What Is DPI? Image Resolution Explained Simply
DPI appears everywhere: in scanner settings, in printing options, and in PDF-to-image converters. It decides how sharp an image is and how large the file becomes. Yet it is often misunderstood. This article explains what DPI really means and how to choose the right value.
DPI in one sentence
DPI means dots per inch: how many dots (or pixels) are used for every inch of the page. More dots per inch means more detail, and also more data.
Strictly speaking, printers use dots and screens use pixels, so you may also see PPI (pixels per inch). In everyday use, and in most software, DPI and PPI are used to mean the same thing.
From DPI to pixels
When you convert a PDF page into an image, the DPI decides the image's size in pixels. The calculation is simple: page width in inches × DPI. An A4 page is about 8.27 × 11.69 inches, so:
| DPI | A4 page in pixels (approx.) | Typical use |
|---|---|---|
| 72 | 595 × 842 | Quick previews, thumbnails |
| 150 | 1240 × 1754 | Screens, email, sharing, most uploads |
| 200 | 1654 × 2339 | Scanned documents with small text |
| 300 | 2480 × 3508 | Printing |
| 600 | 4961 × 7016 | Archiving, fine detail, technical drawings |
Why doubling DPI makes files much bigger
DPI applies to both width and height. Going from 150 to 300 DPI doubles the width and the height, so the image has four times as many pixels. In practice, the file can be roughly three to four times larger. That is why choosing 600 DPI "just to be safe" often creates files that are too big to send.
Does DPI matter on screens?
A screen does not care about the DPI value stored in an image file; it only cares about the number of pixels. A 1240-pixel-wide image looks the same on screen whether its file says 150 or 300 DPI. What matters is whether there are enough pixels for the size it is displayed at. For a page that fills a laptop screen, around 1200–1600 pixels in height is plenty, which is why 150 DPI is a good default.
Does DPI matter for printing?
Yes. A printer spreads the pixels across the paper. If an image has too few pixels for the print size, it looks soft or blocky. The usual target for good print quality is 300 DPI at the final printed size. For large posters viewed from a distance, lower values such as 150 DPI are often acceptable.
Choosing DPI when scanning
- Ordinary text documents: 150–200 DPI is readable and keeps files small.
- Documents with very small print or fine stamps: 300 DPI.
- Photos you want to enlarge or archive: 600 DPI or more.
Scanning above what you need does not make documents more "official"; it just makes them heavier. See How to make a PDF smaller if your files are already too large.
Can you increase DPI later?
Not in a useful way. Changing the DPI number of an existing image, or enlarging it in software, does not add real detail; it only spreads the existing pixels further. If you need higher quality, go back to the source: rescan the document or convert the original PDF again at a higher DPI.
Summary
Use 150 DPI for screens and sharing, 300 DPI for printing, and 150–200 DPI for scanning ordinary documents. Remember that every doubling of DPI quadruples the pixels, and that detail can only be captured at the start, never added later.