Scan DPI: Which Setting for Which Job

Every office scanner asks you to pick a DPI, most people pick whatever it last used, and the result is either a blurry contract or a 40 MB email attachment. This post explains what the number controls and gives you a setting for each job you actually do.

Scan DPI settings: what the number actually means

DPI is dots per inch — how many sample points the scanner records for each inch of the page. At 300 DPI, an A4 sheet becomes roughly 2,500 by 3,500 samples; at 600 DPI, four times as many, because the number applies in both directions.

Two consequences follow from that arithmetic:

• Detail rises with DPI, up to a limit. The limit is the original. A laser-printed page, a rubber stamp, a photo print — each holds a finite amount of detail, and scanning beyond it records nothing new.
• File size rises with the square of DPI. Doubling the DPI roughly quadruples the data. This is why the wrong setting produces attachments that bounce from mail servers.

The skill is matching the number to the original and the purpose, and the good news is that a handful of settings cover essentially all office work.

The setting for each job

Plain documents for reading — 200 to 300 DPI, black and white or greyscale. Invoices, letters, forms you need to keep or forward. 300 is the safe default; 200 saves space when the text is a normal size and you only need it legible.

Documents for OCR — 300 DPI. Optical character recognition works best with cleanly resolved letterforms, and 300 DPI is the setting OCR software is generally built around. Lower settings cost accuracy on small print; much higher settings rarely improve it.

Documents with small print or fine detail — 400 to 600 DPI. Bank statements with tiny footnotes, technical drawings, anything where the smallest text matters. Scan a sample at 300 first; only go up if the small print is actually breaking up.

Photo prints — 600 DPI, colour. Captures what a typical photographic print holds and leaves room to enlarge modestly. For screen sharing only, 300 is fine.

Photos you intend to enlarge — higher, deliberately. Enlargement divides your resolution: a small print scanned at 600 DPI and blown up to twice the size effectively becomes 300. Scale the DPI to the enlargement you plan.

ID cards, cheques, stamps and signatures — 600 DPI. Small originals with fine detail follow the same logic as photos.

Colour mode matters as much as DPI

The colour setting multiplies file size independently of DPI, and choosing it well saves more space than fiddling with resolution.

• Black and white (1-bit) — smallest files, sharp text, correct for plain printed documents. Poor for anything with shading, pencil, or stamps in pale ink, which can vanish entirely.
• Greyscale — the right choice for documents with handwriting, pencil notes, faint stamps or photocopied originals. Meaningfully larger than black and white, far smaller than colour.
• Colour — necessary when colour carries information: highlighted text, coloured stamps and seals, photos, marked-up drawings. Otherwise it is the most expensive way to store a black-and-white page.

A common Indian-office case: documents carrying official stamps and seals. Scan those in colour or greyscale — a pale blue stamp scanned in 1-bit black and white often disappears, and the scan loses exactly the mark that made the document worth scanning.

File format finishes the job

DPI and colour mode decide how much data the scanner captures; the format decides how it is stored. For documents, PDF is the sensible container — multi-page, universally readable, and compressed appropriately when the scanner's document mode is used. For photos, JPEG at a high quality setting. Avoid uncompressed TIFF unless something downstream specifically requires it; it is the format behind most mystery 100 MB scans.

Most office machines also offer a compact or high-compression PDF mode, which is worth using for routine paperwork — it keeps text readable while cutting size dramatically.

Why higher is often worse

It feels safe to scan everything at maximum resolution. In practice:

1. The scan takes longer — at high DPI the carriage physically moves slower, and a multi-page job through the document feeder crawls.
2. Files bloat past what email accepts and storage comfortably holds.
3. Nothing is gained once you pass the detail of the original. An ordinary laser print does not contain 1200 DPI of information; scanning it at 1200 records the same letters with more data per letter.

There is one more ceiling worth knowing: the scanner's own optical resolution. Settings above it are produced by interpolation — the software inventing intermediate values — which adds size but not detail. The optical figure is in your machine's specification sheet.

A default worth setting

If you configure one preset and stop thinking about it: 300 DPI, greyscale, compact PDF handles the overwhelming majority of office scanning, with a colour preset kept for stamped documents and photos. Most office machines let you save these as one-touch presets on the panel, which is five minutes of setup that ends the guessing permanently.

When the scanner itself is the problem

If scans show lines that the original does not have, that is usually a strip of dust on the feeder's scan glass rather than a setting. If pages skew, double-feed or jam, the feeder rollers are worn or dirty. Persistent streaks, grinding from the carriage or scans that fail partway are hardware faults worth professional eyes. Toner Vision services printers and multifunction machines across Delhi NCR with a 4-hour average response — printer repair in Delhi — and if a machine is more down than up, our rental options in Delhi NCR keep an office scanning while it is sorted.