How do I calculate DPI from pixels?

Divide the pixel width by the width of the page in inches. 2550 pixels across an 8.5 inch page is 300 DPI.

My file says 300 DPI but looks bad. Why?

The DPI field is metadata and can survive a resize that removed most of the pixels. Check the pixel dimensions instead; they cannot be faked by a tag.

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

Not meaningfully. Upscaling makes the pixels larger without adding detail. The only real fix is to photograph or scan it again with the page filling the frame.

Does a PDF have a DPI?

The PDF page has a physical size and the image inside it has a pixel count; the effective DPI is one divided by the other. Two PDFs of the same page size can differ enormously.

DOCSUPRA › Check the DPI of a document

How to check the DPI of a document, properly

Every image file can carry a DPI number in its metadata, and that number is a note, not a measurement. Software writes it, software ignores it, and it survives operations that halve the actual detail. The real resolution is arithmetic, and you can do it in your head.

The short answer. Divide the pixel width by the page width in inches. A file that is 2550 pixels wide across an 8.5 inch page is 300 DPI, whatever the metadata says. A file 1000 pixels wide across the same page is 118 DPI, even if it is tagged 300.

The calculation

DPI = pixels ÷ inches. For a US Letter page, 8.5 × 11 inches:

Pixel width	Real DPI across a Letter page	Verdict
2550 px	300 DPI	Scan grade
2125 px	250 DPI	Readable, below most specs
1700 px	200 DPI	Fine for email, short of filing specs
1000 px	118 DPI	Small print starts breaking up

For A4, use 8.27 × 11.69 inches. For a card-sized document, measure the card, not the page.

Why the tag lies

The DPI field is metadata: a note attached to the file saying what physical size it was intended to print at. Nothing enforces it. Resize an image to a quarter of its pixels and the tag can survive untouched, so the file still claims 300 DPI while carrying the detail of 75. Screenshots, exports from chat apps and anything that has passed through a messaging service are the usual offenders.

This matters because a reviewer does not check the tag. They open the file and look at it.

Where to find the pixel dimensions

Windows: right-click the file → Properties → Details → Dimensions.
macOS: select the file → Get Info → More Info → Dimensions.
iPhone / Android: open the photo, swipe up or tap the information icon.
Inside a PDF: harder, because the page and the image can differ. Open it at 100% and compare against the page size, or use a tool that reports the embedded image size.

The one that catches everyone

Sending a document through a messaging app and then downloading it back. Most of them re-compress and resize on the way through, sometimes to a fraction of the original. The file opens, looks fine on a phone screen, and carries a third of the detail it started with. If a document has been through a chat, go back to the original.

What to do when the DPI is too low

Nothing in software can add detail that was never recorded — enlarging the pixels makes them bigger, not more numerous. The only fix is to capture it again: fill the frame with the page, and the resolution follows.

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