Compress Checked before download

Make an MP3 smaller

A voice recording at 192 kbps is three times the size it needs to be. Choose a lower bitrate and the file is re-encoded here, on this device — then re-opened and checked before you are offered anything.

On this device

Choose a bitrate

The bitrate your file already has is shown first, and anything at or above it is refused rather than offered.

    Opened on this device · never transmitted

    What this tool does, exactly

    What it does
    Decodes the audio to raw samples, then encodes it again at the bitrate you choose. Speech is intelligible and pleasant at 64 kbps and unnecessary above 96; music wants 128 at the least and 192 for anything you will listen to properly. The tool shows what the file already is so the choice is a comparison rather than a guess.
    Why lower only
    Because re-encoding at a higher bitrate is a pure loss. What a lossy encoder discarded is gone, and encoding 96 kbps audio at 256 kbps produces a file nearly three times the size containing exactly the same music, slightly degraded. Every request to raise a bitrate is refused, with the bitrate you already have, rather than obliged.
    The generation you cannot get back
    Every MP3 has already been through a lossy encoder once, and this is a second pass over what survived the first. It is the right trade for a file that has to fit in an email or a form, and the wrong one for the copy you are keeping. Keep the original; this is the copy to send.
    What it proves
    Before you are offered anything, the finished file is decoded again from its own bytes. It has to decode at all, at the sample rate and channel count it started with, and to within a frame of the original length. A compressor that hands back a file nothing can play has failed in the one way you would not notice until it mattered.
    Where it stops
    MP3 in, MP3 out. Other formats are decoded by your browser where it can — an M4A or a WAV usually opens — but what comes back is always an MP3, because it is the format everything plays. Anything your browser cannot decode is named rather than silently skipped.

    About this tool

    Which bitrate should I choose?

    For speech — a lecture, an interview, a voice memo — 64 kbps is genuinely enough and 96 is generous. For music, 128 kbps is the usual floor and 192 is where most people stop hearing the difference on ordinary headphones. The tool shows what your file already is, so the useful question is not “what is good” but “how far below what I have can I go for this purpose”.

    Why will it not re-encode at a higher bitrate?

    Because it cannot put back what is not there. A lossy encoder threw detail away permanently the first time, and a second encoder cannot recover it — it can only spend more bytes describing the reduced version. You would get a file two or three times larger that sounds very slightly worse. Rather than take the instruction, the tool refuses it and shows the bitrate you already have.

    Will it sound worse?

    Yes, a little, and how much depends on how far you drop. Going from 320 to 192 kbps is inaudible to almost everyone; going from 128 to 64 is obvious on music and barely noticeable on speech. This is a second lossy pass over material that already lost something, which is why the honest use is a copy to send rather than a copy to keep.

    What does the check before the download do?

    It decodes the finished file again, from its own bytes, and requires three things: that it decodes at all, that the sample rate and channel count still match what went in, and that the length is within a frame of the original. If any of the three fails there is no download and the result says which — the way an audio file usually breaks is by playing as silence, and finding that out from the recipient is the expensive way.

    Does my audio leave the device?

    No. The decoding, the encoding and the check all happen in this tab. The encoder is a WebAssembly module served from this site — about 150 kilobytes, downloaded only when you open this tool — rather than a service somewhere that receives your recording.

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