Getting a photo under an upload form's size limit

What a KB limit means: 1000 or 1024

File sizes are counted in bytes. In the metric system, kilo means 1000, so 1 kB is 1000 bytes. Computers have long also used 'kilobyte' for 1024 bytes, because 1024 is a power of two. To end the confusion, the IEC approved separate binary prefixes in December 1998: 1 kibibyte (KiB) is 1024 bytes, 1 mebibyte (MiB) is 1024 × 1024 bytes.

Forms rarely say which one they mean. The difference is small, but it can decide whether a file right at the limit is accepted.

Pixels and file size

The pixel dimensions of an image set how much picture data it holds. A photo of 4000 × 3000 pixels contains 12 million pixels; halving both sides to 2000 × 1500 leaves 3 million, a quarter as many. Fewer pixels generally means a smaller file, but the exact size also depends on the format, the quality setting and how much detail the picture contains.

That is why two photos with the same pixel dimensions can have quite different file sizes, and why a limit in KB can't be translated into a fixed pixel size.

How to get under a maximum

Do it in this order, and stop as soon as the file fits:

• Save as JPG if the form accepts it. For photos it is much smaller than PNG.
• Lower the quality. Photos usually tolerate a moderate reduction with little visible change.
• Only if that isn't enough, reduce the pixel dimensions, but stay above any minimum width and height the form sets.
• Check the result at full size before uploading: faces and text should still look sharp.

Minimum sizes: don't enlarge

Some forms set a minimum as well as a maximum. GOV.UK asks for an online passport photo of at least 50 KB and no more than 10 MB, at least 600 pixels wide and 750 pixels tall. Canada's online passport renewal asks for 200 KB to 5 MB and at least 1200 × 1800 pixels.

If a photo is below a minimum, go back to the original, full-size file from the camera rather than a copy that has already been resized or compressed. Making a small image bigger doesn't help: enlarging adds pixels without adding detail, so the picture loses sharpness. That is why Allow making images bigger is off by default in the resize tool.

Official photos have extra rules

Passport and visa photos are not ordinary uploads. Each authority publishes its own requirements, and they differ in more than size:

• US visa photos: a square JPEG, 600 × 600 to 1200 × 1200 pixels, 240 kB or less. Compression may be needed to meet the limit, but the compression ratio should be 20:1 or less.
• UK passport: the digital photo must be unaltered by computer software.
• Canada passport renewal: JPEG only, saved directly from the original camera file, and not altered in any way; the page names editing software, filters and even changes to brightness or sharpness.

Format and file name requirements

Read the whole requirement, not just the size. Many official forms accept JPEG only, so an iPhone HEIC photo or a PNG screenshot will be refused even if it is small enough. If a form lists rules for the file name, such as no spaces or a maximum length, rename the downloaded file before uploading.

Scanned documents follow the same steps: JPG first, then quality, then pixels. Zoom in on the smallest text before uploading, because a document that can't be read may be rejected even if it fits the limit.