How big is a merged audio file, and should you save it as MP3 or WAV?
Short answer: the size of a merged file depends on two things only: the total length and the export setting. The files you put in do not matter. In our tests, every minute of the merged track took:
- MP3 128 kbps: 0.96 MB per minute. Three 1-minute clips merged into 2,880,574 bytes; a 1-hour merge came to 57.6 MB.
- MP3 320 kbps: 2.4 MB per minute. The same 3 minutes: 7,201,436 bytes.
- WAV: 10.58 MB per minute for stereo, half that if every clip is mono. The same 3 minutes: 31,752,044 bytes, 11 times the 128 kbps MP3.
Save as MP3 to share or store the result: 128 kbps for speech or the smallest file, 256 or 320 kbps for music. Save as WAV only if you will edit the merged track again. In the audio merger, pick the format and bitrate under Export format; the size shown under it was within 0.2% of our exports.
Why the files you merge do not set the size
An online audio merger in a browser does not glue the original files together. It decodes every clip to plain samples, joins the samples, and writes one new file. The new file's size comes from how that file is written:
- MP3 from this site is constant bitrate: every second gets the same number of bits. 128 kbps is 128,000 bits, or 16,000 bytes, per second, whatever the music sounds like.
- WAV stores every sample as it is: 44,100 samples per second, 2 bytes each, twice over for stereo, plus a 44-byte header.
So the length of the merged track and the setting you pick decide the size. We checked whether anything else moves it.
How we tested
- Tool: this site's audio joiner, run in Chromium through Playwright on a local copy of the page. Each job loaded the clips, set the export format, downloaded the result, and recorded the size the page showed.
- Music: a 30 s excerpt of Mozart's Symphony No. 40, first movement, played by the Musopen Symphony, in the public domain on Wikimedia Commons, converted to 44.1 kHz, 16-bit stereo. We looped it to make three different 60 s clips and four 5-minute clips, and made mono, silent and MP3 copies (MP3 at 128 and 320 kbps with the same encoder the site uses).
- Long merges: to get a 1-hour figure without holding an hour of audio in the test browser, we ran the site's own encoder (lamejs, same settings and same block size) in Node.js. On the 3-minute merge it wrote exactly the same number of bytes as the browser at all four bitrates.
- Checks: exact file size in bytes; mono or stereo and bitrate read with FFprobe (FFmpeg 4.4.2). In this guide 1 MB is 1,000,000 bytes, except where we quote the page's own estimate.
Merged file size by length and format
| Merged length | MP3 128 kbps | MP3 192 kbps | MP3 256 kbps | MP3 320 kbps | WAV stereo |
|---|---|---|---|---|---|
| 3 min (3 clips) | 2.88 MB | 4.32 MB | 5.76 MB | 7.20 MB | 31.75 MB |
| 20 min (4 clips) | 19.20 MB | not tested | not tested | 48.00 MB | 211.68 MB |
| 60 min (encoder in Node.js) | 57.60 MB | not tested | not tested | 144.00 MB | 635.04 MB |
Each MP3 came out a few hundred bytes over its bitrate times its length: 574 bytes over at 3 minutes and 626 bytes over at 20 minutes and at 1 hour for 128 kbps. That is the encoder's start-up delay and the padding of the last frame, not something that grows with length. The WAV files were exactly 44 bytes plus 4 bytes per stereo sample.
What did not change the size
- The music. Three minutes of digital silence merged to 2,880,574 bytes at 128 kbps, the same as three minutes of Mozart.
- The bitrate of the MP3s you put in. Three 128 kbps MP3s and three 320 kbps MP3s of the same music both merged to 2,881,828 bytes at 128 kbps. The 320 kbps inputs added up to 7.20 MB, so the merge was 60% smaller than its parts. Going the other way, three 128 kbps MP3s totalling 2.88 MB merged to 7.20 MB at 320 kbps; the file got bigger, but the sound cannot be better than the 128 kbps sources (see does cutting an MP3 lose quality).
- Mono clips in an MP3. Three mono clips merged to a mono MP3 of 2,880,574 bytes at 128 kbps, the same bytes as stereo. A constant bitrate spends the same bits per second on mono as on stereo.
What did change it
- WAV instead of MP3. Three 128 kbps MP3s (2.88 MB together) merged to a 31.77 MB WAV, 11 times their size. WAV cannot bring back what the MP3s lost; it only stops the merge from adding another round of MP3 compression.
- Mono or stereo, for WAV. Three mono clips merged to a 15.88 MB mono WAV, exactly half the stereo one. But one stereo clip makes the whole merge stereo: one mono and two stereo WAVs (26.46 MB together) merged to a 31.75 MB stereo WAV, because the mono clip is written to both left and right.
- Crossfade. Each crossfade overlaps two clips, so the track gets shorter. Three 1-minute clips with a 2 s crossfade gave a 2:56 track, 4 s shorter, and 2,816,626 bytes at 128 kbps.
- MP3 sources add a little length. Each 60 s MP3 clip decoded to 1,296 samples (29 ms) more than the 60 s WAV it was made from, so the three-MP3 merge was 3:00.088 long. The extra is the encoder delay and end padding stored inside each MP3; why there are gaps between joined MP3 files explains it and how to remove it.
The size estimate on the page, and how long the export takes
The joiner shows the size under the format menu before you merge. For WAV it says the size is exact, and it was: 30.28 MB shown for the 31,752,044-byte file. For MP3 it shows an estimate: ≈ 2.75 MB for the 2,880,574-byte file, and ≈ 18.31 MB for the 20-minute file of 19,200,626 bytes, 0.01% under. The page counts 1 MB as 1,048,576 bytes, so it shows a smaller number than a count in millions of bytes; the same file is 2.75 MB one way and 2.88 MB the other.
| Merge | WAV | MP3 128 kbps | MP3 320 kbps |
|---|---|---|---|
| 3 min | 0.8 s | 22.6 s | 18.5 s |
| 20 min | 5.2 s | 149.7 s | 118.4 s |
WAV export was more than 20 times faster than MP3 on our server, because writing samples is quick and MP3 encoding is the slow part. If a long MP3 merge is taking a while, that is the encoder at work.
MP3 or WAV: which to pick
- To share, upload or keep: MP3. Use 128 kbps for talk, lectures or podcasts, and 256 or 320 kbps for music.
- To edit the merged track again: WAV, then save the final version as MP3. Every MP3 export is one more round of compression.
- When every source is mono (voice memos, for example): WAV comes out at half the stereo size, but an MP3 at the same bitrate does not shrink, so pick a lower bitrate if size matters.
- Before a long merge: read the size under Export format. An hour of stereo WAV is about 635 MB, which may not suit your phone's free space or an email attachment.
Questions
Is the merged MP3 the same size as my files added up?
Only if you export at the same bitrate as the files you added, because the size follows the length and the export bitrate. Our three 128 kbps MP3s added up to 2,881,410 bytes and merged at 128 kbps to 2,881,828 bytes.
Can I make the merged file smaller without losing more quality?
Not with a lower bitrate: that is more compression. Trimming silence or parts you do not need with the audio cutter before merging makes the file smaller in proportion to the time removed.
Is there a size limit?
The page sets none. It holds all decoded clips in memory while it works, so very long merges depend on your device. We merged 20 minutes in the browser; we did not test longer merges there.