Does MP4 conversion use up space?

earlier versions of mp4-tools (for example) didn't delete the demuxed files created while converting, you had to delete them manually (and they ate up just as much space as the original/converted file). look into the preferences of your conversion-tool, maybe there's some folder the temporary files are saved to - or do some digging in your file system for size changes after a conversion.

 

Does this space free up after you reboot?

 

How much storage space are you talking about?

To find out, where you storage capacity is being used, you can use the following free applications:

• DiskInventory X
• OmniDiskSweeper
• JDisk Report
• GrandPerspective

Maybe try a combination of several, if you still can't find the "missing" capacity.

 

Meaning the source files where 3 GB each in size, and the transcoded files where each 3 GB in size, but strangely 9 GB instead of 6 additional GB were taken?

 

Which application did you use and did you try any of the ones I recommended?

Strange.

 

DaisyDisk is my preference for visual organization of drive usage. What conversion app are you using? Does it keep a cache?

 

It may be worth trying Handbrake to see if it is an application-specific issue. I certainly can't say I have ever had such a problem with Handbrake.

 

Do you have that few storage space, that the 3 GB are detriment to your computer experience? And where do you see those numbers?

 

Which app? Maybe it was not updating correctly.

 

Sounds like TRIM on your SSD is freeing up the "free space." Deleting files on SSD's does not actually free up the space, so TRIM works to free it up over time when your computer is idling.