iMovie hard drive space

I was using my macbook air 2013 to create a youtube video on iMovie. When I started I had 55gb of space left. I recorded everything used handbrake to convert it and edited it in iMovie. I then exported it and put it on youtube. I then deleted absolutely every file from iMovie and the event. But my remaining space had gone from 55gb to 44gb. What is taking up this space?

Try looking at your cache. Navigate to ~/Library/Caches/ and look for a folder for iMovie (may also be labeled com.apple.iMovie). Check its size. If it's quite large, then you've found the culprit.

Note: deleting the contents of Caches is safe provided the application isn't running. While not necessary, deleting the contents of the entire folder should prompt a reboot.

You may also elect to give Grand Perspective a try, which will show you the contents of your drive and how files are arranged on it. This will help hunt down the loss of space you are experiencing.

iMovie now has a “Delete Render Files” button right inside its preferences window (opened via Cmd+,).

You should be able to just open your preferences and use that button to clear any unnecessary hard drive space.

Aside from that, after exporting your movie and backing up both the sources and final result somewhere:

• Remove the project from projects list
• Remove the library section with the sources from the iMovie Library

P.S. Here's another approach that explains the current location of cache files and how to delete them manually or with the command line: How to free up iMovie disk space