Convert SB3 to SWF

A legacy Flash application, kiosk, or archive may require an .swf file while the available source is a Scratch 3 project. No direct conversion preserves the Scratch project’s scripts and behavior, so the practical choices are a modern package export or a manual rebuild in a Flash authoring tool.

What the SB3 format is

.sb3 is the Scratch 3 project format used by Scratch and compatible editors such as TurboWarp. It is a ZIP-based package containing project.json, block scripts, sprites, costumes, sounds, and metadata. Renaming .sb3 to .zip lets you inspect and extract its contents; it does not create a Flash movie.

Scratch projects run in the Scratch Virtual Machine, a JavaScript-based runtime. Scratch blocks, extensions, sprite behavior, stage rendering, and event handling have no one-to-one equivalent in ActionScript or the SWF file structure.

What the SWF format is

SWF is the binary movie format historically used by Adobe Flash Player and Adobe Animate. It can store vector graphics, bitmap images, timelines, animation, audio, video, fonts, and ActionScript. Its timeline and vector-graphics support made it suitable for interactive web animations and compact offline applications.

Adobe ended Flash Player support on December 31, 2020, and current browsers block normal SWF playback. An SWF is therefore relevant mainly to a controlled legacy system, an archival workflow, or a player such as Ruffle; Ruffle supports only part of Flash and is not a conversion tool.

How to convert the project

For a usable modern result, use TurboWarp instead of SWF:

1. Open the .sb3 file in Scratch or TurboWarp.
2. Load it in TurboWarp Packager, using the web packager or its desktop application.
3. Choose an HTML export or a standalone package, then test the result in a current browser or its supplied runtime.

TurboWarp generally preserves Scratch behavior better than a format conversion, but it does not produce an SWF.

If an SWF is mandatory, rebuild the project:

1. Work on a copy of the project. Rename it to .zip and extract compatible assets such as SVG and PNG costumes and WAV or MP3 sounds.
2. Create a new document in Adobe Animate with the required stage dimensions and frame rate.
3. Import assets with File → Import → Import to Stage or File → Import → Import to Library.
4. Recreate sprites, costumes, broadcasts, variables, motion, collisions, timing, input handling, and other behavior manually in Animate timelines and ActionScript 3. Scratch blocks cannot be translated reliably into ActionScript automatically.
5. Use File → Publish Settings to enable SWF output, then select File → Publish.

Adobe Animate is an authoring tool for producing a new SWF, not an SB3 converter. There is no reputable general-purpose online service that converts SB3 projects into faithful SWF movies; services claiming otherwise may only rename files, export media, or omit scripts. Uploading a project to an unknown service also exposes its code and assets.

Quality and compatibility limits

A manually rebuilt SWF can differ from the original in timing, collision behavior, coordinate handling, text rendering, sound playback, extensions, pen effects, and input behavior. TurboWarp-specific extensions and Scratch 3 features require separate implementations. A renamed .sb3 remains a ZIP archive and is never a valid SWF.