Guides and test results

These pages explain what happens to a file in PrivateMP3 and what to expect from each output. Measured figures, the times, sizes and codec results labelled as tested, come from running the shipped tool on a known test file and reading the output back with ffprobe. Calculated figures, such as the bitrate arithmetic, are shown with their formula. When a test shows a page was wrong, the page is corrected and the change is listed below.

• MP3 bitrate, size and source quality

  Calculate MP3 size from bitrate and duration, understand why PrivateMP3 uses a fixed 192 kbps target, and separate output bitrate from source quality.

  Last updated 2026-09-24

• Choose an audio format for the next application

  Choose MP3, M4A or WAV from the requirements of the application that receives it, with current Audacity and packet-copy distinctions.

  Last updated 2026-09-24

• How converting without uploading works

  What actually happens when a browser converts a video to audio locally, why the file never leaves your device, and how to verify that claim yourself.

  Last updated 2026-08-14

• No waveform or disabled Convert button

  Follow the exact PrivateMP3 loading sequence when a local file will not open, has no audio track, shows no waveform, lacks a decoder or leaves Convert disabled.

  Last updated 2026-10-05

• Screen recording audio: measured times and sizes

  Six test recordings, from a 4.39 GB one-hour MP4 to a 5.1 MKV, run through PrivateMP3 in desktop Chrome. Open time, export time, output size and ffprobe checks for MP3, M4A and WAV.

  Last updated 2026-10-05

Where to start

• For an hour of lecture or meeting audio, start with the measured test results: they give the time and size of each output for a one-hour 4.39 GB recording.
• To predict a file size before converting, the bitrate guide has the arithmetic. The File check panel in the editor applies the same arithmetic to your file.
• If the waveform never appears or Convert stays grey, the troubleshooting guide maps each message to the stage where it stopped.

Change log

Changes that altered what the tool does or what a page claims. Wording-only edits are not listed.

• 2026-10-06. Tested Apple Lossless (ALAC) M4A files in Chrome 154 and Edge 154: neither can decode ALAC. The M4A page had said ALAC to MP3 worked; it now says it does not in those browsers and gives a documented way to make a convertible copy. File check now names ALAC and states that no output is possible instead of describing an MP3 route. Added measured results to the M4A, WebM, WAV, MP3 cutter and MP4 pages, and added this page.
• 2026-10-05. Added the File check panel: container, every audio track with name, codec, channels, sample rate and measured bitrate, the export path and the size of each output. Fixed the unchanged M4A copy, which failed on MP4 and MOV sources because AAC priming gives the first packet a negative timestamp. Fixed a failed copy leaving the page on Converting with no error. 5.1 audio exported as MP3 is now folded to stereo; before, it stopped with a wrong message about a missing decoder. Published the screen recording test results.
• 2026-08-28. The screen recording page now starts from the fact that a recording is a video file and explains what extraction does and does not do.
• 2026-08-21. Added the page for automated clients and in-page editor tools for browser assistants. Opening a file, Convert and Save file remain visitor actions.
• 2026-08-14. Added the troubleshooting guide and split error messages by processing stage. Rewrote six converter pages with format-specific guidance and corrected a claim about which Audacity builds open M4A.
• 2026-08-12. Volume changes now apply to a selected interval, and the waveform rescales to show them.
• 2026-07-29. Added undo and redo to the editor.
• 2026-07-27. Launched. The ringtone page was fixed the same day so the highlighted clip is the exported file.