What Is DPI? (And When It Matters)
A number people obsess over for screens, where it does nothing at all — and under-think for print, where it decides everything.
DPI is dots per inch: how many dots of ink a printer lays down across one inch of paper. A closely related term, PPI (pixels per inch), describes the same idea for pixels. In everyday use people say DPI for both, and mostly that's fine.
Here's the part that surprises people: for anything displayed on a screen, DPI does nothing at all.
Why DPI is meaningless on screens
An image file is a grid of pixels. That's all it is. A 1200×800 image has 960,000 pixels whether its DPI field says 72, 300, or 6000.
DPI is just a tag stored in the file's metadata. It's an instruction to a printer about how large to render those pixels on paper. A browser ignores it entirely — it takes the pixel grid and shows it. Changing a web image from 72 to 300 DPI changes nothing visible, nothing about the file size, and nothing about the quality. You have relabelled a box.
This is why "save it at 300 DPI for the web" is bad advice that refuses to die. If someone needs a bigger or sharper web image, they need more pixels — see our guide on resizing, and note that enlarging can't invent detail that isn't there.
When it genuinely matters: print
For print, DPI is the whole ballgame, because it's what connects pixels to physical size.
The arithmetic is simple: pixels ÷ DPI = inches. A 1200×1800 pixel image at 300 DPI prints at 4×6 inches. The same file at 150 DPI prints at 8×12 inches — twice the size, half the detail per inch, and it will look noticeably softer.
300 DPI is the long-standing convention for photo printing because it's roughly where a normal person stops resolving individual dots at reading distance. Large formats viewed from further away — posters, banners — get away with much less, since you're not standing six inches from a billboard.
Which sets up the trap: an image that looks perfect on your monitor can be hopeless for print. A 600×400 photo fills a screen nicely and prints at 2×1.3 inches at 300 DPI. Screens are forgiving; paper isn't.
Where it catches people: ID photos
This is the one place the two worlds collide, and it's why passport photo requirements can feel confusing.
A passport photo spec is stated in millimetres, because historically it was printed and glued into a booklet. But you're uploading a digital file, so somewhere the requirement has to be converted into pixels — and DPI is the exchange rate. That's why passport specs quote both a physical size and a pixel count: the pixel count is the physical size, at an assumed print resolution.
You don't need to do that maths. The country pages under the Passport Photo Maker hub output the right pixel dimensions for each spec. But it explains why two numbers that look redundant are both there.
The rule
- Screen? Ignore DPI. Think in pixels, and size to your display width.
- Print? Work backwards: desired inches × 300 = pixels you need. Then check your image actually has them.
- Either way, DPI never adds detail. It's a label on the pixels you already have.
And if your file is too heavy after all that, the fix is compression or fewer pixels — never the DPI field. Our guide on hitting a size limit covers that properly, and why images get big walks through diagnosing which of the four usual causes you actually have.
FAQ
Should I save web images at 72 or 300 DPI?
It makes no difference. Browsers ignore the DPI tag entirely and render the pixel grid. Focus on pixel dimensions instead.
Does increasing DPI improve quality?
No. It changes a metadata label, not the pixels. Printing the same pixels at a higher DPI makes the print smaller, not better.
How many pixels do I need to print at 4×6 inches?
At 300 DPI, 1200×1800. Multiply your target inches by 300 for each dimension.