Image Resolution, DPI and Pixels Explained

A digital image is a grid of pixels, and its resolution is simply how many pixels wide and tall that grid is. DPI, or dots per inch, only matters when the image is printed, because it describes how many of those pixels are squeezed into each inch of paper. Once you grasp that split, most of the confusion around "300 DPI" disappears.

Pixels: the real size of an image

A pixel is the smallest unit of a digital picture, one tiny square holding a single colour. An image that is 1920 pixels wide and 1080 pixels tall contains 1920 x 1080 = 2,073,600 pixels, a little over 2 megapixels.

Megapixels are just pixels divided by one million. Some quick examples with easy numbers:

• 3000 x 2000 is 6,000,000 pixels, so 6 megapixels.
• 4000 x 3000 is 12,000,000 pixels, so 12 megapixels.
• 800 x 600 is 480,000 pixels, so 0.48 megapixels.

The pixel dimensions are the true measure of how much detail an image holds. A file's size in megabytes is something different, because it also depends on format and compression.

DPI and PPI: what they mean

You will see DPI and PPI used almost interchangeably. Strictly, PPI means pixels per inch and refers to a digital image or screen, while DPI means dots per inch and refers to ink dots from a printer. In everyday talk, people say DPI for both, and tools usually do the same.

The important fact is that DPI is not stored as extra detail. An image file can contain a number labelled with a resolution, but changing that label alone does not add or remove a single pixel. A 3000 x 2000 image has the same pixels whether it is tagged 72 or 300.

The one formula you need

Print size depends on pixels and pixels per inch:

print size in inches = pixels divided by pixels per inch

Example with a 3000 x 2000 image:

• At 300 pixels per inch: 3000 / 300 = 10 inches wide, 2000 / 300 = about 6.7 inches tall.
• At 150 pixels per inch: 3000 / 150 = 20 inches wide, 2000 / 150 = about 13.3 inches tall.

So the same picture can be a sharp 10 inch print or a softer 20 inch print. Running the formula backwards tells you how many pixels you need. For a 6 x 4 inch print at 300 pixels per inch, you need 6 x 300 = 1800 pixels by 4 x 300 = 1200 pixels.

How much resolution do you need?

It depends on where the image will be seen.

Screens. Browsers and phones measure images in pixels, not inches. What matters is that the image has enough pixels for the space it fills. A picture shown 800 pixels wide needs about 800 pixels, and some devices with very sharp screens use more than one physical pixel per displayed pixel, so supplying around double the width can look crisper. Beyond that you only add file size.

Print. A commonly used target for photos and documents is around 300 pixels per inch, because at normal reading distance the individual dots become hard to see. Large items viewed from far away, such as banners or posters, can look acceptable at much lower figures because people stand further back. Printers and print shops may state their own requirements, so check them for important jobs.

Documents and forms. Many application portals ask for a pixel size or a file size rather than DPI. Follow whichever they state.

Why enlarging a small image looks bad

Making an image bigger does not create new detail. Software has to invent the extra pixels by guessing from their neighbours, which produces softness or blur. Doubling a 500 x 500 image to 1000 x 1000 gives you four times the pixels (1000 x 1000 = 1,000,000 versus 250,000), but not four times the information.

Shrinking works the other way. Reducing pixels discards detail, which is fine when the final display is small. This is the reason to start from the largest original you have and reduce, never the reverse. The image resizer lets you set exact pixel dimensions and keeps the aspect ratio locked to avoid stretching.

Resolution, file size and compression

More pixels usually means a bigger file, but the relationship is loose. Format and compression change the result. A 12 megapixel photo saved as JPG might be a few megabytes, while the same pixels saved as PNG can be much larger. For the details, see our comparison of PNG, JPG and WebP.

If you need a smaller file without changing pixel dimensions, you can reduce the quality setting with the image compressor. If the image is going into a document, the image to PDF tool lets you place it on a page, and you should size it for the page first so you do not carry unneeded pixels.

Metadata: the stored resolution tag

Image files can carry a resolution value in their metadata. The PNG specification, for instance, includes an optional chunk describing physical pixel dimensions, which tells a program how large the pixels are meant to be when printed. Programs may use it as a hint for default print size. It does not alter the picture itself, and other software may ignore it.

Common mistakes

• Thinking a 72 DPI image is low quality. What counts is the pixel total, not the tag.
• Changing DPI to 300 and expecting a sharper picture. Nothing changes unless pixels are resampled.
• Enlarging a small logo for a banner and expecting it to stay crisp.
• Using a huge original on a web page where a small version would do.
• Confusing megapixels with megabytes.

Quick checklist

• Find the pixel dimensions of your image.
• Decide the final size: pixels for screens, inches and pixels per inch for print.
• Calculate pixels needed with width in inches times pixels per inch.
• Resize down if there are more pixels than needed.
• Never rely on enlarging to add detail.

Tools to get it right

Use the image resizer to hit exact pixel dimensions, the image compressor to trim file size, and the image converter if a format change is needed. For other percentage-based sums, such as scaling a size by a given percent, the percentage calculator can help.