File Signature Checker
Read a file’s magic bytes and catch an extension that is lying.
- Free
- No signup
- Nothing uploaded
- No watermark
Read in this tab. The file is never uploaded — which is the point: you check a download precisely because you do not yet trust it.
How to Use the File Signature Checker Online
File Signature Checker reads the first bytes of a file, the magic number, and matches them against 58 known signatures: %PDF for PDF, PK for ZIP and everything built on ZIP, the eight-byte PNG header, FF D8 FF for JPEG and so on. It reports the format, the evidence it saw, how confident it is, and a red warning when the extension says one thing and the bytes say another.
Reads a file’s magic bytes and catches an extension that is lying. PDFs start with %PDF, PNGs with a fixed eight-byte signature, and ZIP files with PK — and a great many formats including .docx and .xlsx are ZIP archives underneath, which is why they all share that signature.
No software to install Free Works on any device
Step-by-Step Guide to Using the File Signature Checker
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1. Choose a file or folder
It is read in this tab and never uploaded.
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2. The result appears as soon as it has been read
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3. Copy or download whatever you need from it
What is the File Signature Checker?
Reads a file’s magic bytes and catches an extension that is lying. PDFs start with %PDF, PNGs with a fixed eight-byte signature, and ZIP files with PK — and a great many formats including .docx and .xlsx are ZIP archives underneath, which is why they all share that signature.
At a glance
- Takes: Any single file
- Reads: The first 64 KB only, which covers the deepest signature offset
- Signatures: 58, covering documents, images, audio, video, archives and installers
- Verdict: Format, MIME type, evidence bytes, and certain, likely or guess
- Mismatch: Flagged when the extension is not one the format uses
- Runs: On your machine; nothing is sent anywhere
Why Use File and Archive Tools?
A ZIP from a colleague, a download with no extension or a folder full of badly named files can stop work for a while. These tools unpack, identify, hash and rename files inside your browser, so nothing is uploaded to find out what it is.
“Know what a file is before you open it.”
- Archives opened without software
- Downloads checked with hashes
- Duplicate files found
- Big files packed smaller
- Unknown files identified
- Files renamed in bulk
- Folders compared quickly
- Contents never leave your computer
Popular Uses for ZIP and File Tools
These tools help with the small file puzzles that come up every week.
- Opening RAR files See and extract what is inside an archive on a computer with no unzip program.
- Checking a download Compare a file’s SHA-256 checksum with the one on the download page.
- Mystery files Find the real type of a file that has lost its extension or has the wrong one.
- Renaming photos Rename a folder of camera photos with dates or numbers in one go.
- Clearing duplicates Find repeated files taking up space in a downloads or photos folder.
- Sending many files Pack several documents into one ZIP so they travel as a single attachment.
- Comparing folders See which files differ between a backup folder and the working copy.
- Reading file details Check a file’s size, dates and hidden metadata before sharing it.
What You Can Do with the File Signature Checker
Validation happens on your own machine, at your machine's speed
Identifies a file by its actual signature bytes, not by trusting the extension
Hashes are computed over the whole file, so the value can be compared against a published checksum
Why Use ToolXkit for File Signature Checker?
Your files stay with you
Everything runs inside your browser. Nothing you open or type is uploaded to a server.
Free, with no catch
No signup, no watermark and no daily limit on how often you use it.
Fast
Results appear in seconds, because nothing has to travel over the internet and back.
Works on any device
Use it on a computer, tablet or phone, in Chrome, Edge, Firefox or Safari.
Nothing to install
No app, no extension and no desktop program. Open the page and start.
Made for real people
Plain words and clear buttons, built by one developer who wanted tools like this to exist.
Archive Tools for Different Users
Office workers
Open ZIP attachments and bundle reports together without extra software.
Developers
Check signatures, MIME types and hashes, and create TAR or GZIP files.
IT support staff
Identify strange files and verify downloads before installing anything on a machine.
Photographers
Rename shoots in bulk and find duplicates before backing up a card.
Students
Unpack course material and zip up project folders for submission.
Security-minded users
Check what a file really is before trusting its name or icon.
Best Times to Unpack or Check a File
File tools are useful when you are:
- Opening an archive on a new computer
- Verifying a software download
- Stuck with a file that won’t open
- Tidying a messy downloads folder
- Emailing a folder of documents
- Checking a backup is complete
- Renaming lots of photos
- Freeing space on a full drive
Tips and Good to Know
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Files with no signature, such as plain text, CSV and many source files, come back as a guess. The bytes look like text and nothing more can be said.
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A signature says what a file is, not whether it is safe. A real PDF can still carry a malicious link or script.
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Modern Office, EPUB and APK files all start with PK because they are ZIPs. The checker looks further inside to tell them apart, but a badly built one may only show as ZIP.
Frequently Asked Questions
Answers to common questions about this tool.
Do I need to install anything to use the File Signature Checker?
No. File Signature Checker runs entirely in your browser — there is nothing to download, no extension and no desktop program. It works the same on Windows, macOS, Linux, Android and iPhone.
Is it safe to use the File Signature Checker online?
Safer than the usual alternative, yes. Most online tools upload your file to a server you know nothing about, where it sits until someone deletes it. Here the file never leaves your device — the processing happens in the page itself, so there is no server copy to leak, retain or sell.
What is a checksum and why check one?
A checksum is a short code worked out from a file’s contents. If your copy gives the same code as the one the publisher lists, the file arrived complete and unchanged.
Is there a size limit in File Signature Checker?
It depends on your device rather than on the site. Everything is processed in memory in the browser, so a computer handles bigger archives and folders than a phone does.
Is File Signature Checker free to use at work?
Yes. It is free for personal and work use with no account. Because files stay on the machine, it also fits workplaces that do not allow uploading documents to outside sites.
Further reading
- List of file signatures (Wikipedia) The first few bytes that reveal a file’s real type, whatever its name says.
- ZIP file format (Wikipedia) How the ZIP format packs and compresses many files into one.
- RFC 1952: GZIP file format (IETF) The GZIP file format standard, published by the IETF.
More Files & Archives
Other free tools for the same kind of job.
- 7z Viewer List what is inside a 7z archive and extract it, in your browser. Open tool
- Archive Viewer Look inside a ZIP, TAR, GZ, RAR or 7z without extracting it. Open tool
- Bulk File Renamer Rename many files at once with find, replace, prefixes and numbering. Open tool
- Checksum Verifier Check a download against the checksum the publisher listed. Open tool
- Compress Files to ZIP Pack a folder of files into one ZIP, in your browser. Open tool
- Compress Text File Compress a .txt, .log, .csv or .json to GZIP, ZIP or raw deflate. Open tool
- Duplicate File Finder Find byte-identical files in a folder and see how much space they waste. Open tool
- File Extension Finder Look up what an extension means, what opens it, and whether it is risky. Open tool
- File Hash Generator Work out the MD5, SHA-1 and SHA-256 of a file without uploading it. Open tool