Docuex Tools

Scanning DPI Calculator

Not sure what resolution to scan your drawings at? Pick the size and what the scans are for, and we'll recommend a DPI plus estimated file sizes for colour and black & white output.

1. Paper size
A0
841 × 1189 mm
A1
594 × 841 mm
A2
420 × 594 mm
A3
297 × 420 mm
A4
210 × 297 mm
2. What is the scan for?
Archive
long-term storage
CAD conversion
tracing source
OCR / text
searchable PDF
Reprint
good-quality print
Email / screen
view only
Recommended DPI
400dpi
Pixel dimensions
9 360 × 13 240
File size (colour)
42MB
File size (B/W)
5MB

For tracing in CAD we suggest 400 DPI - sharp enough to follow every line, without files becoming unmanageable. Estimates assume JPEG-quality compression.

Quick guide. The right DPI depends entirely on what you'll do with the scan.
150 DPI
fine for viewing on screen or emailing; not enough detail to trace.
200 DPI
OK for ordinary reprints at the same size.
300 DPI
archive standard; supports good OCR on cleanly printed text.
400 DPI
recommended for tracing into CAD; captures fine line work.
600 DPI+
only worth it for very fine detail or photographic originals - file sizes get large fast.

Want us to scan your drawings for you? We do A0, A1 and A2.

See Scanning Service

Choosing the right scanning resolution

DPI (dots per inch) is simply how many samples the scanner takes across each inch of the original. More samples means more detail — and a much larger file, because the pixel count rises with the square of the resolution. Going from 300 to 600 DPI does not double the file, it roughly quadruples it. That trade-off is the whole question: enough resolution to capture the thinnest line and smallest annotation, and no more.

For technical drawings the deciding factor is usually the finest detail on the sheet, not the sheet size. A clean A0 site plan with bold linework can be perfectly usable at 200–300 DPI, while a dense A2 detail full of 1.5 mm hatching and handwritten dimensions may need 600.

Sensible starting points

PurposeDPIWhy
Screen viewing150–200Legible on a monitor, small files, not suitable for reprinting
General archive300The usual default. Captures normal linework and text, prints back at full size acceptably
Reprint at full size300–400Enough to reproduce the sheet without visible softening
CAD conversion / tracing400–600Line edges stay crisp, so dimensions and hatching can be read confidently
OCR of typed text300–400Below 300 recognition accuracy falls away quickly
Fragile or faded originals600Greyscale at high DPI recovers detail that thresholding would lose

Colour, greyscale or black and white?

This affects file size far more than most people expect. A bitonal (black and white) scan stores one bit per pixel; 24-bit colour stores twenty-four. For a typical A1 sheet at 300 DPI that is the difference between a file of a few megabytes and one of several hundred. Use bitonal for clean line drawings destined for archive, greyscale for pencil drawings, faded blueprints or anything with tonal variation, and colour only when the colour carries meaning — service runs, planning overlays, artwork.

Why higher is not always better

Scanning a weathered drawing at 1200 DPI mostly captures paper texture, foxing and fold marks in exquisite detail. It produces an unwieldy file that is slower to open, slower to trace and no more accurate. Past a certain point you are recording the condition of the paper rather than the information on it. For almost all engineering and architectural work, 300–600 DPI is the useful range.

How this fits into digitising a set

If drawings are being scanned as a step towards editable CAD, resolution matters more than for a pure archive job — the person tracing needs to read every dimension with confidence. Use the calculator above to check the file size implications before committing to a whole roll. The paper size reference will confirm what sheet size you are holding, and our large format scanning service covers A0–A2 originals, including fragile and damaged sheets.

Frequently asked questions

What DPI should I scan A1 or A0 drawings at?

300 DPI is the sensible default for archiving and reprinting. If the scan will be traced into CAD, or the drawing contains fine hatching, small handwritten notes or thin linework, use 400–600 DPI. Sheet size itself does not change the required DPI — the finest detail on the sheet does.

Is 600 DPI better than 300 DPI?

Only when the extra detail is actually there to capture. 600 DPI produces roughly four times the file size of 300 DPI. It is worth it for tracing, faded originals and fine detail; it is wasted on clean, bold linework destined for an archive.

What DPI do I need for OCR?

At least 300 DPI, and 400 if the text is small or the original is not crisp. Below 300 the character shapes degrade and recognition accuracy drops sharply. OCR also works far better on greyscale than on a harshly thresholded bitonal scan.

Should I scan in colour or black and white?

Bitonal (black and white) for clean line drawings — the smallest files by a wide margin. Greyscale for pencil drawings, faded blueprints and anything with tonal variation. Colour only where the colour carries information, such as marked-up service runs or planning overlays.

How big will my scan file be?

It scales with the sheet area and the square of the DPI. Doubling DPI roughly quadruples the file. The calculator above estimates the size for your chosen sheet size, resolution and colour mode so you can check before scanning a whole set.

What resolution do you scan at?

We choose the resolution to suit the original and its intended use, typically 300 DPI for archive work and 400–600 DPI where the scan will be converted to CAD or the drawing is fragile or faded. Files are delivered as PDF, JPEG or TIFF.

Need Your Large Drawings Scanned?

We scan A0-A2 originals at the right DPI and can convert them to editable CAD too. Get a quote in hours.

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