Convert SB3 to MP4

A Scratch project must be uploaded to a video platform, submitted as a recording, or edited in video software when its runnable format is not accepted. Because an SB3 file contains code and assets rather than a video timeline, conversion requires running the project and recording its output.

What the SB3 format is

.sb3 is the project format used by Scratch 3.0, the block-based programming environment from the Scratch Foundation. Scratch Desktop and the Scratch website create SB3 files when users save or download projects.

An SB3 file is a ZIP-based package containing project JSON, sprites, costumes, backdrops, sounds, and metadata. Renaming it or extracting its contents does not create a video; the project must be interpreted by Scratch and rendered on its stage.

What the MP4 format is

MP4 is an ISO media container commonly used with H.264 video and AAC audio. This combination works with most phones, browsers, video editors, presentation software, and video platforms.

MP4 is appropriate for a fixed recording that can be watched without Scratch or imported into editing software. It does not preserve scripts, interactivity, clickable controls, editable sprites, keyboard input, or cloud-variable behavior.

How to record an SB3 project with OBS

OBS Studio is the most reliable desktop option because there is no normal file-only conversion: an SB3 package has no predefined video timeline.

1. Install Scratch Desktop from the official Scratch website, or open the Scratch editor at scratch.mit.edu.
2. Load the file with File → Load from your computer and select the .sb3 file.
3. Run the project several times. Check sounds, prompts, keyboard and mouse interactions, microphone requirements, online extensions, and cloud variables before recording.
4. Install OBS Studio from obsproject.com. Create a scene, choose Sources → + → Window Capture, and select the Scratch window. Use Display Capture if Window Capture does not capture the stage correctly.
5. Set the OBS canvas and output resolution to the stage's aspect ratio. Scratch's standard stage is 480 × 360 pixels with a 4:3 aspect ratio. Recording at 960 × 720 preserves that ratio, but enlarging the window does not add detail to bitmap artwork.
6. Verify audio under Settings → Audio. On Windows, confirm that Desktop Audio or an application-audio source receives Scratch's sound. On macOS, grant OBS screen-recording and audio permissions and configure a supported system-audio or application-audio source.
7. Under Settings → Output → Recording, choose a recording location and a stable frame rate such as 30 or 60 fps. Record to .mkv when reliability matters; an interrupted MP4 recording may be unusable, while an MKV recording can usually be recovered.
8. Start the project, begin OBS recording, let the project finish, and stop recording. Trim idle frames and unwanted desktop content in DaVinci Resolve, Kdenlive, Shotcut, or another video editor.
9. Convert the finished MKV recording without re-encoding by selecting File → Remux Recordings in OBS. This produces an MP4 with no additional quality loss.

FFmpeg can perform the same lossless container change from a terminal:

ffmpeg -i recording.mkv -c copy recording.mp4

The command works when the MKV already contains MP4-compatible streams, such as H.264 video and AAC audio. If it fails because of an incompatible codec, re-encode instead:

ffmpeg -i recording.mkv -c:v libx264 -c:a aac -movflags +faststart recording.mp4

Online options

Online converters generally cannot convert an SB3 file directly because they do not reliably execute Scratch code or reproduce its runtime. For a browser-based workflow, open the project in Scratch, use a reputable online screen recorder such as Clipchamp or VEED, then export the captured video as MP4. Browser recording requires screen, tab, and audio permissions and may be less reliable for projects with microphone input or long runtimes.

Services such as CloudConvert and Zamzar are useful for converting an existing video recording, for example MKV or WebM, to MP4; they are not dependable SB3 renderers. Uploading a project or recording exposes it to the service, so use desktop tools for unpublished work or private assets.

Quality and compatibility limits

The result reflects the capture resolution, frame rate, codec, and audio settings rather than a hidden video quality stored in the SB3 package. Preserve the 4:3 stage ratio, avoid stretching it to 16:9, and use integer scaling when possible. Re-encoding reduces quality; remuxing with -c copy does not.

Projects that depend on keyboard input, mouse movement, microphone input, network extensions, or cloud variables may require manual actions during capture and may behave differently outside the Scratch environment. MP4 records only the visible and audible result, so keep the original .sb3 file when the code or interactivity must remain available.