How the calculations work

Print size and effective PPI

Print size in inches is calculated as pixel count divided by PPI. Required pixels are physical inches multiplied by target PPI, rounded upward to the nearest whole pixel. We use the international standard of exactly 25.4 millimeters per inch.

When image and print aspect ratios differ, Fit entire image uses the largest image scale that fits completely within the print bounds. Fill and crop uses the smallest scale that covers the entire area without empty margins. The crop position slider controls which portion of the image is removed without altering the scale or effective PPI. Pixels to fill are calculated for the finished print dimensions only.

Aspect ratios of pixel dimensions are reduced with the greatest common divisor when the result is compact (3000 × 2000 shows as 3:2). Some device panels are close to familiar formats: a 2532 × 1170 landscape panel displays as approximately 19.5:9; in portrait that is 9:19.5. Other large ratios use a rounded decimal form. Non-integer sizes, such as paper in centimeters, are shown as a rounded decimal ratio. For prints that require bleed, the Pixels incl. bleed result applies the same rounding rule to the finished size plus twice the per-side bleed: pixels = ceil((finished + 2 × bleed) × PPI) per side. Bleed amounts vary by print provider, so enter the value from its template.

Screen pixel density

Screen PPI equals the diagonal pixel count divided by the physical screen diagonal in inches: PPI = √(W² + H²) ÷ D. Pixel pitch in millimeters equals 25.4 ÷ PPI, and pixels per square inch equals PPI². We assume square pixels (1:1 pixel aspect ratio) and that the entered diagonal describes the active display area.

Embedded image metadata specifications

The checker reads pixel dimensions for JPEG, PNG, WebP, GIF, and BMP files. Physical density is read from JPEG JFIF (APP0) or EXIF (APP1 IFD0) resolution markers, PNG pHYs chunks, and BMP pixels-per-meter header fields. WebP and GIF have no reliable density tag, so those formats report none. If physical density is absent from the file, the checker explicitly reports that fact rather than guessing. The DPI changer updates JPEG or PNG metadata fields or chunks while keeping pixel data byte-for-byte identical, and it refuses files it cannot edit. PNG stores pixels per meter, so its displayed PPI may vary slightly from a whole-number target due to integer unit conversion.

Limitations and print shop best practices

Image files up to 50 MB are supported. The tool does not evaluate printer ICC color profiles, CMYK separation, paper dot gain, ink absorption, optical focus, or compression artifacts. It does not upscale or resample image pixels. The checker reads file headers only; it does not decode or validate the compressed image data. For mission-critical fine art or commercial offset printing, confirm final requirements with your print service provider.

Primary references

The metadata rules above follow the published format specifications: the PNG specification (pHYs chunk) maintained by W3C/ISO, the CIPA EXIF standard that defines the XResolution, YResolution, and ResolutionUnit tags, and the ITU-T T.871 JFIF specification that defines JPEG density units. The inch-to-millimeter conversion is the exact international definition of 1 inch = 25.4 mm.