Fit to page changes the scale
Fit options resize the entire document to the printer's available area. If an A4 PDF is fitted inside a smaller printable rectangle, every element becomes slightly smaller. This is one of the most common reasons a 50 mm object prints at 48 or 49 mm.
Shrink oversized pages can activate unexpectedly
A print application may treat the document as larger than the printable area because the printer has margins. “Shrink oversized pages” then reduces the whole A4 page even when the PDF page and loaded paper are both A4.
Borderless printing often enlarges
To avoid white slivers at the edge, borderless modes commonly enlarge and crop the page slightly. The exact amount varies by printer and driver. Turn borderless mode off when physical scale matters more than edge-to-edge coverage.
Non-printable margins are a hardware constraint
Many desktop printers cannot print to all four edges. A centred image that stays inside a conservative 10 mm margin is more likely to print without forced scaling, but the actual printable region depends on the device and selected media.
Printer drivers can override applications
The application and the driver may each expose scaling controls. A viewer set to 100% can still be affected by a driver option such as reduce/enlarge or scale to paper size. Review both layers when a measured result is wrong.
Use the 100 mm line to diagnose the scale
Measure the reference line. Divide the measured length by 100 to obtain the scale factor. For example, 97.5 mm means the workflow printed at 97.5%. Correct the print settings rather than changing the requested image dimensions, because the same incorrect setting will affect every future file.
Troubleshooting checklist
- Confirm the loaded paper is A4, not Letter.
- Choose Actual size, 100% or No scaling.
- Disable Fit, Shrink and borderless expansion.
- Check driver-level scaling.
- Use a PDF viewer rather than a photo gallery app.
- Measure the 100 mm line.
- Try another viewer or printer driver if scale remains wrong.