"Please upload a photo at 300 DPI." "Minimum resolution 1200 × 800." "Image too low quality." Resolution and DPI come up constantly — on job portals, passport applications, print shops — and they are often explained in confusing ways. The idea underneath is simple once you separate two things: how many pixels an image has, and how big it will be printed.
Pixels: The Real Measure of Detail
A digital image is a grid of tiny coloured squares called pixels. A photo that is 4000 × 3000 pixels has 12 million of them (12 megapixels). More pixels means more detail — you can crop into it, enlarge it, or print it bigger before it looks blocky.
This is what "resolution" usually means: the pixel dimensions of the image.
DPI: Only Matters When Printing
DPI (dots per inch) tells a printer how many of those pixels to squeeze into each inch of paper. It doesn't add or remove any detail on its own:
- The same 3000-pixel-wide image printed at 300 DPI comes out 10 inches wide.
- Printed at 150 DPI, it comes out 20 inches wide — bigger, but less sharp up close.
On a screen, DPI is basically ignored. A 1000-pixel image displays the same on your phone whether its file says 72 DPI or 300 DPI.
The Simple Formula
Print size (inches) = pixels ÷ DPI
| Pixels (width) | At 300 DPI | At 150 DPI |
|---|---|---|
| 600 | 2 inches | 4 inches |
| 1800 | 6 inches | 12 inches |
| 3000 | 10 inches | 20 inches |
So if a print shop wants a sharp A4 print (about 8.3 inches wide) at 300 DPI, you need roughly 2500 pixels across. Use the Aspect Ratio Calculator to work out matching dimensions when you resize.
Recommended Values
| Purpose | What matters | Typical target |
|---|---|---|
| Website images | Pixel width | 1200–2000 px wide for full-width images |
| Social media | Pixel dimensions | As specified by the platform |
| Online forms and portals | Pixels + file size | Exactly what the form states |
| Photo prints | DPI at print size | 300 DPI |
| Scanning documents | Scanner DPI | 300 DPI for text; 600 for fine detail |
| Posters seen from far away | DPI at print size | 150 DPI is often enough |
Why "Changing DPI" Doesn't Fix a Blurry Photo
If an image is too small, changing its DPI setting from 72 to 300 just changes the label — the pixels stay the same, so it still prints small or blurry. Enlarging it adds pixels but invents detail that wasn't there, which is why upscaled images often look soft. Tools like Image Enhancer can improve the appearance of small images, but the best result always comes from starting with a larger original.
Resolution and File Size
More pixels usually means a bigger file. That's why a phone photo can be 4–8 MB while a form might only accept 200 KB. You rarely need all those pixels for an upload:
- Reduce the pixel dimensions to what's needed (the Image Editor can help with basic edits).
- Compress the result with Image Compress.
For a practical walkthrough, see how to reduce photo size for online forms.
For Scanned PDFs
The same idea applies to documents. Scanning at 300 DPI gives clear, readable text and works well with OCR; going higher mostly creates huge files. If your scanned PDF is too large, see how to compress a scanned PDF.
