Image resolution and DPI explained

18 August 2026 · 7 min read

Someone asks for your image "at 300 DPI" and you have no idea whether the file you have qualifies. Or a print shop rejects a photo as too low resolution when it looked perfectly sharp on screen. Both confusions come from treating two different things as one.

Resolution is a count of pixels

An image is a grid of coloured squares. Its resolution is simply how many there are: 4000 across by 3000 down, say. That is the whole of it — the total amount of visual information the file contains.

This number is fixed. It does not change when you view the image larger or smaller, print it, or email it. It changes only when you resize the file, which either discards pixels or invents new ones.

DPI is a physical measurement, and it needs paper

DPI — dots per inch — describes how densely those pixels are packed when the image exists physically. It is a ratio between pixels and inches, so it only becomes meaningful once you decide how big the printed version will be.

A 3000-pixel-wide image printed at 10 inches wide is 300 DPI.
The same file printed at 30 inches wide is 100 DPI.
Nothing about the file changed.

Which is why DPI on its own tells you nothing. It is a property of a printed result, not of a file.

The number stored inside the file is mostly fiction

Image files can carry a DPI value in their metadata, and this is where most of the confusion originates. Your camera writes 72, or 240, or 300, and software dutifully displays it as though it means something.

It does not, on its own. It is a suggestion about intended print size, and changing it alters no pixels whatsoever. You can set an image to 300 DPI in an editor and the file will be byte-for-byte identical in every way that affects how it looks.

So when someone asks for "300 DPI" and you flip that metadata field, you have technically satisfied the request and changed nothing at all. If the image was too small to print well before, it still is.

What they actually meant

Almost always: enough pixels to print at the size I need without looking blocky. 300 DPI is the print industry's rule of thumb for the point where individual dots stop being visible at normal viewing distance.

The useful conversion is one multiplication:

pixels needed = print size in inches × 300

An A4 page is roughly 8.3 × 11.7 inches, so printing one edge-to-edge wants around 2500 × 3500 pixels. A 6 × 4 inch photo print wants 1800 × 1200. A business card at 3.5 × 2 inches wants 1050 × 600.

Compare that with what you have. A 12-megapixel phone photo is around 4000 × 3000, which prints beautifully at A4 and is more than enough for anything smaller.

Screens do not work this way at all

For anything displayed on a screen, DPI is irrelevant. What matters is pixels versus the pixels of the slot the image sits in.

A complication: modern displays pack more physical pixels into the same space, so a "400 pixel wide" area in your layout may be 800 real pixels on a high-density screen. That is why serving roughly twice the CSS width is a sensible ceiling for web images — and why going beyond that is waste, since nobody can see it.

If a web designer asks for 300 DPI images, they have picked up print vocabulary for a screen problem. Ask what pixel dimensions they need.

Why PDFs measure things in points

PDFs do not use pixels at all. They use points, where one point is exactly 1/72 of an inch — a typographic unit far older than computers. A US Letter page is 612 × 792 points, and A4 is 595 × 842.

This explains a behaviour that otherwise looks like a bug. Convert images to a PDF in "fit to image" mode and each page is sized to the image's pixel dimensions treated as points. A 3000-pixel photo therefore becomes a page over 40 inches across. It looks fine on screen, because screens do not care, and prints unpredictably.

The same arithmetic runs in reverse when converting a PDF to images. Rendering at 150 DPI means scaling by 150 ÷ 72, or about 2.08. An A4 page at 595 × 842 points comes out at roughly 1240 × 1754 pixels.

Enlarging does not add detail

If an image is too small for the print you want, resizing upward will not fix it. The detail was never captured, and no amount of interpolation invents it — you get a larger, softer version of the same information.

AI upscaling tools genuinely improve on naive enlargement by predicting plausible detail, but "plausible" is the operative word. For a photograph destined for print, starting from a bigger original is the only real answer.

Practical numbers

Web images: up to twice the display width in pixels. Ignore DPI entirely.

Printed photos: long edge in inches × 300. Below about 150 DPI at the final size, softness becomes visible.

Scanned documents: 150 to 300 DPI. Above that you get much larger files with no readability gain.

Large-format posters: lower is fine, often 100 to 150 DPI, because they are viewed from further away. The rule of thumb scales with viewing distance.

The short version

Resolution is how many pixels exist. DPI is how tightly they are packed once printed, and means nothing without a physical size. The DPI number inside a file is a suggestion that changes no pixels. When someone asks for 300 DPI, work out the print size, multiply by 300, and check whether your image has that many pixels.

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