How many DPI does a scanned document need

I write these guides from the experience of preparing real filings, not as a lawyer or an immigration adviser. Every figure I quote links to its official source with the date I checked it. If something here is out of date, write to me and I will fix it.

The number that decides whether an official can read your document is not its weight in megabytes. It is the resolution. And it is the one number no tool shows you.

What DPI means, without the jargon

DPI is dots per inch: how many points of information exist in each inch of scanned paper. More dots, more detail survives.

A letter-size sheet is about 11 inches tall. Scanned at 300 DPI the file is roughly 3,300 pixels high. At 150 DPI, about 1,650. At 72 DPI, about 790. All three look fine as thumbnails. Only one lets you read the fine print on a seal.

The important part is that DPI describes captured information, not file size. You can have a heavy file with poor resolution and a light one with plenty. Which is why weight misleads.

The numbers that matter in practice

• 300 DPI is the archival standard, what a professional scanner defaults to and what you use if the document will be printed again.
• 200 DPI is the comfortable point for an application. Everything reads, seals and signatures included, at a fraction of the weight.
• 150 DPI is still legible for normal text, but a seal with fine lettering starts to struggle.
• Below about 115 DPI is where a reviewer may call it illegible. Large text survives, detail dissolves.
• 72 DPI is old screen resolution. Fine for looking at, useless for verifying.

This tool works around 200 DPI at its normal setting, and will not go below about 115 without warning you.

Why you do not see the problem until it is too late

When you open a PDF, your screen shows the whole page shrunk down. At that size, a 72 DPI document and a 300 DPI one look nearly identical. Everything seems fine, you submit, and the problem surfaces on the other side when somebody enlarges the seal to verify it and finds a smudge.

The check that actually works takes ten seconds: zoom to 200 percent and look at three specific things. The small text on a seal. The signature. A long number, like a passport or reference number. If you have to guess a character, so does the reviewer.

The rule almost everyone gets backwards

Scan high, compress afterwards. Never the reverse.

No tool on earth recovers resolution that was not captured. If the scan came out at 100 DPI, enlarging it only produces a blurrier, heavier version of the same problem. A 300 DPI scan, on the other hand, reduces to 200 without drama, and to 150 if you genuinely have to.

When the original already arrived damaged

It happens constantly: someone sends you a document that has already been through a generic compressor, or was scanned in an economy mode. It is degraded before you touch it.

Compressing it again does not fix it and does make it worse. This tool detects those cases: if a scanned page sits below about 115 DPI, it leaves it alone rather than degrading it further, and flags the file. When you see that warning, the only real fix is going back to the paper.

Colour versus black and white

Many applications explicitly require documents scanned in colour, particularly passports and identity documents. Black and white produces far lighter files and is tempting when fighting a size limit, but it can invalidate the document.

If the application says nothing, stay in colour. If space is tight, reduce resolution before dropping colour.

Operational summary

Capture at 300 DPI or your phone's maximum. Keep colour. Compress at the end, against the portal's real limit, not before. Check at 200 percent on a seal and a signature. And if a document arrived blurry, go back to the paper rather than trying to rescue it.

Common questions

How many DPI does a document need for an official application?

Around 200 DPI is comfortable. Below roughly 115 DPI a reviewer may consider it illegible.

Can I increase the DPI of a scan I already have?

Not usefully. Enlarging does not recover information that was never captured, it just produces a heavier version of the same blur.

How do I know if my document has enough resolution?

Open it, zoom to 200 percent and look at a seal, a signature and a long number. If they are hard to read for you, they will be for the reviewer.