What is the PART file format?
A .part file stores data downloaded by a program from the internet. It contains a partially downloaded file, whether due to a suspended download process or a download error. .part files are used by various applications for downloading files, including Mozilla Firefox, Free Download Manager, JDownloader, wget, and aria2. If the download has not been completed, such a file may not work. When the download is complete, the .part extension is replaced with the file's original extension.
A .part file has no format of its own: it holds exactly the bytes received so far from the server. A partially downloaded MP4 video therefore opens with a valid MP4 header, while a partial ZIP archive starts with a valid ZIP header. An empty, zero-byte .part file means the transfer never truly began. Some download tools write a companion control file alongside it - aria2 creates an .aria2 sidecar tracking which byte ranges have been completed, allowing finer-grained resumption.
Downloading of .part files can be resumed, but it will not always work. Download errors most often require the download process to be restarted. Partially downloaded video or audio files can be opened despite not having been downloaded completely - in such a case the user must open the file in an appropriate program, or rename it by changing the .part suffix back to the original extension. The .part suffix is typically added to the end of the file's name - for example, movie.avi would be named movie.avi.part during the download. Similar incomplete-download extensions include .crdownload (Google Chrome) and .download (Safari on macOS).
Security & safety
RISK: LOWA .part file cannot execute and is exactly as risky as the file being downloaded - no more, no less. The .part suffix actually helps: it stops the OS from treating an incomplete executable as runnable, which is part of its purpose. Two practical notes: if you didn't initiate the download that created the .part, treat the underlying source file as suspicious and delete it; and orphaned .part files left over from old or cancelled downloads are safe to delete to reclaim disk space.
Format details
in a nutshellFULL NAMEPartial Download File
DEVELOPERMozilla (Firefox) and various download managers - no single ownersince Used by Mozilla Firefox/Mozilla Suite (early 2000s); also wget, aria2, KGet and other download managers
CATEGORYWeb Files
MIME TYPEapplication/octet-stream
TYPEIncomplete copy of whatever file is being downloaded - no format of its own
Programs that open PART files
Windows4 apps
Free Download Manager
Free Resume the stalled task from its queue; FDM is built around resumable downloads. 
VLC media player
Open-source For partial video/audio only: drag the .part into VLC to play the portion that downloaded. 
Mozilla Firefox
Open-source Press Ctrl+J (Downloads), find the interrupted item and click the retry/resume arrow; if it can't resume, restart the download from the source. 
macOS3 apps
Mozilla Firefox
Open-source Cmd+J to open Downloads, then retry/resume the interrupted item, or restart it. 
Linux3 apps
Mozilla Firefox
Open-source Ctrl+J > resume the interrupted download; same behavior as other platforms. 
wget / aria2
Open-source Re-run the download with resume: 'wget -c URL' or 'aria2c -c URL' continues into the existing .part. Technical details
deep spec| Format type | Not a format in itself - the file holds the raw bytes of the partially-downloaded target file, with no structure of its own |
| File naming convention | Original filename with `.part` appended as an extra extension, e.g. `movie.avi` becomes `movie.avi.part` |
| MIME type | `application/octet-stream` |
| Magic bytes / file signature | None intrinsic - the header mirrors the target file type (a partial ZIP begins with `PK\x03\x04`; a partial MP4 opens with an `ftyp` box) |
| File size range | 0 bytes (transfer never started) up to just under the total size of the target file |
| Used by | Mozilla Firefox, Free Download Manager, JDownloader, `wget`, `aria2`, KGet, and other download managers |
| Resume support | Server must support HTTP `Range` requests; if it does, the download tool can continue from the last received byte rather than restarting |
| Companion control file | `aria2` writes a `.aria2` sidecar tracking completed byte ranges; older Firefox versions created a JSON sidecar alongside the `.part` file |
| On download completion | The download tool automatically strips the `.part` suffix and renames the file to its correct target extension |
| Empty file meaning | A zero-byte `.part` file indicates the transfer was initiated but no data was received from the server |
| Related partial-download extensions | `.crdownload` (Google Chrome), `.!ut` (uTorrent), `.download` (Safari/macOS), `.opdownload` (Opera), `.partial` (various tools) |
| Developer / owner | No single owner - the convention is used independently by Mozilla, download managers, and CLI tools such as `wget` and `aria2` |
| Platform support | Windows, macOS, Linux - any platform where download managers or browsers operate |
| Content integrity | No checksum or integrity data within the `.part` file itself; validation happens only after the download completes and the tool renames the file |
| Released | Used by Mozilla Firefox/Mozilla Suite (early 2000s); also wget, aria2, KGet and other download managers |
PART conversions
Possible
.PART .MP4GUIDE
No conversion needed - if the download finished, remove the .part suffix; if the file is still incomplete,…
How to convert
.PART .ZIPGUIDE
You can get a working ZIP from a .part file only by resuming or re-downloading -- renaming helps only if the…
How to convert
.PART .PDFGUIDE
A PART file that was downloading a PDF becomes a working PDF by resuming or re-downloading in Firefox; there…
How to convert
Not possible
Community Q&A
asked by users
М
Frequently asked questions
How do I open or finish a .part file?
You don't open a .part - you finish the download that made it. In Firefox press Ctrl+J, find the interrupted item, and click resume; with wget/aria2 re-run using -c to continue. If it can't resume (history cleared or the server doesn't support it), download the file again.
Can I just rename a .part file to the real extension?
Only if the download actually completed - compare the file size with the expected size. Renaming changes nothing about the data, so an incomplete file stays corrupt no matter what you call it.
What program creates .part files?
Mozilla Firefox is the most common, plus download managers and tools like wget (--continue), aria2, KGet, and JDownloader. It's the Firefox-world equivalent of Chrome/Edge's .crdownload and Safari's .download.
Can I delete .part files?
Yes, if no download is currently running. Deleting one also gives up any chance of resuming that download, so resume first if you still want the file. Leftover .part files from old downloads are safe to remove.
Why is my download stuck as a .part file?
The transfer didn't finish or the final rename failed - common causes are an interrupted connection, the browser closing mid-download, antivirus scanning, or disk/permission issues. If the size matches the expected size, rename away the .part; otherwise resume or re-download.
Can I play a video that's still a .part file?
Often, partially. VLC can play the portion of an incomplete MP4/MKV that already downloaded. The rest won't appear until the download finishes - VLC can't invent the missing data.
References
1GNU Wget manual - resuming downloads (--continue)www.gnu.org
2Mozilla Support - Where to find and manage downloaded files in Firefoxsupport.mozilla.org
