What is the SB3 file format?
Projects created in Scratch 3 are saved as .sb3 files. This file format was developed at MIT (Massachusetts Institute of Technology) for a block-based programming learning platform. Scratch projects can take the form of a story, animation, or simple game, and the whole project is saved as a ZIP archive with a renamed extension.
The .sb3 format replaced .SB2 files used by Scratch 2.0, and the original .SB format introduced with the first Scratch release. The experimental ScratchX version used .sbx for its projects.
What data can .sb3 files hold?
Project files created with Scratch 3.0 and higher contain complete project data and can hold such objects as:
- scripts in JSON format (all blocks, sprites, and variables),
- graphics, icons, costumes and other sprite images (PNG and SVG),
- audio files (WAV and MP3), each named by its MD5 content hash inside the archive.
To access .sb3 data without Scratch, a user can unpack the archive using any tool that supports ZIP format. Sometimes it may be necessary to rename the .sb3 extension to .zip before the extraction tool recognises it.
Managing .sb3 files in Scratch
.sb3 is the default project file format used by Scratch 3.0, available both as an online application and as Scratch Desktop for Windows and macOS. When using the online version, select File → Download to your computer to save the project to disk. To open a .sb3 file from disk, choose File → Upload from your computer from the same menu.
Security & safety
RISK: LOWSB3 is a ZIP of JSON and media files with no executable code; it cannot run outside the Scratch environment. The Scratch VM sandboxes project execution. No known exploits via malicious SB3 files. Safe to share and inspect.
Format details
in a nutshellFULL NAMEScratch 3.0 Project Fileaka Scratch project, Scratch 3 file
DEVELOPERMIT Media Lab (Scratch Foundation)since January 2, 2019 (Scratch 3.0 public launch)
CATEGORYMisc Files
MIME TYPEapplication/x.scratch.sb3
TYPEZIP-based JSON project package
MAGIC BYTES · FILE SIGNATURE
OFFSET
00010203
HEX
504B0304
ASCII
PK··
Identical to any ZIP archive. Distinguish by presence of project.json inside.
Programs that open SB3 files
Windows3 apps
Scratch (web - scratch.mit.edu)
Free Go to scratch.mit.edu, click Create, then File → Load from your computer. 
macOS2 apps
Linux1 app
Scratch Desktop (AppImage/snap)
Free Download AppImage or install via snap, then File → Load from your computer. 
Android1 app
Scratch (web - mobile browser)
Free Open scratch.mit.edu in Chrome/Firefox, use Create → File → Load. Limited editing on mobile.
iOS1 app
Web1 app
Technical details
deep spec| Container format | ZIP archive (magic bytes 50 4B 03 04 at offset 0, same as any `.zip` file) |
| Root manifest | `project.json` - complete project data (all blocks, sprites, variables, stage) |
| Asset naming convention | Costumes and sounds stored as `{md5hash}.{ext}` (e.g., `.png`, `.svg`, `.wav`, `.mp3`) |
| Compression | DEFLATE per ZIP entry |
| Encoding | Binary ZIP container with UTF-8 JSON inside |
| Stage dimensions | Fixed at 480 × 360 px |
| MIME type | `application/x.scratch.sb3` |
| Integrity | ZIP CRC-32 per entry; asset filenames are MD5 hashes of file content |
| Typical file size | 10 KB - 50 MB (depends on sprite costumes and sound assets) |
| Maximum asset entries | ZIP-constrained (up to 65,535 files without ZIP64 extension) |
| Format version | Scratch 3.0 JSON schema (sb3); incompatible with Scratch 2.0 (.sb2) without conversion |
| Introduced | 2 January 2019 (Scratch 3.0 public launch) |
| Backward compatibility | Scratch 3.0 opens .sb2 files with auto-conversion; cannot open Scratch 1.x `.sb` files |
| OS compatibility | Windows, macOS, Linux, ChromeOS, Android, iOS (via web browser) |
| Released | January 2, 2019 (Scratch 3.0 public launch) |
| Latest version | Scratch 3.0 (ongoing; format stable since 2019) |
| Specification | en.scratch-wiki.info |
SB3 conversions
Possible
.SB3 .SB2GUIDE
Yes, but only with a third-party tool such as the sb3tob2 utility, since Scratch itself never exports back to…
How to convert
.SB3 .MP4GUIDE
Yes, you can get an MP4 from a Scratch 3.0 Project File, but only by playing the project and recording your…
How to convert
.SB3 .HTMLGUIDE
Yes, you can convert an SB3 to HTML with the free TurboWarp Packager, which wraps your Scratch project into a…
How to convert
Not possible
.SB3 .APKWHY NOT?
No. An SB3 file is a Scratch 3.0 project, not an app. Scratch 3.0 removed the experimental Android export…
.SB3 .EXEWHY NOT?
No. Scratch 3.0 has no EXE export, and an .sb3 file holds project data rather than a runnable program.
.SB3 .SWFWHY NOT?
No. There is no way to turn an .sb3 file into a .swf file. Adobe Flash reached end of life in December 2020,…
Community Q&A
asked by usersFrequently asked questions
How do I open an .sb3 file?
At scratch.mit.edu click Create → File → Load from your computer, or use the Scratch Desktop app (free download from scratch.mit.edu/download).
Can I open an .sb3 file without the internet?
Yes - download the free Scratch Desktop app, which works offline.
How do I convert an .sb3 to a video (MP4)?
Scratch has no direct video export. Play the project and record your screen with OBS Studio or Windows' Xbox Game Bar.
How do I extract images and sounds from an .sb3 file?
Rename .sb3 to .zip and extract with 7-Zip or WinRAR. Costumes and sounds are inside as PNG/SVG/WAV/MP3 files named by their MD5 hash.
Can I convert .sb3 to EXE or APK?
No direct conversion exists. Turbowarp Packager (packager.turbowarp.org) can export to HTML (which runs in browsers), but not to a native EXE or APK.
What is the difference between .sb, .sb2, and .sb3?
.sb is Scratch 1.x (very old). .sb2 is Scratch 2.0 (Flash-era, 2013-2018). .sb3 is Scratch 3.0 (2019-present, ZIP+JSON, no Flash). Scratch 3 opens .sb2 files automatically.
Can I edit project.json directly to modify a Scratch project?
Yes - rename to .zip, extract, edit project.json (following the sb3 schema), re-zip, and rename back to .sb3. Useful for programmatic project generation.
References
1Scratch File Format - Scratch Wikien.scratch-wiki.info
2scratch-vm - SB3 format (DeepWiki)deepwiki.com
