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DPI vs PPI: what they mean and when they matter

One is a printing measurement, the other is pixel density. Neither has any effect on how an image looks on a screen, and the 72 DPI rule you were taught is a myth.

The distinction

PPI
Pixels per inch. How densely pixels are packed — on a screen, or when printed.
DPI
Dots per inch. How many ink dots a printer lays down. A hardware property.
In an image file
A metadata number. It changes nothing about the pixels.
On screen
Completely irrelevant. Browsers ignore it entirely.
The only real property
Pixel dimensions. Width × height.
For print
Supply enough pixels: roughly 300 × the final size in inches.

What each one actually is

An image has one real size

A digital image has exactly one intrinsic property relevant here: its pixel dimensions. A photograph of 3000 × 2000 contains six million pixels, and that is true whether its metadata claims 72 PPI or 300 PPI.

The PPI field is an instruction about printing. It says how many of those pixels to place per inch of paper, which determines how large the print comes out: 3000 pixels at 300 PPI is a 10-inch-wide print; the same file at 72 PPI is a 41-inch-wide print of the same six million pixels, spread thinner and looking correspondingly worse.

Changing the number changes nothing except a few bytes in a header. Nothing is added, nothing is removed, and the file size is unaffected.

The 72 DPI myth

"Save it at 72 DPI for the web" is one of the most persistent pieces of bad advice in the field, and it does nothing whatsoever.

The number comes from early Macintosh displays, which happened to be about 72 pixels per inch, so a document at 72 PPI appeared on screen at roughly its printed size. That correspondence stopped being true decades ago and was never true of the web.

Browsers ignore the field entirely. An image is rendered at its pixel dimensions, scaled by CSS. Setting it to 72 does not reduce the file size, does not make the page faster and does not change how the image appears. What actually makes a web image smaller is fewer pixels and better compression.

DPI properly means something a printer does

Strictly, PPI describes pixels and DPI describes ink dots. A printer builds each pixel out of many tiny dots of cyan, magenta, yellow and black, which is why an inkjet advertising 4800 DPI is not asking you for a 4800 PPI image — it is describing its own mechanism.

In everyday use the two terms are swapped constantly, including by software: Photoshop's "Resolution" field is PPI but is labelled DPI in half the tutorials about it. In practice, when someone asks for "300 DPI", they mean 300 pixels per inch of final printed size.

What to supply for print

Work from the final printed size, not from the number.

Formula
Pixels needed = printed inches × 300
6 × 4 inch photo
1800 × 1200 pixels
A4 page (8.3 × 11.7 in)
about 2480 × 3508 pixels
A3 poster
about 3508 × 4961 pixels
Large format, viewed from a distance
150 PPI or even less is often fine — a billboard is nowhere near 300

If you have enough pixels, the PPI tag can be corrected in seconds. If you do not, no tag will help, and the print will be soft.

Screen PPI is a different conversation

Display density is genuinely measured in PPI, and it does matter — but it is a property of the hardware, not of your file.

A standard monitor is around 96 PPI; a high-density laptop or phone screen is roughly double that. This is why images for the web are prepared at twice their layout size: a 2× screen draws two physical pixels for every CSS pixel, so an image displayed in an 800-pixel column looks sharpest if the file is 1600 pixels wide.

That is about pixel dimensions again. The metadata field is still irrelevant.

Try it yourself

Everything below runs in your browser, so you can test any of this on your own files without uploading them.

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Questions

Frequently asked questions

Every answer below is present in the page source, expanded, so it can be read without opening anything.

Does changing DPI make a file smaller?

No. The DPI or PPI value is a metadata field describing how to print the image — changing it alters a few bytes in a header and nothing else. Only reducing the pixel dimensions or compressing harder makes a file smaller.

Do I need 72 DPI for the web?

No, and it makes no difference whatsoever. Browsers ignore the field completely and render images at their pixel dimensions. The advice is a leftover from 1980s Macintosh displays and has never applied to the web.

What DPI do I need for printing?

Around 300 pixels per inch of final printed size, so a 6 × 4 inch photo needs about 1800 × 1200 pixels. Large prints viewed from a distance need far less — the correct question is always how many pixels you have, not what the tag says.

What is the difference between DPI and PPI?

PPI counts pixels per inch, DPI counts printer ink dots per inch. They describe different things, but everyday usage swaps them constantly. When a print shop asks for 300 DPI they mean 300 pixels per inch of final size.

My image is 72 DPI. Is it low quality?

The tag tells you nothing about quality. A 6000 × 4000 image tagged 72 PPI has exactly as much detail as the same file tagged 300 — only the pixel dimensions matter. Check those instead.

How do I change the DPI of an image?

Most editors expose it in an image-size dialogue; make sure resampling is off, or you will change the pixel count as well. Note that our tools re-encode from raw pixels, so they do not preserve or set this field — it is a print instruction, and these are web tools.

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