How to Resize a Video on Mac

The fastest way to resize a video on Mac is to set the frame shape with the Crop tool in Photos, then pick the pixel dimensions from the Resolution pop-up when you export from iMovie. Here's why those are two separate steps, and where the built-in tools push back.

Open the free Video Resizer →

"Smaller" means three unrelated things on a Mac

"Make this video smaller" means at least three unrelated things, and picking the wrong one wastes an afternoon.

The aspect ratio is the shape of the frame. 16:9 wide, 9:16 tall, 1:1 square. The resolution is how many pixels fill that shape, like 1080 by 1920. The file size is how many megabytes the result weighs, which depends far more on the codec and bit rate than on either of the other two. Resizing is the first two. If what you actually need is a smaller file, that's a different job with different levers.

Resize a video on Mac by setting the frame shape first

In Photos, double-click the clip, click Edit, then Crop. The aspect presets include Square, 4:5 and 16:9, with Custom for anything else. Setting the shape here, before you open an editor, saves you from discovering the constraint below halfway through.

That constraint: in iMovie, the Crop to Fill frame is constrained to a 16:9 aspect ratio. iMovie is built around widescreen, and it will keep pulling a clip back toward that shape. Fit restores a clip's original aspect ratio and Reset clears crop and rotation changes, but neither of them makes the crop frame taller than wide. For 16:9 work that's invisible. For vertical work it's the whole problem.

Choosing pixel dimensions at export

Once the shape is settled, the dimensions get decided at export. In iMovie, Share > Export File gives you a Resolution pop-up: lower resolutions produce smaller files that upload to the web more quickly, higher ones produce larger files better suited to watching on a Mac or a television.

Editing in the frame you'll post in

Montaj exports at a fixed 1080 by 1920. Vertical, and the same every time. There's no resolution menu to get wrong and no widescreen frame quietly reasserting itself, because the whole editing surface is already the shape the clip is going to be published in. What you see while you're cutting is the frame, not a preview of a crop that will happen later.

It runs on your device, so nothing is uploaded to be re-framed. If you need a shape other than vertical, resize a video on Mac in Photos and iMovie as above. This one is for people whose output is always tall.