Pixels, KB, DPI and mm: What Photo Size Requirements Really Mean

Four measurements, four different questions

Pixels tell you how many tiny dots wide and tall the picture is. Kilobytes (KB) tell you how much storage the file takes. Millimetres and inches tell you how big the picture is when printed on paper. DPI tells you how many pixels are used for each inch of paper, and so connects the pixels to the printed size.

A form can ask for any of them, and a file has to pass each one separately. A picture can have the right shape and still be too heavy as a file.

Pixels: the size on a screen

For an online upload, pixels are usually what counts. A picture of 600 × 600 pixels is 600 dots wide and 600 tall, however you view it. The shape, also called the aspect ratio, comes from the two numbers: a square has equal width and height.

If a form states a size in pixels, your picture needs exactly that size, or fall within the range given. Changing other settings does not change the number of pixels.

Kilobytes: the weight of the file

The file size depends on three things: how many pixels the picture has, how strongly it is compressed, and how much detail it contains. A plain background with a face takes less space than a busy scene of the same size.

A size limit on a form is nearly always a maximum, such as under 100 KB. Some forms also set a minimum, such as at least 20 KB. A tool that compresses to a target size makes files smaller, not larger, so it cannot raise a small file up to a minimum. For that, start again from your original at a higher pixel size or quality.

DPI: a bridge between pixels and print size

DPI matters when you print. The rule is pixels = inches × DPI. A photo of 2 × 2 inches at 300 DPI has 600 × 600 pixels, and a photo of 35 × 45 mm at 300 DPI has about 413 × 531 pixels, since 25.4 millimetres make an inch.

On its own, a DPI number stored in a file does not change how many pixels the picture has or how many KB it takes. Many online forms look only at pixels. The tools on GetTaskCraft set the pixel size of the picture. They do not write a DPI value into the file's information, so if a form reads that value, you would need other software to set it.

Why a form rejects a file that looks fine

• The picture has the wrong size in pixels, or the wrong shape.
• The file is above the maximum size, or below the minimum, in KB.
• The file type is not accepted. Many forms take only JPG, and some do not accept WebP or PNG.
• The picture is correct but was saved again and again, and the form reads a different version than you think. Always check the file you actually upload.

An order of steps that works

1. Read the requirement once, and write down the size in pixels or millimetres, the file type and the size limit in KB.
2. Crop the picture to the right shape first, so that nothing is stretched later.
3. Resize it to the exact pixel size. If the form gives millimetres, use the DPI to work out the pixels.
4. Save it in the file type that the form accepts, usually JPG.
5. Compress it to the KB limit as the last step, then check the final size on your device.

Doing this on GetTaskCraft

The Crop Image tool gives you the shape, Resize Image sets an exact size in pixels, and Compress Image or the Passport Photo Maker brings the file under a limit such as 50 or 100 KB. All of it runs in your browser, so your photos are not uploaded to a server.