Use the Source DPI fields. In v1.2.0 and later, the Margins panel of the Page Layout stage has inline Source DPI fields, so you can check and adjust the value right where page sizes are computed. If you are on an older release, updating from the download page makes this much easier to audit.
- Make the whole project consistent. A book scanned in two sessions often mixes DPI values. Bring every image in the project to its correct value before running the final output — mixed source DPI is the classic cause of pages coming out at inconsistent sizes.
- Re-run the affected stages. After changing DPI, let page layout and output run again so dimensions are recomputed from the corrected values, then check the new output size before processing the whole book.
Prevention
- Set your scanner software to the resolution you intend (300 DPI for standard text, 600 DPI for archival work) and confirm the saved files carry that value before importing.
- For camera captures, correct the DPI in ScanTailor as a standard first step for every project — never trust the camera’s default.
- Keep capture settings identical across all sessions of one book so the project starts consistent.
More output problems and fixes: the troubleshooting archive.
FAQ
Does changing the DPI value change my image quality?
No. Correcting the DPI number only changes the size interpretation of the same pixels. Quality only changes if you actually re-scan at a different resolution or resample the image.
Why do pages from the same book come out different sizes?
Almost always because the images carry different source DPI values — typically a mix of scanner-stamped and camera-default values. Audit the DPI of every image in the project and make them consistent.
What DPI should my scans be for ScanTailor?
300 DPI is the standard working resolution for text pages and 600 DPI suits archival work and small print. What matters most for this problem is that the value stored in the file matches the resolution you truly captured at.