Pixels, Millimetres and DPI: Sizing a Photo for an Online Form

What each unit means

Pixels are the dots a digital image is made of. A size in pixels fully describes an image file, and nothing else is needed.

Millimetres and inches describe a printed size. An image file has no physical size of its own, so a size in millimetres only becomes a number of pixels once you know how densely it will be printed.

DPI, dots per inch, is that density. At 300 DPI, one inch of paper holds 300 pixels.

Converting between them

Pixels equal inches multiplied by DPI. For millimetres, divide by 25.4 first to get inches.

A 35 by 45 mm photo at 300 DPI is 413 by 531 pixels. A 2 by 2 inch photo at 300 DPI is 600 by 600 pixels. The same 35 by 45 mm photo at 200 DPI is 276 by 354 pixels.

If a form gives millimetres but no DPI, 300 is the usual choice for anything that may be printed, and it is a safe default.

Steps

1. 1Open the Image Compressor and add your photo.
2. 2Under Resize, choose the unit the form uses: px, mm or inch.
3. 3Enter the width and height. For mm or inch, also enter the DPI.
4. 4Choose the target file size the form allows, then press Compress.
5. 5Check the pixel size shown under the result and download it.

One side or both

Fill in only the width or only the height and the other side is worked out so the picture keeps its proportions. Nothing is cut off.

Fill in both and you get exactly that size. Because your photo rarely has the same proportions as the form's box, the tool scales the picture to cover the box and trims the overflow from the centre. Keep your face near the middle of the frame, or crop the photo yourself first.

Signatures

Signature images are usually wide and short, with a small size limit. Sign with a dark pen on white paper, photograph it straight on in good light, crop closely around the signature, and then apply the form's dimensions and size limit in the same way as for the photo.