Setting an image's DPI on a Mac updates the print-density instruction stored inside the file rather than resizing the actual pixels, so a 3000 by 2400 pixel image stays 3000 by 2400 pixels no matter what DPI value you set. At 150 DPI that same file is intended to print at 20 by 16 inches; at 300 DPI it is intended to print at 10 by 8 inches. macOS itself stores this information in two different places depending on the format. A JPEG carries a JFIF APP0 segment with Xdensity and Ydensity fields using dots per inch, while a PNG carries a pHYs chunk with an integer pixels-per-metre value derived from your requested DPI. Image DPI Changer writes exactly those standard fields, validates the file structure first, and downloads a local copy whose pixel dimensions are byte-for-byte identical to the source. The whole operation runs in your browser on macOS without uploading the file, without installing anything, and without re-encoding the compressed image data.
What "Changing DPI" Actually Changes in the File
DPI is often confused with resolution. Raising a 72 DPI image to 300 DPI does not invent new pixels, sharpen a blurry photo, make Safari render the image more crisply, or turn a low-resolution scan into a high-resolution master. The pixel dimensions of the file stay exactly the same; only the print-density instruction changes. Likewise, lowering a value does not delete pixels. A printer, layout program, or export pipeline may use that instruction to calculate a physical size, but a website generally displays the image according to its CSS or layout dimensions instead, so the DPI label on a file rarely affects how the file looks on a web page.
The same rule applies on Mac as on any other platform. When a print shop, a Lightroom export preset, or an InDesign document asks for a specific DPI, they are asking how many of the file's existing pixels should occupy one inch of paper. If the source file does not have enough pixels for the physical size they want, no DPI label can compensate. That is why every native Mac method and every third-party metadata tool, including Image DPI Changer, describes itself as a metadata update, not a resolution boost.
Mac Methods at a Glance
macOS gives you three native ways to touch DPI: the Preview app, the Terminal command sips, and a Shortcuts workflow. Each method changes something slightly different, and not all of them write the JFIF or pHYs metadata the way downstream print software expects.
| Mac method | What it changes | Pixel dimensions | Typical side effect |
|---|---|---|---|
| Preview — Adjust Size with "Scale proportionally" off | Pixel dimensions, DPI label, file size | Resamples the image | Adds or removes pixels; degrades the image on reduction |
| Preview — Adjust Size with "Scale proportionally" on | Document size only | Stays the same | Updates the DPI label embedded in the file |
| Terminal — sips -d dpiValue file | DPI metadata in supported formats | Stays the same | Rewrites the JFIF or EXIF resolution fields broadly |
| Shortcuts — "Adjust Image Size" action | Pixel dimensions, DPI label | Resamples by default | Same resampling exposure as Preview's default dialog |
| Image DPI Changer (browser) | JFIF density (JPEG) or pHYs chunk (PNG) | Stays the same | No re-encoding, no upload, no resampling |
The first and fourth rows resample. The middle two are closer to a true metadata update, but each one rewrites more than the focused metadata fields a print workflow actually reads. Image DPI Changer sticks to the two specific fields a print pipeline uses: the JFIF APP0 density fields for JPEG and the pHYs physical-pixel chunk for PNG.
Change Image DPI on Mac with Image DPI Changer
The tool runs entirely in your browser on macOS. Nothing is installed, nothing is uploaded, and the source file is read as bytes directly from your Mac. Follow these steps to set a new DPI on a single JPEG or PNG.
- Open Image DPI Changer in your browser and choose one JPEG or PNG up to 25 MiB from Finder. After the choice, the page reports whether it detected any existing print-density metadata, including a JFIF density block, a PNG pHYs chunk, or standard EXIF resolution fields.
- Enter a whole-number target DPI such as 72, 150, 300, or 600. This value is a print-density instruction only and does not change the pixel dimensions of your image.
- Select "Update image DPI". The page validates the file signature and bounded header structure, then writes a new JFIF APP0 block for JPEG using the standard dots-per-inch unit, or a new pHYs chunk for PNG using the exact 0.0254 metre per inch relation to convert the requested DPI into an integer pixels-per-metre value.
- Download the resulting copy. The downloaded file has the same pixel dimensions, the same compressed image data, and only the metadata field you wanted has been updated.
- Open the downloaded file in Preview, InDesign, or whatever destination layout program you intend to use, and verify the intended physical size at the new DPI.
If the page reports that it could not detect any existing JFIF or pHYs density, that simply means the source file had no print metadata to begin with. The downloaded copy will still contain a freshly written JFIF block or pHYs chunk with the value you entered. Truncated marker segments, malformed PNG chunks, duplicate physical-pixel chunks, or a pHYs chunk placed after image data are rejected rather than patched optimistically, so a corrupt input will not produce a broken output.
Calculate the DPI You Need Before You Change It
Before you reach for any metadata tool, decide the physical size you actually want on paper and work the math backwards from the pixels you have. The formula is straightforward:
Intended DPI = pixel count ÷ intended print size in inches
For example, a 3600 pixel wide image that should print at 12 inches wide on paper gives:
3600 pixels ÷ 12 inches = 300 DPI
So you would set 300 DPI in Image DPI Changer, download the copy, and place it in the target layout program at exactly 12 inches wide. If the layout program asks for a physical size, give it 12 inches. If it asks for the DPI, give it 300. Both values describe the same relationship between the same 3600 pixels and the same 12 inch strip of paper. The DPI label is the more portable of the two: most print pipelines read the DPI metadata first and use it to derive the inches, so updating that single field is often enough.
Verify the New DPI on Your Mac
After downloading the updated file, open it in Preview and choose Tools → Show Inspector, or open the Info window with Command-I. The DPI value stored in the file should now match what you typed. For a JPEG, the value comes from the JFIF density block. For a PNG, the value is reconstructed from the integer pixels-per-metre field inside the pHYs chunk, so a readback can occasionally differ from your requested value by a small rounding fraction. That difference is a storage precision limit in the PNG specification, not a change in the actual image pixels. The underlying pixel data is still untouched.
If you want to verify the file outside of Preview, a metadata viewer or a layout application such as InDesign will report the same DPI. Different programs can choose to display different metadata families (JFIF, EXIF, XMP, or pHYs), so if one program shows a different value, check whether it is reading the same metadata family Image DPI Changer wrote. The generated JFIF field and the pHYs chunk are both documented, portable metadata per the W3C JFIF specification and the W3C PNG specification, which means a wide range of print software will read them consistently.
When You Need a Different Tool Instead
If your real problem is that the source image does not have enough pixels for the physical size you want, no DPI metadata update can fix it. That is a resize problem, not a metadata problem. On Mac, you would reach for a true pixel resizer in that case, for example the Image Resizer browser tool, which changes the actual pixel count for a new physical size while still running locally in your browser. Knowing which job you are trying to do — change the print-density label versus change the pixel count — is the difference between a metadata tool and a resampling tool.
For most Mac users, the workflow is short: decide the physical size, calculate the DPI from the available pixels, run the file through Image DPI Changer, and place the downloaded copy into the destination layout program at its intended size. If you also need to convert between inches and centimetres, or want to see the physical size for a given DPI before you change anything, the DPI Converter walks through those calculations without modifying anything. Keep the original file on disk so you can return to it whenever the print provider changes their requirements, and always follow the print provider's documented pixel count, bleed, and file format requirements rather than assuming a single DPI label is enough on its own.