Image DPI explained: what resolution to scan at
DPI is the least understood setting on any scanner, and picking it badly costs you either legibility or gigabytes. It is simpler than it sounds once you separate the physical page from the pixels.
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What DPI actually measures
DPI is dots per inch: how many samples the scanner takes across each inch of the page. An A4 page scanned at 300 DPI is roughly 2480 by 3508 pixels. Double the DPI and you quadruple the pixel count, because it applies in both directions.
That quadrupling is why a 600 DPI colour scan of a few pages can reach hundreds of megabytes while a 200 DPI scan of the same pages fits in an email.
What to use, by document type
- 150 DPI: reading on screen only, and you are certain nothing will ever be printed or enlarged.
- 200 DPI: everyday paperwork — invoices, letters, receipts. Small print stays readable and files stay manageable.
- 300 DPI: the standard for anything that may be printed, filed formally, or run through text recognition. This is the safe default.
- 600 DPI: fine print, engineering drawings, old documents with faint handwriting, or anything being enlarged.
- Above 600 DPI: photographic originals and archival work only. For ordinary paper you are recording the texture of the fibres.
DPI, screens and printing
Screens do not have DPI in a meaningful sense — an image is displayed at whatever pixel size the viewer chooses. DPI only becomes real when the image is printed, where it fixes the physical size of each pixel.
So an image is 'high enough resolution' relative to its printed size. A 1000-pixel-wide image is generous for a 3 cm logo and inadequate for a full-page photo.
Step by step
- 1Decide whether the scan will ever be printed or enlarged.
- 2Pick 200 DPI for screen-only paperwork and 300 DPI for anything else.
- 3Scan in greyscale for text documents — colour triples the size for no benefit.
- 4If a file is already too large, use the resizer below rather than re-scanning.
- 5Check the smallest text at 100% zoom before you file or send the result.
Frequently asked questions
- Does increasing DPI after scanning improve quality?
- No. Upscaling invents pixels by interpolation; the detail was never captured. Re-scan at a higher resolution if you need more.
- What DPI does text recognition need?
- 300 DPI is the usual minimum for reliable results. Below that, accuracy drops sharply on small or condensed type.
- Why is my 600 DPI scan so slow to open?
- Because it may be tens of millions of pixels per page, and the viewer has to decode all of them. For plain paperwork, 300 DPI opens instantly and looks the same on screen.