Image size & photo info

How many megapixels do you need for printing?

A quality print needs about 300 dots per inch. A standard 4×6 inch print therefore calls for at least 1200×1800 pixels, just over 2 megapixels. An 8×10 inch print needs roughly 8 megapixels, and for a 12×18 inch poster 15 or more is recommended. Below those thresholds a print can look grainy or soft, even if the photo looks perfect on screen.

Aspect ratio and social media

The aspect ratio is the proportion between width and height: 16:9 is the format of videos and YouTube thumbnails, 4:5 and 1:1 work well in the Instagram feed, and 9:16 is the vertical standard for Stories, Reels and TikTok. Knowing your photo's ratio tells you right away whether it will get cropped when you post it and which platform it will look best on.

Pixels, centimetres and DPI: three different things

An image file contains only pixels: how many across and how many down. Centimetres do not exist inside the file, they exist only when the image is printed, and they come from the ratio between the pixels and the dots per inch chosen for printing.

From which follows something that confuses many people: changing the DPI written in the metadata does not change the quality of an image, it only changes the size it will be printed at. A 1200-pixel image printed at 300 DPI is ten centimetres wide, the same one at 150 DPI is twenty centimetres wide and has half the definition. The pixels are the same.

How many pixels are actually needed

For print you count about 300 dots per inch: a 10 by 15 centimetre photo wants at least 1200 by 1800 pixels, an A4 about 2500 by 3500. For a screen far fewer are enough: an image 1200 pixels wide covers the width of nearly every web page, and for high-density screens you export at twice the display size.

The calculation worth remembering is that megapixels grow with the square: doubling the sides quadruples the pixels and the size. Which is why a 48-megapixel phone does not produce photos four times better than a 12-megapixel one: on the long side the difference is only double.

Aspect ratio, and why it ruins photos

It is the proportion between width and height, and it hides almost every unwanted crop. A photo taken in 4:3 published as a square post loses the two side strips, and uploaded as a 9:16 story loses half the subject. Automatic cropping takes the centre, which is hardly ever where the important thing is.

The recurring formats are few: 16:9 for videos and covers, 1:1 for square posts, 4:5 for vertical posts, 9:16 for stories and reels. Knowing what format a photo will end up in before taking it is the difference between framing wide and having to crop it badly afterwards.

The data that says more than the dimensions

The size against the pixel count says how heavily it was compressed: a 4000-pixel image weighing 200 kilobytes has been compressed a great deal, and enlarging it will show the artefacts. The format says whether there is transparency and whether lossy compression was used.

And the metadata says when and with what it was taken, often with GPS coordinates: that is the part worth checking before publishing a photo, and it is removed with a dedicated tool.

Nearby tools

To change dimensions there is Resize photo, for size Compress image and Compress to a target size. For shooting data Photo EXIF data and Remove metadata, and for format Convert image.