iMovie, the home-built powerful video editor for all the macOS, iOS, and iPadOS devices, comes with many great editing features, including adding text, adding music, cropping a video, splitting a video, etc.
But if you want to change the video aspect ratio to have a pleasant viewing experience or make it best suits video sharing platforms, iMovie is far from the perfect tool. Because you can only crop video into 16:9 in iMovie. If you want to make your video aspect ratio in 1:1, 9:16, or 21:9, then iMovie is not to tool to use in those instances, then you will need another video editing tool, like FlexClip.
Content:
Also Read: iMovie Vertical Video with Black Bars! How to Fix? >
Part 1: A Better Way to Change Video Aspect Ratio to More Than 16:9
As we said before, iMovie is far from the perfect tool for changing the aspect ratio. When you change a video's aspect ratio in iMovie using the crop feature, it auto-adds pillarbox (black bars placed on the sides) or letterbox (black bars above and below) to your video to keep your video in the fixed aspect ratio at 16:9.
So we want to introduce a free alternative video aspect changers to iMovie - FlexClip - to help you change the aspect ratio to 4:5 or 1:1 and more.
FlexClip is a powerful online video editor offering rich editing tools to help you make a stunning video. It can easily help you change the aspect ratio to 16:9, 4:5, 1:1, 21:9, and 9:16 in a few clicks.
How to Change Aspect Ratio with iMovie Alternative - FlexClip

Upload Your Video to FlexClip

Change Video Aspect Ratio with FlexClip
You can also zoom out the video or drag it to highlight the center part and not miss the important information in your video.
While you're in the editor, you can use FlexClip's tools to make any other edits you'd like to your video:
- Add & edit text animation to the video;
- Add transition effects, filters to the video;
- Add watermark, overlays & voice over;
- Split, trim, and rotate the video;
Also, you can make a stunning video with FlexClip's pre-made video templates:


Part 2: How to Change Aspect Ratio in iMovie on iPhone/iPad
Frankly speaking, the video aspect ratio conversion option isn't available at the moment in iMovie on the iPhone/iPad. But here is a workaround to help you change the video aspect ratio in iMovie on iPhone/iPad - Crop.
Launch iMovie on your iPhone, click the Plus icon, select Movie, and then tap Create Movie to import video from iPhone to iMovie.

Import Video to iMovie on iPhone
Once your input is successfully imported, tap the video in the timeline, and select the magnifier icon with "Pinch to zoom" text at the upper right corner to start cropping.

Change Aspect Ratio in iMovie on iPhone
After clicking the magnifier icon, you'll be allowed to pinch to zoom your video and change aspect ratio using your fingers. Once you finished, just click Done, and next Save Video.
Part 3: How to Change Aspect Ratio in iMovie on Mac
It takes only a few steps to change the iMovie aspect ratio in old versions on Mac, simply go to "File > Project Properties > Aspect Ratio", and change the aspect ratio to 16:9 or 4:3.

Change Aspect Ratio in iMovie Old Version
However, Apple has already canceled the Aspect Ratio option in iMovie 10 (and later )Project Properties. The only workable way to change the video aspect ratio in iMovie on Mac is to enable the crop feature, but the aspect ratio is fixed at 16:9.
Open iMovie and tap the plus sign to start a new movie, then import your video you want to change the aspect ratio.
Drag the video to the timeline and head to the Crop icon, then Crop to Fill in the toolbar above the viewer. Now you can select an area you want to crop with the white rectangle on the viewer. After you set the cropping area, click the "√" button to apply the crop.

Change Aspect Ratio in iMovie on Mac
Final Words
That's all for how to change the aspect ratio in iMovie and the best iMovie alternative to change the video aspect ratio freely - change aspect ratio online with FlexClip's online video editor. By the way, FlexClip is a powerful video editor/maker that helps you create stunning videos with great ease. Try it FREE!