What DPI actually means (and which number to pick)
72, 96, 150, 300 — DPI settings decide between blurry prints and bloated files. What the number controls, and which value fits screen, print and OCR.
Published
Every export dialog eventually asks the same question: 72, 96, 150, 200 or 300 DPI? Pick too low and print comes out blurry. Pick too high and the file quadruples for no visible benefit. The setting is easy to get right once you know what the number actually controls.
The definition
DPI is dots per inch: how many pixels represent one inch of physical page. It ties together the two sizes an image has — its pixel dimensions and its real-world dimensions:
pixels = inches × DPIAn A4 page is 8.27 × 11.69 inches. Rendered at different DPI:
| DPI | Pixel size of an A4 page | Rough use |
|---|---|---|
| 72 | 595 × 842 | Thumbnails, quick previews |
| 96 | 794 × 1123 | Screen reading |
| 150 | 1240 × 1754 | Good screens, casual print |
| 200 | 1654 × 2339 | Quality print, small text |
| 300 | 2481 × 3508 | Professional print, OCR, archiving |
Doubling the DPI quadruples the pixel count — it scales in both dimensions. That is why 300 DPI files are so much larger than 150 DPI files, not just twice the size.
Where 72 and 96 come from
The odd-looking numbers are history, not engineering. Early Macintosh screens showed 72 pixels per inch, chosen so one pixel matched one typographic point (1/72 inch) — which is also why PDF’s internal unit is 1/72 inch, and why a PDF page rendered at “scale 1” comes out at 72 DPI. Windows standardised on 96 instead. Both numbers outlived the hardware that justified them and now survive as the conventional “screen resolution” presets in every export dialog.
Modern screens are far denser than either — a phone display exceeds 400 pixels per inch — which is why a 72 DPI export that “was fine for screens” in 2005 can look soft on today’s devices. For anything meant to be read on screen at full size, 96 is the floor and 150 is the safe choice.
Choosing in practice
Exporting PDF pages as images — as the PDF to images tool does — 150 DPI is the sensible default: sharp on any screen, files a fraction of the 300 DPI size. Go to 300 when the image will be printed, or when you need to zoom into fine print or dense diagrams.
Compressing a scan. This is the same decision in reverse: how much resolution can be removed? For a document that will only ever be read on screen, re-rendering at 96 DPI is usually indistinguishable and is where the large savings come from. Keep 150 if the original contains small print. The PDF compressor defaults to 96 for exactly this reason.
Scanning for OCR or archiving. Feed OCR engines 300 DPI. Recognition accuracy degrades quickly below that, because the engine is reading letter shapes and low resolution blurs exactly the distinctions it needs — do the OCR first at full resolution, compress afterwards if needed.
Print. Commercial printers ask for 300 DPI at final physical size. Home inkjets are forgiving; 200 usually looks fine. Below 150, printed text has visibly ragged edges.
The classic misconception
DPI on its own says nothing about quality — it is only meaningful together with a physical size. A 3000-pixel-wide image is a 3000-pixel-wide image; at 10 inches wide it is 300 DPI, at 40 inches it is 75 DPI. Changing the DPI number in a file’s metadata without resampling changes nothing except how big software suggests printing it.
The question worth asking is always: how many pixels does this use case need? Screen reading of an A4 page needs roughly 800–1200 pixels of width. Print needs about 2500. OCR wants everything it can get. Work back from there and the DPI setting picks itself.