What DPI should you use when converting a PDF to images?
A PDF page has no pixels — it is measured in points, 72 to the inch. DPI (dots per inch) decides how many pixels each inch becomes when you convert it to an image. Too low and text is blurry; too high and you get huge files that are slow to create and to open. Here are the exact numbers and a setting for every use.
How DPI turns a page into pixels
Pixel width = page width in inches × DPI. An A4 page is 8.27 × 11.69 inches (595 × 842 points), so at 150 DPI it becomes 8.27 × 150 ≈ 1240 × 1754 px. Doubling the DPI doubles both sides — and quadruples the number of pixels, the memory needed and (roughly) the file size.
Image sizes for common paper formats
| DPI | A4 (210 × 297 mm) | US Letter (8.5 × 11 in) | Megapixels (A4) |
|---|---|---|---|
| 72 | 595 × 842 | 612 × 792 | 0.5 MP |
| 150 | 1240 × 1754 | 1275 × 1650 | 2.2 MP |
| 200 | 1654 × 2339 | 1700 × 2200 | 3.9 MP |
| 300 | 2480 × 3508 | 2550 × 3300 | 8.7 MP |
| 600 | 4961 × 7016 | 5100 × 6600 | 34.8 MP |
Which DPI for which job
| Use | DPI | Why |
|---|---|---|
| Thumbnails, quick previews | 72 | Small and fast; text is readable only when large |
| Screens, email, chat, web pages | 150 | Sharp on normal and most high-DPI screens at full width |
| Zooming in, small print, OCR on clean documents | 200 | Small text stays crisp; files stay reasonable |
| Printing, OCR on difficult scans, archiving as image | 300 | The classic print resolution; what most OCR engines expect |
| Fine line art, enlargement, professional print | 400–600 | Only when you know you need it — files get very large |
Higher DPI cannot add detail that is not there
For vector content (text and drawings made in Word, InDesign, Illustrator, LaTeX…) every DPI is rendered fresh, so higher DPI really is sharper. For scanned pages the PDF just contains a photo — usually scanned at 200–300 DPI. Converting at 600 DPI only upsamples that photo, making the file larger without making it clearer. Match the scan resolution or go a little below it.
Memory and the safety limit
Before encoding, every page is held as an uncompressed bitmap of 4 bytes per pixel. An A4 page at 300 DPI needs about 35 MB; at 600 DPI about 140 MB. Posters and engineering drawings (A0 and larger) at 300 DPI would need well over 500 MB, which can crash a browser tab — or a phone.
That is why our PDF to Image converter caps each page at about 40 megapixels on desktop and 16 megapixels on phones and tablets. Pages above the cap are scaled down proportionally and marked in the results. Pages are also converted one at a time, and each bitmap is released before the next page starts.
DPI metadata vs. pixel size
Image files can store a DPI value, but most apps ignore it — what counts is the pixel size. If a print shop asks for “300 DPI”, they mean enough pixels for the printed size: for an A4 print that is about 2480 × 3508 pixels.
FAQ
Is 300 DPI always better?
Only for printing and difficult OCR. For screens 150 DPI looks the same and the files are about four times smaller.
Why is my image smaller than expected?
Either the page is smaller than A4/Letter, or it hit the memory safety limit and was scaled down. The result list shows the actual pixel size of every image.