Guides · updated October 22, 2026

MP4 to MP3 at 320 kbps

320 kbps is the maximum MP3 bitrate. What it does to an MP4's AAC track, when it is wasted, and how to set it without guessing.

Tested: 22 October 2026 · Chrome 156, Firefox 157, WebKit 27.2 · MP4/AAC 128 kbps fixture

Short answer

320 kbps is the highest MP3 bitrate, but it cannot add detail the MP4's audio track never had. If the source AAC is 128 kbps, a 320 kbps MP3 is larger without sounding better. Use 320 for music from a high-bitrate source; use 128–192 kbps for speech.

What 320 kbps actually means

MP3 is defined by MPEG-1 Audio Layer III. For 32, 44.1 and 48 kHz audio, the standard allows bitrates from 32 to 320 kbps. 320 is the ceiling; there is no “MP3 512” or “MP3 lossless” mode.

A constant 320 kbps stream carries about 2.4 MB per minute:

320,000 bits/s × 60 s ÷ 8 = 2,400,000 bytes ≈ 2.4 MB

At 192 kbps the same minute is about 1.44 MB. The 320 file is roughly two-thirds larger. That extra space is useful only if the encoder has detail to spend it on.

The part most converters skip: the source is already lossy

An MP4 usually carries AAC audio, not PCM. AAC is also lossy. When you convert to MP3, the pipeline is:

Decode the AAC packets into PCM samples.
Encode those samples with LAME at the bitrate you chose.

The second step cannot recover anything the AAC encoder removed. If the source is a 128 kbps AAC track, a 320 kbps MP3 is a larger file built from the same reduced signal. It may sound slightly different because the two encoders make different choices, but it will not sound more detailed.

This is why the useful rule is: choose an MP3 bitrate at or above the source bitrate, then stop. Going far above it mostly buys file size.

When 320 kbps is worth it

Music with a high-bitrate source. A 256–320 kbps AAC track, or a lossless master, gives the MP3 encoder real detail to preserve.
Material with cymbals, applause, strings or dense mixes. These are the passages where low-bitrate MP3 artefacts are easiest to hear.
A delivery copy you will keep. If the MP3 is the only copy on a device, a higher rate is cheap insurance.

When it is wasted

Speech, interviews and lectures. 96–128 kbps mono is usually enough; 320 kbps stereo spends bits on silence and room noise.
A 128 kbps source. You cannot add detail that is not there.
A file you will delete after one listen. The size difference is real, especially on a phone.

How to set it here

Drop the MP4 into the converter on the home page.
Open the settings row. The panel shows the source codec, channel count, sample rate and, when the container exposes it, the average bitrate.
Choose 320 kbps from the bitrate menu.
Leave the sample rate on Auto unless the destination needs 44.1 or 48 kHz specifically. A 96 kHz source is resampled to 48 kHz because MP3 cannot carry 96 kHz.
Convert. The result panel shows the actual output size and the measured average bitrate.

The panel does not offer VBR. That is deliberate: the encoder is CBR, and a VBR menu would be a promise the tool cannot keep.

Verify the result

Use ffprobe on the downloaded MP3:

ffprobe -v error -select_streams a:0 \
  -show_entries stream=codec_name,bit_rate,sample_rate,channels \
  -of default=nw=1 output.mp3

You should see codec_name=mp3, the bitrate you chose (within frame overhead), and the sample rate the converter reported. If the source was 96 kHz, the output will be 48 kHz; that is the format limit, not a failure.

The honest summary

320 kbps is the best MP3 can do. It is not lossless, and it cannot improve a low-bitrate source. Use it when the source deserves it; use 128–192 kbps when it does not.

FAQ

Is 320 kbps MP3 lossless?

No. MP3 is a lossy format at every bitrate. 320 kbps is the maximum MPEG-1 Layer III rate, not a lossless mode. Converting an AAC track to MP3 always adds a second lossy generation.

Will 320 kbps make my MP4 sound better?

No. A higher bitrate gives the encoder more room, but it cannot reconstruct detail the source encoder discarded. If the MP4 audio is 128 kbps AAC, a 320 kbps MP3 cannot sound better than the source.

Why is the 320 kbps MP3 so much bigger?

At 320 kbps the file is about 2.4 MB per minute. At 192 kbps it is about 1.44 MB per minute. The difference is roughly 67% more data for a second generation that may not be audible.

Does the converter offer VBR?

No. The encoder exposes constant bitrate only. CBR makes the output size predictable, and the page does not offer a VBR control it cannot honour.

How do I check the source bitrate?

With ffprobe: ffprobe -v error -select_streams a:0 -show_entries stream=codec_name,bit_rate,sample_rate,channels -of default=nw=1 input.mp4. If the result is 128 kbps AAC, a 192 kbps MP3 is already above the source.