What Affects OCR Accuracy?
OCR accuracy is determined mostly by the quality of the image going in. Even the best OCR engine can't reliably read blurry, skewed, or low-contrast text. Here's what matters most:
- • Resolution (DPI) — the single biggest factor
- • Page alignment — straight vs. tilted/crooked
- • Contrast — dark text on light background vs. faded or shadowed
- • Font type — standard printed fonts vs. handwriting or decorative fonts
- • Language setting — matching the document's language
7 Tips to Improve OCR Accuracy
Scan at 300 DPI minimum
Most scanner apps default to 150 or 200 DPI — change this before scanning. For documents with small fonts (legal footnotes, fine print), use 400 DPI. Going above 600 DPI adds file size without improving accuracy.
Keep pages flat and straight
A 5-degree tilt reduces OCR accuracy noticeably. Use a flatbed scanner rather than a phone camera when possible. If you're working with an existing crooked scan, use the Rotate PDF tool to correct the angle before running OCR.
Ensure good contrast
Dark text on a white background is ideal. Faded documents, colored paper, or light pencil marks reduce accuracy. If you're re-scanning, increase the scanner's contrast setting.
Clean up phone photos first
Phone cameras introduce shadows, perspective distortion, and glare that hurt OCR. Run your photo through PDF.it's Phone Scan Cleanup before OCR — it removes shadows, corrects perspective, and boosts contrast automatically.
Select the correct language
OCR engines use language models to resolve ambiguous characters. Selecting the right language (especially for non-English documents) can improve accuracy by 5–15%. This matters most for accented characters in Spanish, French, Portuguese, and German.
Split large PDFs for faster processing
Very large scanned PDFs (100+ pages) can time out. Split the PDF into smaller sections first using the Split PDF tool, OCR each section, then merge the results back.
Try grayscale instead of color for text documents
For black-and-white text documents, grayscale scans are smaller and process faster without sacrificing accuracy. Only use color scanning if the document has colored text or important colored graphics.
OCR Accuracy by Document Type
| Document Type | Typical Accuracy | Best Practice |
|---|---|---|
| Printed text, 300+ DPI | 98–99% | Ready to use |
| Phone photo, good lighting | 90–95% | Run Phone Scan Cleanup first |
| Old/faded typewritten document | 80–90% | Boost contrast before scanning |
| Neat printed handwriting | 70–85% | Manual review recommended |
| Cursive handwriting | 40–60% | Manual transcription may be needed |