file format for space over quality

Hi. I have a big and growing music library. My iPhone can hold all of it as of now, but I am running out of space and I will soon have more music then space. Right now I have a mixture of .m4as and .mp3s. I have started converting all my music to .mp3s trying to get rid of all the .m4as, but I am wondering if there is a format that will make smaller files at the same bitrate and frequency as my .mp3s. I don't want my music to sound all tinny, but I don't need CD or .wav quality either. I tried an experiment of taking some .m4as and .mp3s and converted the .mp3s to .m4as without changing bitrate or frequency and converting the .m4as to .mp3s without changing the bitrate or frequency. I was trying to figure out what files would come out smaller, but sometimes the .mp3s came out smaller and sometimes the .m4as came out smaller so I was confused. I could not figure out which one would truly be smaller. I did not bother testing a third format, because I figured what happened with the first 2 formats I tested would happen again with the third format. That is, I would end up with mixed results. I figured there would be people who would know more about file formats then me, so that's why I am asking here.

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#1 Space saving will come at a price

Because all of your files are already in a lossy format, there’s nothing to be gained from converting them. In fact, quite the opposite. Converting between lossy formats (such as mp3 and m4a) will only result in a further degradation in audio quality with each conversion. If you were to convert to a lossless format, the audio quality would remain the same, but you would end up with larger file sizes. This is clearly the opposite to what you want.

So, if you really do need to save on disk space, then you will need to accept that the price will be a degradation in audio quality.

#2 I know

Yes. I understand that converting between lossy formats is not generally a good idea. I know it reduces the audio quality. Lots of people are sensitive to this, but I am not. You could for example play a .m4a file and that same file in .mp3 that has been converted from m4a to .mp3 and I would not know which one is which. So, what's the best format to save space?

#3 Try HEAAC to have the best

Try HEAAC to have the best of both worlds. I love HEAAC personally. You can use many programs including itunes to convert to HEAAC