ChapterForge Guide
Turn a folder of MP3 chapter files into a single, fully-chaptered M4B audiobook with cover art and rich metadata. Available on macOS and Windows.
What is ChapterForge?
ChapterForge is a desktop application for macOS and Windows that takes individual MP3 files — one per chapter — and assembles them into a single M4B audiobook file with proper chapter markers, embedded metadata, and optional cover art. The resulting file plays natively in Apple Books, Audiobookshelf, and any MPEG-4 audio player that understands chapter navigation.
The app keeps the project-based approach: you create a project, import your MP3 files, fill in the metadata you care about (title, author, narrator, publisher, genre, ISBN, and more), arrange chapters in order, and then convert. FFmpeg handles all the heavy lifting and ships bundled inside the app, so there is nothing extra to install.
Organise each audiobook as a named project. Drag-and-drop MP3 files, reorder chapters, and save the project to come back later.
Edit title, author, narrator, publisher, genre, year, ISBN, language, and comment fields — per-project and per-chapter.
Queue multiple audiobook projects and convert them sequentially without babysitting the process. Pro feature on macOS.
System requirements
- macOS 13.0 (Ventura) or later
- MP3 input files; M4B output
- 150 MB free disk space, 4 GB RAM recommended
- FFmpeg bundled — no separate install
- Available on the Mac App Store
- Windows 10 (version 1809) or later, 64-bit
- MP3 input files; M4B output
- 150 MB free disk space
- FFmpeg and FFprobe bundled — no separate install
- Available on the Microsoft Store
🍎 How to use ChapterForge on macOS
The macOS version is a native SwiftUI app built to feel at home on your Mac, with a sidebar-and-detail layout that keeps all your projects a single click away.
Launch and create a new project
After launch, the sidebar lists all your existing audiobook projects. Click "New Project" at the bottom of the sidebar to create a fresh one. Give it a title — ChapterForge will use this as the M4B filename by default.
Add your MP3 files
With a project selected, drag MP3 files from Finder into the chapter list, or click the folder import button. Files are sorted alphabetically by filename on import, which works perfectly when your files are named numerically (e.g., 01-chapter.mp3, 02-chapter.mp3). You can drag rows to reorder manually at any time.
Fill in project metadata
The top of the detail pane has fields for title, author, narrator, genre, year, language, publisher, and description. This metadata is written into the M4B file and appears in Apple Books and other players. At minimum, fill in title and author — everything else is optional.
Edit chapter metadata (optional but recommended)
Click any chapter row to open the Chapter Metadata Editor in a dedicated window. Here you can set the chapter title, artist, album, narrator, publisher, genre, ISBN, comment, language, track number, and total tracks. Use the Apply To Bar to propagate fields from one chapter to all others, or to a selection, in a single click. The Undo and Redo buttons in the editor track every metadata change independently.
Add cover art
Open Output Settings (the gear icon in the toolbar or ⌘,) and drag a JPEG or PNG into the cover art well. The image is embedded in the M4B and displayed in Apple Books. Square images at 1400×1400 px or above look best.
Choose an output folder
Set a Master Output Folder from the sidebar using the Choose button. macOS asks for folder permission once, then stores a security-scoped bookmark so ChapterForge remembers that location across app restarts — no repeated permission prompts. Each converted project saves its M4B into this folder automatically.
Convert
Click the Convert button in the toolbar. A full-screen progress modal appears, showing the current encoding phase (preparing, converting, muxing, finalizing), an overall progress bar, the current chapter being processed, encoding speed, and an ETA. You can cancel at any point without corrupting the output — ChapterForge writes to a temporary file and only moves it to the output folder when conversion succeeds.
Find your M4B
When conversion finishes, a completion sheet shows the output file path and size. Click "Show in Finder" to jump straight to the file. The project stays saved in the sidebar so you can re-export with different settings any time.
🪟 How to use ChapterForge on Windows
The Windows version is built with Electron, React, and Tailwind CSS. It uses a four-panel layout — sidebar, projects view, queue, and settings — accessible from a compact icon sidebar on the left.
Launch and check the welcome screen
On first launch, the Welcome view walks you through the core workflow. After that, the Projects page is your home base. A status indicator confirms FFmpeg is bundled and ready — you never need to download it separately.
Create a new project
Click "New Project" in the projects sidebar to create an untitled audiobook project. Give it a title, author, and narrator. You can also import an entire folder in one step using the "Import Folder" option, which scans for MP3 files and creates a project with all chapters pre-populated.
Add MP3 files to the chapter list
With a project open, drag MP3 files from Explorer into the file drop zone, or click "Browse Files". Each file appears in the chapter list with its detected title and duration. Drag rows to reorder. Remove any file using the × button. Files auto-sort by numeric filename prefix on import, which keeps multi-disc audiobooks in order.
Edit project metadata
The metadata editor sits to the right of the chapter list. Fill in the book title, author, narrator, publisher, genre, year, language, and description. These fields write directly into the M4B. The chapter list also supports inline title editing — click any chapter title to rename it without opening a separate dialog.
Configure output settings
Open Settings from the sidebar icon. Set your default output directory (it pre-fills for every future project), choose audio quality (the app auto-selects the best AAC encoder available, including hardware-accelerated encoding when your system supports it), and pick a sample rate. You can also set a default cover image path that applies to all new projects.
Send to the queue and convert
Click "Convert" to send the project to the Queue. The Queue page shows every job with a phase indicator: queued, preparing, converting, muxing, finalizing, done, or failed. Each job has pause, resume, cancel, retry, and remove controls. A live log tail shows FFmpeg output line-by-line for diagnostics. The queue processes one job at a time and starts the next automatically.
Monitor progress
The Queue page shows a detailed progress bar for each active job, with the current phase label, percentage, encoding speed (e.g., 3.1×), and an ETA countdown. Completed jobs show their output file path and file size. Click a job row to open the Job Detail drawer with a full log and all job metadata.
Find your M4B
Completed jobs show the output path in the job card. Click the path to open the containing folder in Windows Explorer. If a job fails, the error message appears in the job card and in the log tail — most failures are either a missing source file or insufficient disk space.
Metadata fields reference
ChapterForge writes standard M4B/iTunes metadata tags. Here is what each field does and where it appears in compatible players.
| Field | Written to M4B as | Notes |
|---|---|---|
| Title | title | Book title. Used as the M4B filename if none is specified. |
| Author | artist / album artist | Displays as the primary creator in Apple Books and Audiobookshelf. |
| Narrator | composer | Convention for audiobooks. Appears in the 'Read by' field in Apple Books. |
| Genre | genre | Use 'Audiobook' or a specific sub-genre like 'Science Fiction'. |
| Year | year / date | Publication year. Four-digit integer. |
| Publisher | publisher | Label or publisher name. |
| Language | language | ISO 639-1 language code recommended (e.g., 'en', 'fr', 'de'). |
| Description | description / comment | Longer text shown in the player's info panel. |
| ISBN | custom tag (©isbn) | Stored as a custom iTunes atom. Supported in Audiobookshelf. |
| Cover Art | covr | JPEG or PNG. 1400×1400 px minimum recommended for Apple compliance. |
Audio encoding
ChapterForge uses AAC audio for the M4B container. AAC is the standard codec for M4B files and is supported by every player that understands the format. The app picks the best encoder available on your system automatically.
Microsoft Media Foundation AAC encoder. Efficient, high quality, ships with Windows 10+.
FFmpeg's native AAC encoder. Reliable cross-platform fallback with good quality.
Highest quality AAC encoder, but not always bundled. Used automatically when present.
Metadata editing tools (macOS)
The macOS version ships a particularly deep set of metadata editing tools. These are in addition to the standard per-chapter editor.
Available in the All Chapters Editor, this tool works across any metadata field. Plain text mode handles the simple cases. Enable wildcards (* and ?) for pattern-based replacements, with an option for shortest-match (non-greedy) behavior. Enable regex for full power — capture groups like (\d+) work with $1 replacement tokens. All modes respect a Match Case toggle.
The All Chapters Editor includes a set of field checkboxes that control what gets propagated when you use Copy Down. Tick the fields you want (Album, Artist, Genre, Year, Language, Publisher, Narrator, Composer, ISBN, Total Tracks) and the values copy from the first row to every row below it in a single action.
Add a text prefix and/or suffix to chapter titles for a selection of rows. Useful when your import named chapters as bare numbers ('1', '2', '3') and you want to rename them to 'Chapter 1', 'Chapter 2', 'Chapter 3' without editing each one individually.
Replaces all chapter titles with 'Chapter 1', 'Chapter 2', and so on. Destructive by design — it overwrites whatever titles are already there — so it is specifically positioned as a bulk reset tool. Fill Track # does the same for track number fields.
The Apply To Bar in the individual chapter editor lets you push the current chapter's field values to a scope of your choice (current chapter only, selected chapters, or all chapters). Propagate Book Fields does the reverse: it copies from the project level down into chapters, mapping project title to album, project author to artist, and project narrator to narrator.
Troubleshooting
Conversion fails with no output file
This usually means one of the source MP3 files could not be read, the output folder is not writable, or there is not enough free disk space. Check each of the following in order:
- Verify all source files still exist at their original paths — ChapterForge does not copy files on import, it references them in place.
- Check that the output folder exists and that you have write permission.
- Ensure you have at least 1.2× the combined size of your input files free on the output drive. The app checks this before converting but the estimate is conservative.
- On Windows, check logs at
%APPDATA%\ChapterForge\logs\for a detailed error message from FFmpeg.
macOS keeps asking for folder permission
ChapterForge uses security-scoped bookmarks to remember folder permissions across restarts without re-asking. If you are still seeing the prompt, ensure you are running the current App Store version. Grant permission once for both the input folder and the Master Output Folder and ChapterForge will not ask again for those locations.
Chapter titles appear in the wrong order
ChapterForge sorts MP3 files alphabetically by filename on import. If your files use zero-padded numbers (01, 02, 03) this works perfectly. If they use non-padded numbers (1, 2, 10, 11), alphabetical sort will put 10 before 2. Either rename the source files to use zero-padding, or drag the chapter rows into the correct order manually after import.
The M4B file plays but shows no chapters in Apple Books
This means the chapter metadata was not written into the file correctly. Make sure you have at least one named chapter in the project before converting. If chapter titles are blank, Apple Books may still load the file but will not display a chapter list. Add a title to each chapter (even a simple numbered one using Auto-number) and re-export.
Conversion is very slow
Encoding speed depends on your CPU, the number of input files, and total audio duration. Most systems process at 2× to 5× realtime — a 10-hour audiobook typically converts in 2 to 5 minutes. On Windows, hardware-accelerated encoding engages automatically when your GPU supports it. On both platforms, closing other CPU-intensive apps during conversion will help.
Job fails on Windows with “encoder not found”
ChapterForge probes for aac_mf, aac, and libfdk_aac at startup and selects whichever is available. If all three are missing from the bundled FFmpeg, the job will fail. This should not happen with the Store version, which ships a fully-featured FFmpeg build. If you see this error, reinstalling from the Microsoft Store will restore the correct binaries.
Frequently asked questions
There is a free tier on macOS that converts a limited number of chapters per project, so you can verify the output before committing. Pro unlocks unlimited conversion via a monthly subscription ($2.99/month with a 3-day free trial) or a one-time lifetime purchase ($34.99). The Windows version uses a one-time purchase model (pricing in the Microsoft Store). Purchases are managed through the respective app stores.
Yes. Converting MP3 files into an M4B container requires transcoding from MP3 to AAC, because MP3 audio cannot be stored in an MPEG-4 container without re-encoding. ChapterForge uses the highest quality AAC encoder available on your system to minimise any audible quality loss. For spoken-word content at standard audiobook bitrates, the difference is inaudible.
Yes. On macOS, Pro users can add multiple projects to the Batch Queue and convert them sequentially. On Windows, the Queue accepts multiple jobs from any number of projects and processes them one at a time. You can add new jobs to the queue while others are already running.
ChapterForge references files by their original path. If you move or rename a source file, the project will show a missing-file warning and conversion will fail for that chapter. Resolve this by removing the chapter from the project and re-adding the file from its new location.
The easiest approach is to update the project metadata in ChapterForge and re-export. For minor edits to an already-exported file, tools like mp4tag (Windows) or Meta (macOS) can edit M4B tags without re-encoding the audio.
The macOS version is a native Swift/SwiftUI app with a more mature feature set, including fields for ISBN, language, composer, and all the bulk-editing tools. The Windows version covers all the core fields and is updated continuously — the gap is narrowing with each release.
Currently, ChapterForge is optimised for MP3 input files on both platforms. M4A and AAC support is on the roadmap. If your source files are in M4A or AAC, convert them to MP3 first using a tool like Audiobook Converter Pro, then import into ChapterForge.
Ready to build your audiobook?
Download ChapterForge for your platform. The free tier on macOS lets you test the full workflow before committing.