An online form says: "JPG, 3.5 × 4.5 cm, 413 × 531 pixels, under 100 KB". That is three different rules about one photo, and each one measures something else. Knowing which is which makes the error messages much easier to understand.
Three different sizes
| What it measures | Unit | What it depends on |
|---|---|---|
| Pixels (width × height) | pixels | The shape and detail of the picture. Cropping or resizing changes it. |
| Print size | mm, cm or inches | Pixels divided by the DPI. It only matters when you print. |
| File size | KB or MB | The pixels, plus how strongly the file is compressed. |
A photo can be right on one and wrong on the others. A 413 × 531 pixel photo can be 30 KB or 300 KB, depending on how it was saved. A file of 100 KB can be 200 pixels wide or 2000.
How they connect
- Pixels and print size are linked by the DPI: pixels = inches × DPI. At 300 DPI, 35 × 45 mm is 413 × 531 pixels. The full sum is in how many pixels a passport photo is.
- Pixels and file size move together. Fewer pixels means a smaller file. So does saving a JPG at a lower quality.
- File size and print size are not linked at all. Changing the print size in a photo editor only changes a number stored in the file.
For a photo you only upload, websites check the pixels and the file size. The print size (cm or mm) is a way to describe the shape, and it matters only if you print.
The order that works
When a form gives all three, fix them in this order:
- Shape first. Crop the photo to the shape the form wants, for example 3.5 × 4.5 cm is a portrait shape. Cropping first means you do not shrink parts of the picture you are going to throw away.
- Then the pixels. Set the width and height. If the form only gives centimetres or millimetres and no pixels, 300 DPI is a safe choice.
- Last, the file size. Bring the file under the KB limit. If it has a minimum too, make sure it reaches it.
If you make the file small first and resize afterwards, the size changes again, and you have to start over. Resize a photo does all three in one go: pick the size in pixels, mm, cm or inches, frame the picture, and open "More options" to add a KB limit.
When the rules seem to fight each other
- Exact pixels and a small KB limit. This works for most photos, because a small photo of plain colours compresses well. If it does not fit, the photo has too much noise or detail: use a better-lit original with an even background.
- A photo with too few pixels. If the form wants at least 600 pixels and your photo has 400, look for a bigger original first. If there is none, Upscale an image can make it bigger, but it adds no real detail, so it is no use for an official passport or visa photo.
- A minimum size as well as a maximum. A very plain, small photo can end up below the minimum. Make a photo smaller has a "Must be at least" box, and adds a hidden note to the file to reach the minimum without changing the picture. The page on increasing a photo size in KB opens it with a minimum filled in.
- The right numbers, but the website still refuses it. Look at the type of file. A PNG renamed to .jpg is still a PNG. A photo saved with print colours (CMYK), or one stored sideways, can be refused too. Check my file shows all of these.
- The size looks right on your computer. Windows and phones count a kilobyte differently, so the same file shows two sizes. Read KB, MB and why the same file shows two different sizes.
How to see the numbers on your own file
- Windows: right-click the file, choose Properties, then open the Details tab for the width and height in pixels. The size is on the General tab.
- Mac: select the file and press Command + I. "More Info" shows the dimensions, and "Size" shows the file size.
- Any device: choose the file in Check my file. It shows the real file type, the size, the pixels and the DPI saved in the file, and compares them with the rules you type in.
An example, step by step
The rule: "JPG, 3.5 × 4.5 cm, under 100 KB".
- Open resize a photo to 35 × 45 mm. It starts with the size already typed in. 3.5 × 4.5 cm is the same as 35 × 45 mm.
- Choose your photo. On the size screen, open "More options" and type 100 in "Also keep the file under (KB)".
- Press "Next", then drag the picture until the face is in the middle.
- Download the photo. The result page shows the exact pixels (413 × 531 at 300 DPI) and the file size, and tells you whether it meets the rule.
For a rule with only a KB limit, use compress a photo to 100 KB. For an official passport or visa photo, use the page for your country: it also shows the head-size guide and the background rule, and the size table in passport photo size by country shows every size side by side.