MP3 Compressor

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MP3 too big to email or upload? Drop it here and pick how small. It never leaves your computer. An MP3 is already a compressed file, so making it smaller means re-encoding it at a lower bitrate — and that is where most compressors get it wrong. They offer you a bitrate dropdown that goes up to 320 kbps and will happily re-encode your 128 kbps podcast at 320, handing back a file that is five times bigger and sounds slightly worse than the one you started with. This tool measures what your file's bitrate actually is — from its size and its length, not from the header, which lies about variable-bitrate files — and only ever picks a bitrate below it. Drop the file, choose Light, Balanced, Strong, or a size to fit under, and the result appears with the before and after side by side. If your MP3 is already as small as MP3 goes, it says so and offers nothing, because a bigger file is not a compression.

Built by Bob QA by Ben Shipped September 21, 2026

How to use

1
Drop your MP3 onto the box, or click to choose one. The file is read on your device — nothing is uploaded, and there is no queue to wait in. The tool checks the header rather than the file extension, so a WAV somebody renamed to .mp3 is caught right away.
2
Pick how small. Light trims about a quarter off, Balanced roughly halves it and is the one most people want, and Strong squeezes it down to around a third for speech. Compression starts the moment you drop the file and re-runs whenever you change the level — there is no Compress button to hunt for.
3
Or pick Fit a size when you have a hard limit — a 25 MB email attachment, a 10 MB upload cap, an 8 MB chat limit. Choose the size and the tool works out the highest quality that still fits underneath it.
4
Read the two result lines: how the size changed, and what the bitrate, channels, and sample rate came out as. Play the preview to hear it. Then click Download MP3 — podcast.mp3 comes back as podcast-compressed.mp3.

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Frequently asked questions

Is my audio uploaded anywhere?

No. The compression runs inside your browser tab. The page reads the MP3 from your disk in pieces, decodes it, and encodes the smaller file on your own device. That matters for unreleased tracks, client recordings, and interviews you promised to keep private — and it means there is no upload wait and no daily cap on how many files you can do. Open your browser's network panel while it runs and watch: none of the audio is sent anywhere.

How much smaller will my file get?

It depends on what bitrate your MP3 is already at, which is why the tool measures it first. Balanced halves the bitrate, so a 5 MB file comes back around 2.4 MB. Strong takes it to about a third. A 233 kbps file on Balanced becomes 112 kbps; the same setting on a file that is already 140 kbps gives you 64 kbps. The exact before-and-after is shown on screen before you download, and it matches the file you get.

Will compressing hurt the sound quality?

Yes, some — and anyone who tells you otherwise is selling something. MP3 is a lossy format, so re-encoding one throws away a second round of detail on top of what the first encode already discarded. This is called generation loss. At Light and Balanced most people will not notice on phone speakers, laptop speakers, or earbuds. Strong is audibly thinner on music but perfectly clear for speech. There is a preview player on the result so you can hear the damage before you commit to the download.

Why won't it let me pick 320 kbps?

Because that would not be compression. Most online MP3 compressors give you a bitrate menu that goes all the way up, and if your file is 128 kbps and you pick 320, you get a file two and a half times bigger that also sounds slightly worse — you paid for a re-encode and got nothing. This tool only ever picks a bitrate below your file's real measured average, so the output is always smaller than the input. If no valid MP3 bitrate is below yours, it tells you to keep the file you have.

What does 'measured bitrate' mean, and why not just read the header?

An MP3's frame header states a bitrate, but on a variable-bitrate file that number only describes one frame — often the loudest one. A VBR file whose header says 320 kbps can easily average 140 kbps across its full length. Dividing the file size by the duration gives the true average, which is the only honest basis for deciding what to compress to. Trusting the header on a VBR file is how other tools end up re-encoding upward without noticing.

How do I get a file under a specific size, like 25 MB for email?

Choose Fit a size and pick 5, 10, 16, or 25 MB. The tool divides your size budget by the length of the audio, works out the bitrate that fits, and snaps down to the nearest real MP3 rate so the result lands under the limit rather than just over it. If even the smallest bitrate MP3 supports still will not fit — a two-hour recording will not squeeze into 5 MB — it gives you the smallest version it managed and tells you how far over you still are, so you know to trim the recording instead.

Why did my stereo file come back as mono?

Below about 64 kbps, splitting the bits across two channels leaves too few for either one, and the result sounds worse than the same bitrate spent on a single channel. So when the target lands at 64 kbps or under and the source is stereo, the audio is mixed down to mono. It is never done quietly — the result line says so. For speech, podcasts, lectures, and voice memos this is almost always the right trade; for music, pick Light or Balanced instead and it stays stereo.

What are the limits, and what happens to my tags and cover art?

One MP3 at a time, up to 1 GB and 3 hours. The sample rate carries through unchanged. Tags and cover art are not carried over yet — the file you get back is the audio, so if artwork and track names matter, re-add them in your music app afterwards. A one-hour show takes well under a minute on a mid-range laptop, and you can cancel mid-run at any point.

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MP3 Compressor

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