Is it safe to delete .etl files?

Mostly yes. .etl files are diagnostic trace logs, and Windows doesn't need old ones to work. Deleting them won't break anything, but you lose information that could help troubleshoot a problem later. Some are locked because Windows is writing to them right now, and most live in protected folders, so how you delete them matters.

Quick verdict:

• Safe: old .etl files in C:\Windows\System32\LogFiles\WMI, C:\Windows\Logs and C:\ProgramData\Microsoft\Diagnosis\ETLLogs, once nothing is writing to them.
• Safe, but use the Windows tool: error reports and diagnostic files, through Settings → System → Storage → Temporary files.
• Leave alone: files that are in use (Windows refuses anyway), and the folders themselves.
• Not worth it: most ETL files are tiny. Only delete the big ones if you're after space.

What are .etl files?

ETL stands for Event Trace Log. It's the file format of Event Tracing for Windows (ETW), the logging system that Windows components, drivers and many apps use to record what they're doing. The files are binary, so you can't open them in Notepad.

A normal Windows PC has hundreds of them scattered around. The places where they usually take up space:

Apps and drivers (graphics drivers, antivirus, VPN clients) can also write their own .etl files in their folders under ProgramData or AppData.

They grow when a trace session logs a lot, for example a component that keeps failing and writing errors, or when old files are never cleaned up. Microsoft's documentation says AutoLogger files default to a 100 MB maximum each, and sessions can keep several numbered copies (<name>.etl.0001, .0002 and so on).

How much space do they use?

Usually not much. Most .etl files are a few kilobytes. Now and then one grows to hundreds of megabytes or more, and that's the one worth finding.

To check with Windows only, open File Explorer, go to one of the folders above, switch to Details view and sort by Size. Right-click the folder and choose Properties for its total. Some folders ask for administrator permission to open.

You can also search the whole drive from PowerShell (run as administrator) for the largest ones:

Get-ChildItem C:\ -Filter *.etl -Recurse -ErrorAction SilentlyContinue |
  Sort-Object Length -Descending | Select-Object -First 20 FullName, @{n='MB';e={[math]::Round($_.Length/1MB,1)}}

A disk space analyzer gives you the bigger picture. With SquirrelDisk, scan C: or click A folder and paste C:\Windows\Logs or C:\ProgramData. The chart shows which folders are heavy, so you can see whether logs are really your problem. SquirrelDisk runs as your normal user without admin rights, so folders your account can't read are skipped and some system log folders may look smaller than they are. It also won't delete anything inside C:\Windows or C:\ProgramData\Microsoft: for those, use the steps below.

How to delete them safely

1. Start with the Windows tools

1. Open Settings → System → Storage → Temporary files.
2. Tick the categories you want. Depending on your PC, the list includes error reports and diagnostic files, plus log files left by Windows upgrades.
3. Click Remove files.

The classic Disk Cleanup (search for it in Start, then click Clean up system files) offers similar options. These tools only touch files Windows knows are safe to remove.

2. Delete the old .etl files by hand

For the folders Windows' own tools don't cover, like LogFiles\WMI or Diagnosis\ETLLogs:

1. Restart the PC. Some sessions start a fresh file at boot, which frees the old ones.
2. Open the folder in File Explorer. Accept the administrator prompt if asked.
3. Sort by Date modified and Size. Select old or large .etl files, not the folder.
4. Press Delete. If Windows says a file is open in another program, skip it: that trace is running right now.
5. Don't delete subfolders or other file types in these folders.

If you want to know which trace sessions are running, logman query -ets in an administrator terminal lists them.

3. Windows Update logs

The files in C:\Windows\Logs\WindowsUpdate are only useful if an update is failing. If you need them, run Get-WindowsUpdateLog in PowerShell first: it converts them into a readable WindowsUpdate.log on your desktop. Otherwise old ones can go. If updates are taking a lot of space, the bigger culprit is usually the SoftwareDistribution folder.

What you should not delete

• Files in use. Windows blocks them, and forcing it (with unlocker tools or by booting another system) gains little: they come back.
• The folders themselves. Some trace sessions expect their folder to exist. Empty the old files, keep the folder.
• Logs you need right now. If you're troubleshooting a crash, a failed update or a slow boot, or support asked you for logs, collect them first.
• Event Viewer logs. Those are .evtx files in C:\Windows\System32\winevt\Logs, a different thing. Clear them from Event Viewer (right-click a log, Clear Log) if you must, not in File Explorer.
• Other files in ProgramData\Microsoft. Only remove the .etl files you came for.

If something does go wrong, it's rarely more than a missing log. Restarting the PC recreates the files Windows needs.

How to stop them growing back

• Diagnostic data. In Settings → Privacy & security → Diagnostics & feedback, turning off optional diagnostic data reduces what the telemetry services log. On that page you can also delete the diagnostic data Microsoft has collected from your device.
• Find the noisy component. A single huge .etl file that keeps coming back means something is logging errors non-stop. Its name usually tells you what: a driver, a VPN client or a Windows component. Updating or reinstalling that software fixes the cause.
• Turn on Storage Sense in Settings → System → Storage so temporary files are cleared on a schedule.

ETL files are rarely what fills a disk. If you're short on space, look at the big folders first: the Windows Temp folders, crash dumps and error reports, or leftovers like Windows.old.