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Photo Printing

Change the DPI of an Image

Set the DPI tag of a JPEG or PNG to 300 in your browser. The pixels stay the same, and the page shows the print size before and after.

Built and written by Jernej Letica

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Changing the tag keeps the picture data of a JPEG or PNG unchanged. It does not add pixels.

How to change image DPI to 300

The page opens in tag mode, so the photo and its DPI come first. The size fields follow below.

  1. Choose a JPEG or PNG photo. The page reads its pixel size and any DPI tag it already stores.
  2. Read the Before and After line. Before is the print size at the stored DPI; After is the print size at the DPI in the field.
  3. Leave the DPI at 300, or type the value your lab asks for. The print size changes; the pixel fields do not.
  4. Press Download with this DPI. The saved file has the same pixels and a new tag, and its name ends with the DPI.

Why the pixels stay the same

A DPI tag tells a program how large to print the pixels, and nothing more. It adds no detail, so this page never resamples the photo. A frame of 6000 × 4000 px prints at 50.80 × 33.87 cm at 300 DPI. Set the tag to 150 DPI and the same file prints at 101.60 × 67.73 cm, with each pixel twice as wide on paper.

That is why the page shows both sizes. The Before line is what the file already claims, and the After line is what you are about to save. If they match, the tag was already right.

How JPEG and PNG files store the tag

A JPEG keeps its density in a JFIF header, in the EXIF resolution pair, or in both. The page updates whichever of them is there, and writes whole DPI values, so 300 is stored exactly. A JPEG with no JFIF header gets a small one of 18 bytes. Nothing else in the picture changes, and the compressed blocks are copied as they were.

A PNG keeps its density in a pHYs chunk, counted in pixels per metre. The page rewrites that chunk, places it right after the header as the format asks, and copies every other chunk unchanged. Transparency stays, and the decoded pixels are the same values as before.

What a program does with the tag

Print dialogues and layout programs read the tag as the default size of the picture. Many let you choose their own page setup instead, so the size on paper can differ from the figures on this page. Some ignore the tag completely. A lab usually wants the pixel size and the paper, and sets the scale in its own workflow.

So treat the After line as the size the tag asks for, and check the print size in the dialogue before you send the file.

Checks before you download

  • Ask the lab for its DPI and paper size first. Type that DPI, then read the After line to confirm the print size.
  • Work from the original file. Tagging a web copy at 300 DPI does not give it the pixels a print needs.
  • Watch the Before line. A photo with no tag says no DPI tag stored, and its print size then depends on the program.
  • Keep the saved copy apart from the original. Its name ends with the DPI, so the two never overwrite each other.

Privacy

Your photo never leaves your device. The page reads its size and DPI in your browser, and a changed photo is saved straight from this page.

Frequently asked questions

Does changing the DPI make an image sharper?

No. The tag only tells a program how large to print the pixels. A 1200 × 800 px photo tagged at 300 DPI still holds 1200 × 800 px. It prints at 4.00 × 2.67 inches, and at 72 DPI the same file would print at 16.67 × 11.11 inches.

How do I change the DPI of an image, JPEG or PNG?

Set the DPI field, then press Download with this DPI. The page writes the density into the JPEG header or the PNG pHYs chunk and leaves the picture data unchanged, so the pixels decode to the same values. A PNG without a density chunk gets one, and its transparency is kept.

Why does the print size change when the pixels do not?

The print size is the pixel count divided by the DPI. The pixel count stays the same, so changing the DPI changes the division. At 300 DPI, 6000 px is 20 inches, or 50.80 cm. At 150 DPI the same 6000 px is 40 inches, or 101.60 cm.

Is 300 DPI the right setting for every print?

Not always. The page starts at 300 DPI because that is the value the calculator uses in its print rows. Your lab decides which DPI suits its paper and the size you order, so type that number into the field. The After line shows the result before you save.

Does the saved file keep my camera details?

Yes. Only the density notes change: the JFIF header and, when the file has one, the EXIF resolution pair. The rest of the JPEG is copied as it was, and a PNG keeps every chunk except its density. Use a metadata tool if you want the camera details removed.

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