Some notifications say the photograph must be scanned at 200 DPI. Then they also give pixel dimensions, and it is not obvious how the two relate — or what you are supposed to do about it.
DPI is a label, not a property of the image
An image file contains a grid of pixels. That is the real content: 200 across and 230 down, say.
DPI — dots per inch — is a separate number stored in the file's header that says how large those pixels are meant to be when printed. At 200 DPI, 200 pixels across means one inch wide. At 100 DPI, the same 200 pixels means two inches.
Changing the DPI value does not add or remove a single pixel. It changes the intended print size, nothing else.
This is why the two numbers can both appear in a notification without contradiction. The pixel dimensions say how much image there is; the DPI says how big it is supposed to be on paper.
Where the requirement comes from
The instruction to scan at 200 DPI predates online forms. It was written for a physical photograph on a flatbed scanner, and it was a sensible way to say "capture enough detail" before anyone thought in pixels.
Many notifications have carried the wording forward without updating it. A 3.5 x 4.5 cm photograph scanned at 200 DPI produces roughly 276 x 354 pixels — which is close to the 275 x 354 that several exams now specify directly.
So in most cases, hitting the pixel dimensions is the substance of the requirement, and DPI is a legacy way of expressing it.
When it actually matters
Sometimes the portal reads the header.
Browsers write 96 DPI into any JPEG produced from a canvas — which is essentially every online resizer, including this one, unless it deliberately does otherwise. If a portal checks that field and expects 200, it rejects a file whose pixels are perfectly correct.
This is an unusual failure and a maddening one, because the image looks right and every visible number matches.
What this tool does
Our resizer writes the DPI you ask for directly into the file, defaulting to 200.
For JPEG that means the density fields in the JFIF header, splicing in a JFIF segment when the encoder did not produce one. For PNG it means a pHYs chunk, which is the PNG equivalent and stores resolution in pixels per metre.
You can see the value in the result panel after creating a file, and change it in Required size if your notification asks for something else.
Checking it yourself
You do not have to take our word for it:
- Windows — right-click the file, Properties, Details, look for Horizontal and Vertical Resolution.
- macOS — open in Preview, then Tools, Show Inspector.
- Any platform — most online EXIF viewers show it, though that means uploading your photo to someone else's server, which rather defeats the point.
What to do in practice
Match the pixel dimensions first. That is what nearly every portal actually validates, and it is what determines whether your photo looks right.
Leave DPI at 200 unless your notification says otherwise. It costs nothing, it satisfies the portals that do check, and it is invisible to the ones that do not.
Do not try to hit a physical size in centimetres by changing DPI. If a notification asks for 3.5 x 4.5 cm at 200 DPI, the answer is 276 x 354 pixels, and setting DPI without setting the pixels changes nothing about the file's content.