Image Resolution, DPI and Pixels Explained for Print and Web

A print shop asks for “300 DPI”. A form wants “3.5 × 4.5 cm”. A website theme wants “1200 × 630 pixels”. And when you open your photo’s properties, it says 72 DPI, which sounds terrible, even though it looks perfectly sharp on your screen.

These numbers describe related but different things, and mixing them up is behind most “why is my print blurry” and “why was my upload rejected” problems. Once the relationship clicks, every one of these requests becomes simple arithmetic.

Pixels: what an image actually contains

A digital photo is a grid of coloured squares called pixels. A typical 12-megapixel phone photo is 4032 pixels wide and 3024 pixels tall, about 12 million pixels in total. That pixel count is the image’s real resolution. It is fixed when the photo is taken or the image is created, and you can’t add genuine detail later.

When people say an image is “high resolution”, they mean it has a lot of pixels.

DPI and PPI: how pixels become inches

PPI (pixels per inch) says how many of those pixels are placed side by side in each inch when the image is printed. DPI (dots per inch) strictly describes printer ink dots, but in everyday use, and in most software, people say DPI when they mean PPI. This article does the same.

The key relationship is:

So the same 2400-pixel-wide image prints 8 inches wide at 300 DPI, or 16 inches wide at 150 DPI. The image hasn’t changed; only the printed size and its sharpness have.

Why changing the DPI number often does nothing

Every image file carries a DPI value in its metadata, often 72 or 96. That number is only a suggestion for printing. Changing it from 72 to 300 without changing the pixels doesn’t add any detail; it just tells the printer to print the same pixels smaller.

That is also why the “72 DPI” on your phone photo is not a problem. A 4032 × 3024 photo has enough pixels to print about 13 × 10 inches at 300 DPI, whatever the DPI field says.

On screens it matters even less. Browsers, phones and websites display images by their pixel dimensions and ignore the DPI value completely.

How many pixels do you need for print?

For prints viewed up close, such as photos, documents and brochures, 300 pixels per inch is the long-standing standard. Posters and banners seen from a distance can get away with 150 DPI or less.

An A4 page at 300 DPI needs about 8.7 million pixels, which is why modern phone photos print beautifully at that size. A 1080-pixel-wide image from social media, on the other hand, is only 3.6 inches (about 9 cm) wide at 300 DPI. Print it on a full A4 page and it will look soft.

How many pixels for screens and websites?

For screens, think in pixels only:

• Website photos rarely need to be wider than 1600–2000 pixels. A 4000-pixel photo shown 800 pixels wide wastes bandwidth and slows the page.
• Social sharing images are commonly 1200 × 630 pixels.
• Square posts are often 1080 × 1080 pixels.
• Full HD screens are 1920 × 1080 pixels.

High-density (“retina”) screens show more pixels in the same space, so images displayed at 800 pixels wide can benefit from a 1600-pixel version. Beyond that, extra pixels mostly add file size. Resize Image has presets for these common sizes, and also lets you limit the file size at the same time.

Online forms that give centimetres

When a form asks for a photo of “3.5 × 4.5 cm”, it is describing how the photo would print. Convert with the formula: 3.5 cm is 1.38 inches, and at 300 DPI that is 413 pixels; 4.5 cm becomes 531 pixels. If the form also gives a pixel size, use that instead; it is what the upload checker measures. Our guide to reducing photo size for online forms covers the KB side of the same problem.

Can you increase resolution?

You can make an image larger in pixels, but the software has to invent the new pixels by blending existing ones. A small image enlarged a lot looks soft and smeary. Modern AI upscalers do better than simple resizing, but they guess detail rather than recovering it, which is not acceptable for documents, IDs or anything that needs to be accurate.

The honest fix for a low-resolution image is a better original: the full-size photo instead of the WhatsApp copy, the original logo file instead of a screenshot, or a vector logo (SVG) that can be printed at any size.

Images inside PDFs

The same rules apply to images placed in a PDF. A photo placed on an A4 page at 300 DPI needs around 2480 pixels across the page width to print sharply. When a PDF is compressed for email, images are usually reduced to 150 DPI or less, which is fine for screens but not for professional printing. Keep a high-quality version for the print shop and a compressed version for sharing.

Common mistakes

• Changing 72 to 300 DPI in an editor and assuming the image is now print quality.
• Printing a screenshot or social media image large. Check the pixel count first.
• Uploading a 6000-pixel photo to a website that shows it 800 pixels wide.
• Upscaling ID or document photos to meet a pixel requirement. Take or scan a new one.
• Confusing file size with resolution. A 50 KB image can be 200 × 230 or 1200 × 900 pixels, depending on compression.

Frequently asked questions