Image DPI Changer

What this tool does

The Image DPI Changer patches the resolution metadata in a PNG or JPEG file so that print software, document editors, and PDF tools interpret the image at the correct physical size. It works at the byte level: for PNG files it writes or replaces the pHYs chunk with the correct pixels-per-metre value; for JPEG files it patches the JFIF APP0 header to set the density value in dots-per-inch. The pixel data is never touched, so the image looks the same but is physically sized differently when placed in a print layout.

Why you might need it

A photo from a phone or digital camera arrives labelled as 72 DPI — the default for many devices. When you insert it into a Word document, InDesign layout or PDF form, the application reads the DPI label and sizes the image to fill an enormous area of paper. The same photo re-labelled as 300 DPI will be placed at roughly one-quarter the size, matching what the pixel count can actually support.

The reverse is also common: images exported from vector tools or screen-capture software at 96 DPI need to be re-labelled as 300 DPI before delivery to a print shop, because the printer’s preflight system checks the DPI metadata and may reject files that read as low-resolution even if the pixel count is high enough.

How to use it

1. Drop a PNG or JPEG onto the drop zone, or click to browse. The tool reads the file and shows the current DPI if one is stored in the metadata.
2. Choose a preset — 72, 96, 150, 300 or 600 DPI — or type a custom value.
3. Click Apply DPI. The tool patches the metadata and the download button appears immediately.
4. Click Download to save the patched file. The pixel content is identical to the original.

Format and quality notes

For PNG, the tool inserts a pHYs chunk immediately before the first IDAT data chunk, which is the position the PNG specification recommends. Any existing pHYs chunk is removed before the new one is inserted. The chunk includes a correct CRC-32 checksum so every conforming PNG parser will accept the result.

For JPEG, the tool writes to the JFIF APP0 segment immediately after the SOI (start-of-image) marker. If a JFIF APP0 already exists, the density fields are patched in-place. If no APP0 is present, a minimal one is inserted. The pixel data, Huffman tables, quantisation tables, and all other segments are preserved exactly.

WebP and AVIF are not supported because their metadata structures differ from the two-byte-stream formats above and would require different handling.

Tips for best results

Always keep the original file and treat the DPI-patched version as a derivative. If you later need to re-print at a different size, you can re-apply this tool to the original rather than re-patching an already-patched file.

When submitting images to a print shop, ask for their required DPI. A standard A4 print at 300 DPI requires a pixel count of at least 2480 × 3508. If your image has fewer pixels, increasing the DPI label does not add resolution — it just tells the printer to print the pixels larger, which can result in visible pixelation. Match DPI to the available pixel count; do not set DPI higher than the image can support.