BYTETOOLS

Image Format Checker

Identify what an image file really is from its magic bytes, then decode the header for dimensions, bit depth, subsampling and encoding mode.

Drop any image file to identify its real formatRead from the magic bytes, not the file name — nothing is uploaded

Choose a file to see which format it actually is, plus the header fields decoded from its first bytes.

What is the Image Format Checker?

The ByteTools Image Format Checker ignores the file name and reads the first bytes of the file itself.

  • Format detected from magic bytes, never from the file extension
  • Recognises PNG, JPEG, GIF, WebP, AVIF, HEIC, BMP, TIFF, ICO, PSD and more
  • PNG IHDR decode with bit depth, color type, interlacing and a CRC pass over every chunk
  • JPEG baseline versus progressive plus chroma subsampling from the SOF marker
  • Clear warning when the extension or reported MIME type contradicts the real format
  • Hex dump of the first 24 bytes and a one-click copyable report

How to use the Image Format Checker

  1. 1

    Drop any image file onto the upload area, or click it to browse — the extension is ignored.

  2. 2

    Read the summary tiles for the detected format, its MIME type, the extension on disk and the file size.

  3. 3

    Watch for the amber warning that appears when the extension or the browser-reported type disagrees with the real bytes.

  4. 4

    Open the Decoded header table for format-specific fields such as bit depth, color type, subsampling or interlacing.

  5. 5

    Click Copy report to put the signature, hex dump and every decoded field on your clipboard.

About the Image Format Checker

The ByteTools Image Format Checker ignores the file name and reads the first bytes of the file itself. Every image format stamps a fixed signature at the start — 89 50 4E 47 for PNG, FF D8 FF for JPEG, a RIFF header followed by WEBP — so the tool can tell you what a file truly is even when someone renamed a .webp to .jpg to slip it past an upload form.

Once the format is known, the header is decoded too. PNG files report their IHDR bit depth, color type, interlace method and per-chunk CRC results. JPEGs report baseline or progressive encoding, chroma subsampling read from the component sampling factors, and any embedded EXIF, ICC or XMP metadata. GIF, WebP, BMP, TIFF, ICO, AVIF and HEIC each get their own decoded fields.

Processing is 100% local: the file is read straight from the drop zone with the browser File API and parsed byte by byte in JavaScript on your own device. Nothing is uploaded to any server, so you can inspect confidential artwork, client assets or a suspicious download safely. Close the tab and no copy of the file remains anywhere.

Frequently asked questions

How do I find out what format an image really is?

Drop the file into this tool and it reads the bytes at the very start of the file, where every format writes a fixed signature. That signature is what image software actually looks at, so it identifies the true format even when the file carries the wrong extension.

Does renaming a .webp file to .jpg convert it?

No. Renaming only changes the label on the file, and the compressed data inside stays exactly the same. Any program that reads the header will still see WebP and may refuse to open it, which is why this checker flags that mismatch for you.

What are magic bytes in an image file?

Magic bytes are the fixed sequence at the beginning of a file that identifies its format, such as 89 50 4E 47 for PNG or FF D8 FF for JPEG. They are written into the format specification itself, which makes them far more reliable than a file extension.

How can I tell if a JPEG is progressive or baseline?

This tool reads the JPEG start-of-frame marker: SOF0 means baseline sequential and SOF2 means progressive. Progressive JPEGs arrive in increasingly sharp passes and are often slightly smaller, while baseline files decode from top to bottom in a single pass.

Is my image uploaded when I check its format?

No. The file is parsed entirely inside your browser with JavaScript and nothing is sent over the network. You can safely inspect private, unreleased or confidential images because they never leave your device at any point.

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