80 min ≠ 700 MB???!!! What gives?!

Hi there, guys, hope I’m not breaking any rules here, first-time user and whatnot. My question is not strictly about Audacity, but I hope you can at least give me a push in the right direction. Anyway, I’m trying to make an audio CD out of several MP3 files already in my hard drive. After a painstaking selection process, I’ve managed to narrow the compilation down to just under 80 minutes (79 minutes and 18 seconds, to be precise). So naturally I thought that converting those files to the proper uncompressed format would give me just under 700 MB worth of data, that I could fit it into a regular 700 MB/80 min CD-R.

I didn’t even use Audacity at first, I thought any CD-burning software would carry out the conversion without a hitch, so I initially decided to make a BIN/CUE image using UltraISO and then burn that same image using the same program (a bit convoluted, yeah, but I don’t have any other burning software installed at the moment and I didn’t feel like installing any for how often I have to burn discs). What I didn’t expect is that the resulting image’s size would not be 700 MB, but something around 750 MB, which UltraISO then refused to burn.

I was flabbergasted, but I assumed UltraISO just happened to have a shoddy conversion algorhythm, so I turned to Audacity instead. I went on to export all of the 18 MP3 files, one by one, into WAV 44100 Hz 16 bit yadda yadda… and the combined 18 WAV files’s size turned out to be… around 800 MB. I’m really stumped here: shouldn’t the correlation between duration and size be exact and invariable? Am I missing something? Am I doing something wrong? Some of the MP3 might have inconsistent bit rate, but that should be evened out in the conversion anyway, and the duration is definitely within the right boundaries. So what could I try next?

Please be aware that while I’d appreciate learning about a program that can do this one-off thing without further hassle, I’d appreciate it even more if I could understand the cause for the weirdness. And since I only use Audacity occasionally and I’m definitely no audiophile, please bear with me if I ask something obvious.

You may have run into a different kind of problem. The Audio CD process runs from time and, in general, takes no notice of the format, compression, etc etc.

But it’s not 80 minutes. We tell people about 78 minutes and the number can go down if you have a complex show or a great many sound files (up to 99).

Audio CDs don’t carry song titles. Did you run into that yet?


I don’t have any other burning software installed at the moment

Other than Windows Media?


Important: Windows 8 no longer supports native DVD playback. So, you won’t be able to play DVDs through WMP like you can in Windows 7 and earlier. You can still play, rip, and burn CDs though.


I don’t know the “exact” capacity of an 700MB/80 Min CD… And, there’s more than one way to count the [u]size of one MB[/u].

It may depend on the particular CD and/or burner, and some CD players might have trouble if you “push it” too far.

Although the underlying PCM audio data is the same (assuming 16-bit, 44.1kHz, stereo WAV files) I believe WAV files are larger than the audio on the CD because of the details of file-formatting on the disk (maybe related to error-checking/correction?). So if you write WAV files to a CD, they will take-up more space than an audio CD with the same music.

WAV files have a small header (44 bytes) which is not on an audio CD, and audio CDs have a Table Of Contents (and I don’t know how big that is).

Have you ever bought an external hard drive for backup or transpo?

!!!-200 GIGABYTES-!!!

(183GB actual usable size…)


I guess knowing the actual capacities of both 650 MB/74 min and 700 MB/80 min would help, if they are indeed not as advertised. What still has me clueless is the difference in size between the UltraISO-created image and the Audacity-exported WAVs. Shouldn’t the process and the results be pretty much the same for both programs?

Unfortunately Windows Media Player can’t help me here (already tried that yesterday). It seems to only be able to burn 650 MB/74 min discs. Idiotic, to be sure, but that’s how it rolls.

I guess knowing the actual capacities of both 650 MB/74 min and 700 MB/80 min would help, if they are indeed not as advertised.


Capacity and Actual Usable may be different and it could be normal. I’m betting if you had one 80 minute song it might fit on an 80 minute CD. Nobody has that. There’s always some difference or variation and variations take up room.

Anyway, I think you’re assigning surgical accuracy to a service that’s not surgically accurate. Traditionally, when you run out of a CD, you start another one.

No, huh?

You can make a Data DVD. It won’t play music in your Buick, but there’s 4.7GB on there.

(4.52GB actual usable space)


80 minutes of CD quality audio data:

80 (minutes)
60 (seconds per minute_
44100 (samples per second)
2 (bytes per sample)
2 (audio channels - stereo)

= 846720000 bytes of CD Quality audio data.

WAV files also have “headers” (that may include metadata), add a little to the file size, but usually not a significant amount.

Digital storage usually uses 1MB = 1,000,000 bytes (https://en.wikipedia.org/wiki/Megabyte), so that’s about 847 MB

Well, thanks, guys; can’t say I reached the epiphany I was looking for, but I managed to solve my problem. I caved in and used Nero, which burned my compilation (adjusted to slightly under 78 minutes, 78:20 including additional pauses between tracks) just fine. While the “total size on disc” shown under Nero’s main window was around 693 MB, the “info” tab under “settings” showed the compilation’s size to be 790 MB. Not sure if this means that Nero downsamples the files slightly so they’ll fit or if it’s using different definitions of “MB” across different screen, but the bottom line is that it got the job done, so I can’t argue with the results.