How PNG DPI Works

PNG files store physical pixel density in an optional chunk called pHYs (physical pixel dimensions). This chunk holds the pixel-per-unit values for X and Y axes, plus a unit identifier (1 = meter).

To store 300 DPI in a PNG, we convert: 300 ÷ 0.0254 = 11,811 pixels per meter. That number is written as a 4-byte big-endian integer into the pHYs chunk. We also recalculate the CRC checksum so the file remains valid.

If no pHYs chunk exists, we insert one before the first IDAT chunk. The PNG spec requires this ordering.

PNG vs JPEG for Printing

PNG supports transparency (alpha channel), which makes it essential for logos, stickers, and overlays. When printing:

Use PNG when your image has a transparent background
Use JPG for photos — smaller file size, same print quality
Both formats need 300 DPI metadata for most print services

Note about transparency: Some print shops flatten transparency to white when printing PNGs. Always check your print service's spec before submitting.

Convert PNG to 300 DPI

How PNG DPI Works

PNG vs JPEG for Printing