Image Format Detector

How to use the image format detector

1. Choose any image file.
2. Read the detected format.
3. Check the print table for the size you need.

About this tool

Reads the file's first bytes to determine what it actually is. Every image format begins with a distinctive signature — JPEG with FF D8 FF, PNG with an eight-byte marker, GIF with the letters GIF8 — and that signature is what a decoder reads.

The extension is not evidence. It is a label anyone can change, and renaming a JPEG to `.png` changes nothing about the file. The type your browser reports is no better, because it is usually derived from the extension. When the two disagree, this says so — that mismatch is worth knowing, since most software follows the bytes and opens the file regardless, while some follows the extension and refuses.

The print table converts pixel dimensions into an effective DPI at common sizes. 300 DPI is the traditional standard for photographic printing and 200 is acceptable for most purposes; below about 150 the result looks visibly soft. This is the calculation that answers whether an image is large enough to print at a given size, which the pixel count alone does not.

Two formats are identified but cannot be displayed: HEIC, which browsers do not decode, and TIFF, likewise. Recognising them is still useful — it explains why the image will not open.

Common uses

• Finding out what a file really is when the extension looks wrong.
• Checking whether an image is big enough to print.
• Diagnosing why an image will not open somewhere.

Frequently asked questions

Why not just trust the file extension?

It is a label, not evidence. Renaming a JPEG to .png changes nothing about the file, and the type your browser reports is usually derived from that same extension.

What resolution do I need to print?

300 DPI is the traditional photographic standard, 200 is acceptable for most purposes, and below about 150 the print looks visibly soft. The table shows the figure for common sizes.

Why can it identify HEIC but not display it?

Identifying a format needs only its first bytes; displaying it needs a decoder, and browsers do not include one for HEIC or TIFF.