How to Fix “Unknown Compression Method” When Extracting Backups

If you have ever downloaded a WordPress backup ZIP, TAR, RAR, or GZIP file and tried to extract it locally or on a server, you may have seen an annoying warning:

Unknown Compression Method

It stops the entire extraction process, prevents access to your database or files, and in some cases, leaves the backup completely unusable. This error occurs in cPanel, Plesk, phpMyAdmin, 7-Zip, WinRAR, macOS Archive Utility, and even Linux command line unzip.

For WordPress users, this situation is not just inconvenient—it can be devastating. You may need to restore your website after malware, server crash, hosting migration, plugin update failure, or corrupt core files. But when the backup fails to extract, the entire purpose of having one is defeated.

The good news? No matter how complicated this error looks, it is fixable.

This guide explains why the error appears, how to fix it, how to prevent it permanently, how the compression system works, and how to recover damaged files even when unzip reports unknown compression method. Everything is written for beginners, website owners, and technical WordPress administrators.

Let’s begin.

What Does “Unknown Compression Method” Mean?

This error appears when the extraction software cannot understand the compression format stored inside the backup archive. In simple words, the software has no idea how the file was compressed, so it fails to extract the data. If you want to diagnose exactly why your backup extraction or restoration fails, enable debugging by following our guide on how to enable WordPress debug mode to find errors and identify underlying PHP or plugin issues.

For example:

• Extracting with WinRAR → fails

• Trying 7-Zip → fails

• Linux unzip → fails

• Hosting File Manager → fails

But another tool might open it.

This means the archive file does not match the decoding engine being used.

Why Does This Error Happen?

It generally occurs due to one of the following:

1. Backup file is partially corrupted

Interrupted download, server timeout, or drive failure can corrupt archives.

2. Wrong file extension

Maybe the file looks like .zip but is actually .gz.

3. Archive created using unsupported algorithm

Some backup plugins use advanced formats like:

• Deflate64

• BZIP2

• LZMA

• PPMD

• ZSTD

• Brotli

Many unzip tools do not support them.

4. File headers have changed

Server compression layers may add metadata.

5. Wrong transfer mode

Downloading ZIP over FTP in ASCII mode damages the file.

6. Incomplete upload

Backup file only contains half of the archive chunks.

Where Do WordPress Users Face This Error Most?

• Hostinger File Manager

• cPanel Extract feature

• DirectAdmin Extract

• Localhost using XAMPP

• Linux command line

• 7-Zip

• WinRAR

• phpMyAdmin gzip extraction

• Mac Archive Utility

Solutions: How to Fix the “Unknown Compression Method” Error

Below are the most reliable fixes.

Fix 1: Try Using a Different Extraction Tool

Your extraction program may not support the algorithm used.

Recommended tools that support more formats:

7-Zip (Windows)

Open source. Reads LZMA, ZIPX, TAR, ISO, and more.

PeaZip (Windows/Linux)

Supports Zstandard and Brotli.

WinRAR (Windows)

Extracts Deflate64 correctly.

Keka (macOS)

Extracts formats macOS cannot open.

Linux CLI (all)

Using tar or pv.

Unarchiver (macOS)

Handles ZIPX compression.

Fix 2: Use Linux Command Line to Force Extract

Upload the file to your server via SSH and run:

If unzip fails, try this:

If still failing:

Linux tools understand more compression formats than GUIs.

Fix 3: Correct Damaged File Headers

Often the issue is because the uploaded file has a broken header.

Run:

Then extract recovered.zip.

This command rebuilds the archive structure.

Fix 4: File Is Not Actually ZIP—Identify It

Run:

It may show output like:

Then extract using:

Sometimes WordPress exporters rename .gz to .zip.

Fix 5: Split Archive Recovery

Some backup plugins produce chunk files like:

First merge:

Fix 6: FTP Transfer Fix

If your file was uploaded to server using ASCII mode, reupload using Binary mode.

ASCII removes line endings—ruins binary archives.

Fix 7: Open in WinRAR if Method is Deflate64

ZIP files using Deflate64 cannot be opened by:

• 7-Zip (older builds)

• macOS Archive Utility

• Windows built-in unzip

Open using:

1. WinRAR

2. PeaZip

3. Keka

Fix 8: Extract Through Backup Plugin Interface

If your WordPress backup came from:

• UpdraftPlus

• Duplicator

• JetBackup

• Softaculous

• All-In-One WP Migration

Use their restore wizard instead of manual extraction.

They bypass compression issues. If you want to learn the detailed manual restoration method step by step, check out our guide on how to restore a WordPress site from backup manually for complete instructions.

Fix 9: Redownload or Re-Export the Backup

If file size differs from original:

• File is incomplete

• Download interrupted

• Check MD5 hash

Example:

Compare results.

If mismatch → redownload. If your backup extract failed due to damaged database tables, read our full guide on WordPress database corruption causes, prevention, and fixes to repair MySQL issues safely.

Fix 10: Fix GZIP Database Extraction Errors

Database .sql.gz fails?

Convert using:

Then import using phpMyAdmin.

Most Common WordPress Backup Compression Methods

ZIP
TAR
GZ
XZ
BZIP2
LZMA
ZSTD
RAR

Hosting panels support only the first three.

How to Avoid This Error in the Future

For stronger long-term protection and to reduce the chances of backup corruption caused by malware, check our guide on the best WordPress security plugins to secure your website properly.

1. Stop using weak hosting file managers

Use SSH or desktop extraction.

2. Use plugins that produce standard ZIP

Duplicator Pro
UpdraftPlus
WPvivid

3. Avoid Windows built-in unzip

Not designed for large archives.

4. Upload backups in Binary mode

Fixes corruption issues.

5. Store backups on cloud instead of local PC

Google Drive
Dropbox
Wasabi

To make sure you never lose data again, follow our tutorial on how to backup WordPress to Google Drive automatically and store secure cloud copies of your website.

Understanding Backup Compression Internally (Simple Terms)

When you create a ZIP file, WordPress backup plugins compress data using algorithms. Every unzip tool has to know how to read that algorithm. If the extraction program does not support it, you see:

Unknown Compression Method

That means:

“This program cannot decode what created this archive.”

Advanced Repair Techniques

If your backup extraction failed during server processing or ZIP building because the machine ran out of memory, read our guide on how to fix WordPress using too much RAM on shared hosting to optimise resources and prevent crashes.

Repair ZIP dictionary:

Display ZIP metadata:

Extract broken files:

Force ignore compression flags:

Real Scenario: WordPress Host Migration Failure

If attempting to extract or restore your backup causes a blank page, check out our guide on how to fix the WordPress White Screen of Death to quickly regain access to your website.

Example:

You moved your site from SiteGround to Hostinger.

Downloaded site.zip.

When extracting:

Error: Unknown Compression Method

Cause:

SiteGround uses ZSTD compression.

Fix:

Install PeaZip or 7-Zip latest build. If restoring your backup leads to front-end errors or a white screen, you may be facing PHP or plugin conflicts, so follow our troubleshooting guide to fix “This site is experiencing technical difficulties” in WordPress and get your website running again.

Real Scenario: .zip Upload Corrupted by Browser

Slow internet connections sometimes break downloads.

Firefox and Chrome partially store the file.

Solution:

Download using curl:

Full integrity guaranteed.

Real Scenario: File Extension Label Changed

A user renamed:

backup.tar.gz → backup.zip

Extraction failed.

Fix: rename back to:

backup.tar.gz

and execute:

Code: Convert ZIP to TAR

If ZIP method unsupported:

Code: Extract with PHP Script

Create extract.php:

Works even if hosting fails to extract.

What If Nothing Works?

Then the archive is unreadable.

Possible options:

• contact hosting support

• regenerate backup from original server

• recover partial files manually

• restore WordPress database using SQL dumps

• rebuild website using stored HTML snapshots

You can get emergency WordPress support from our expert team if your backup is broken and you want fast, safe website restoration without losing data.

Prevention for Professionals

To reduce restore failures and prevent outdated plugin conflicts, follow our guide on how to set up automatic updates for WordPress safely so your site stays healthy without breaking.

Use CLI backup

Instead of compressing full backup, store raw folders and database dumps.

Monthly test restores

Never trust backups unless tested.

Avoid ZIP size over 4GB

Large archives break easily.

Should You Use RAR for WordPress Backups?

No.

RAR is proprietary, unsupported on hosting panels.

ZIP or TAR are recommended.

WordPress Backup Plugin Comparison (Compression Level)

Final Advice

The “Unknown Compression Method” error looks scary, but it does not mean your data is lost. It only means:

Your extraction software cannot read the algorithm.

Switch tool, repair archive, extract using command line, or regenerate backup.

If nothing works:

Your backup file is damaged — request a fresh copy. If your backup extraction fails due to malware or suspicious files, follow our detailed guide on how to remove malware from WordPress manually to clean your website safely before restoring backups.

FAQs

What causes “Unknown Compression Method” errors?

The extraction software does not support the compression algorithm used to create the archive.

Is my backup corrupted?

Not always. Often the issue is wrong tool or wrong extension.

Can I fix incomplete ZIP downloads?

Yes, using zip -FF.

Why does 7-Zip fail to open some ZIP files?

Because older builds do not support Deflate64 or ZIPX.

Why does WinRAR extract ZIP but 7-Zip does not?

Different decoding engines → different support levels.

How to avoid this error permanently?

Use widely supported ZIP compression.