MP3 quality: 128 vs 192 vs 320kbps and file size

The 128, 192 and 320kbps you see for MP3 are bitrates: how much data one second of audio uses. With a constant bitrate, you can work out the file size from the bitrate and the length. At the same length, a 192kbps file is 1.5 times the size of a 128kbps file, and a 320kbps file is 2.5 times. A higher bitrate leaves room for better sound, but it can’t make the audio better than the original. So start with the default 192kbps and listen before you decide.

kbps is data per second

kbps stands for kilobits per second. In this guide, 1kbps means 1,000 bits per second, and 8 bits make 1 byte.

MP3 (MPEG-1 Audio Layer III) is a lossy format. According to MDN, MPEG-1 MP3 uses one of a fixed set of bitrates from 32kbps to 320kbps. 128, 192 and 320kbps are all on that list, and 320kbps is the highest. The same guide notes that the higher the bitrate, the higher the quality can be, and the larger the file is likely to be.

A constant bitrate (CBR) uses the same bitrate from start to finish. A variable bitrate (VBR) aims for a set quality level and lets the bitrate change from part to part. With CBR, it’s easy to work out the size in advance.

Working out file size by length

For a CBR MP3, use this formula:

size in bytes = bitrate in kbps × 1,000 × length in seconds ÷ 8

Bitrate	1 second	1 minute	10 minutes
128kbps	16,000 bytes	960,000 bytes (about 938 KB)	9,600,000 bytes (about 9.2 MB)
192kbps	24,000 bytes	1,440,000 bytes (about 1.4 MB)	14,400,000 bytes (about 13.7 MB)
320kbps	40,000 bytes	2,400,000 bytes (about 2.3 MB)	24,000,000 bytes (about 22.9 MB)

These are calculated values, and real files can differ slightly. KB and MB here use steps of 1,024, the same as the result card on this site. Counting in steps of 1,000, one minute comes to 0.96 MB, 1.44 MB and 2.4 MB.

A real test lines up with the math. The audio from a 10-second test video (MOV, AAC stereo 48 kHz, 1.1 MB), saved as a 192kbps MP3, came to 235 KB (measured in desktop Chrome; results vary by device and browser). The formula gives 240,000 bytes, or about 234 KB.

For comparison, MDN explains that uncompressed 48 kHz, 16-bit stereo audio takes up 192,000 bytes (192 kB) per second. In bits, that’s 1,536kbps. So a 192kbps MP3 uses one-eighth of that, and a 320kbps MP3 about one-fifth. Watch the units: 192 kB is bytes and 192kbps is bits, so they aren’t the same amount.

Choosing between 128, 192 and 320kbps

Bitrate	Size per minute (calculated)	vs. 128kbps	Choose it when
128kbps	about 938 KB	1×	You want a smaller file
192kbps (default)	about 1.4 MB	1.5×	You want a starting point to listen to
320kbps	about 2.3 MB	2.5×	You want to put sound quality first

• For MP3, MDN lists 128kbps (at a 48 kHz sample rate) as the recommended minimum bitrate for stereo sound.
• The result can’t sound better than the original. If the original audio is low quality, a higher bitrate may only make the file larger.
• Whether you notice a difference depends on the material, your playback device and your ears. Save the same part at two bitrates and compare them yourself.

Keep the original audio without re-encoding

The audio inside a video may already be compressed with a codec such as AAC or Opus, and MDN classes both as lossy. Saving as MP3 decodes that audio and re-encodes it as MP3. With “Original audio (as is)”, the audio is taken out of the video without re-encoding, so it’s identical to the original. The format follows the original codec: AAC is saved as .m4a, MP3 as .mp3, and Opus or Vorbis as .ogg.

Input	Save as	Result
10-second MOV (AAC stereo 48 kHz), 1.1 MB	MP3 192kbps	MP3, 235 KB
Same MOV	Original audio (as is)	M4A (AAC, unchanged), 159 KB
10-second WebM (Opus), 829 KB	Original audio (as is)	OGG (Opus, unchanged), 146 KB

Measured in desktop Chrome; results vary by device and browser. If the device or app you’ll play the file on can’t open M4A or OGG, choose MP3. If you’d rather keep the picture and sound together as an MP4, use Convert to MP4. When the video is H.264 and the audio is AAC or MP3, it only changes the format without re-encoding. Other audio is converted to AAC at 160kbps.

How to do it with ZEKILO Studio

1. Open Video to MP3 and drag and drop one video or audio file, or pick it with [Choose files]. If a video has no sound, “This video has no audio” appears as soon as you add it.
2. Under Save as, choose “MP3” or “Original audio (as is)”. For MP3, pick 128, 192 or 320kbps under Audio quality (bitrate), then press [Start].
3. Listen to the result to check it, then save it with [Download].

For MP3, the sample rate and channel count follow the original, but anything above 48 kHz is brought down to 48 kHz, and 3 or more channels are mixed down to 2 (stereo). Tags such as the title, and metadata such as shooting location and device info, aren’t carried over. Your file is processed in this browser and never leaves your device.

Summary

• 128, 192 and 320kbps are amounts of data per second. With CBR, size = bitrate × length ÷ 8.
• Per minute, that works out to about 938 KB, 1.4 MB and 2.3 MB, and 320kbps is 2.5 times 128kbps.
• A higher bitrate leaves room for better sound, but it can’t beat the original.
• To avoid re-encoding, choose “Original audio (as is)”. If your device or app can’t open M4A or OGG, save as MP3.