Which players actually show these chapters?

Any player that reads ID3 CHAP frames, which is the convention Apple Podcasts established and most podcast apps follow. Chapters appear as a list with skip points. Players that only understand MP4 chapter tracks will not see frames written into an MP3, and the plain MP3 hardware players ignore them entirely.

Will writing chapters keep my existing title and artist tags?

No, and this is worth knowing before you start. The writer rebuilds the whole tag block from scratch, and this tool supplies only the chapter frames, so previously set title, artist and album text is dropped. Set those fields again with the tag editor after writing chapters if you need them.

What does the tool do with a file that already has chapters?

It parses the existing CHAP frames and loads them into the table as editable rows, numbering any untitled chapter as Chapter 1, Chapter 2 and so on. You can then retitle, re-time, add or remove entries and write the file back out, so re-editing an episode does not mean retyping every chapter.

How are chapter end times decided?

Automatically. Each chapter ends where the following one begins, and the final chapter ends at the measured duration of the decoded file. That is why the tool decodes the audio on load, and why an undecodable file leaves the last chapter's end at zero instead of guessing.

Can I use it on an M4A or M4B file?

No. This tool accepts MP3 only, and the file picker is restricted to MPEG audio. Audiobooks distributed as M4B use a different chapter mechanism inside an MP4 container, so an MP3-specific chapter writer cannot write into one without re-muxing the audio.

Podcast Chapter Marker Tool

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Podcast Chapter Marker Tool writes real, player-readable chapter markers into an MP3 by building ID3 CHAP frames from titles and start times you type. Players such as Apple Podcasts then offer skip points inside the episode.

What is Podcast Chapter Marker Tool?

Drop an MP3 onto the panel, add a start time and a title for each chapter, and download the same audio with chapter markers written into its tag block. The tool reads any chapters already present in the file and loads them into the editor, so you can adjust an episode rather than starting over. Existing ID3 CHAP frames are parsed on load, and the duration is measured by decoding the file so the final chapter can be closed off at the true end of the audio.

What the editor gives you:

A chapter title field and a start time in seconds, with the Add button appending the row and advancing the next start by 60 seconds as a convenience.
End times that are worked out automatically: each chapter runs until the next one begins, and the last runs to the measured file duration.
A sortable chapter table showing each start as an h:mm:ss timestamp with an editable title and a remove control.
Automatic loading of existing chapters, so an already-chaptered episode opens with its markers in place.
A single write step that produces a new file named after the original with a .chaptered.mp3 suffix.
ID3 CHAP frames are written under a fresh ID3v2.3 tag, matched against Apple's expectations.

On the technical side the file is held as raw bytes and the tag block is rebuilt rather than patched. The tool locates the existing ID3 header, strips it, and prepends a newly constructed tag containing one CHAP frame per chapter. Each CHAP frame carries an embedded TIT2 subframe for its title plus start and end times in milliseconds, and the tag declares ID3v2.3 with 512 bytes of padding appended so a later edit can rewrite in place without moving the audio. Because the whole tag is rebuilt, the writer is also what makes version upgrades automatic: a file carrying an older tag simply comes out with a clean v2.3 block.

Honest limits, all confirmed against the code:

It accepts MP3 only; there is no support for M4A, M4B or other containers here.
Writing chapters produces a fresh tag that contains the CHAP frames and nothing else, so pre-existing title, artist and album text frames are not carried across.
If the file cannot be decoded, the duration is unknown and the last chapter's end time falls back to zero, which some players read as an open-ended final chapter.
Chapter starts are sorted by time but are not validated against the duration, so a start past the end of the audio is written exactly as typed.
There is no waveform or scrubber; you type start times rather than clicking them on a timeline.
Chapters are ID3 CHAP frames, not MP4 chapter tracks, so players that only read MP4 chapters on another container will not see them.

The practical workflow is to add chapters from the intro outward, giving each segment a descriptive title and a start time rounded to the nearest second, then check the table's timestamps against your notes before writing. If the episode already had a title and artist tag you care about, set those again with the tag editor after writing the chapters, since the chapter write starts from a clean tag.

How to use Podcast Chapter Marker Tool

Drop an MP3 onto the panel or click to choose one; existing chapters are detected and listed automatically.
Type a chapter title and its start time in seconds, then press Add; the next start time pre-fills 60 seconds later.
Edit or remove rows in the chapter table until the start times and titles are right; end times update automatically.
Click Write chapters and download to save the file as name.chaptered.mp3 with the CHAP frames inside.

When to use Podcast Chapter Marker Tool vs related tools

Reach for Podcast Chapter Marker Tool when listeners should be able to skip to a segment inside the player, and use Show Notes Timestamp Generator when you only need readable times in a written description. The two are built from different inputs on purpose: one writes binary markers into the file, the other formats text. If the download needs its title, artist or album fixed as well, ID3 Tag Editor handles those fields, and Podcast Recorder is the step before either of them.

Privacy & Security

This tool runs entirely in your browser — no data ever leaves your device. There is no server round-trip, no upload, no logging, and no account required. Your input is processed locally using client-side JavaScript and is never stored, transmitted, or accessible to anyone else. When you close the tab, everything disappears.