How do I blur faces in a video on iPhone?

iMovie: not possible

iMovie for iPhone has filters, titles and a picture-in-picture overlay, but no blur, mosaic or censor effect, and no way to track a region. People sometimes stack an opaque shape over a face with picture-in-picture; it does not follow the face and looks like a mistake. If you need a blur, skip iMovie.

Anonimo: automatic, on the phone

1. Open Anonimo and pick the video from your library.
2. Wait while it finds every face and groups them by person. A one-minute clip takes about a minute on a recent iPhone.
3. Tap the people who should stay visible. Everyone else is covered in every frame.
4. Choose pixelation, blur or an emoji, then export. The result is saved to Photos; the original is untouched.

Free exports are limited to 30 seconds at 720p with a small watermark; Pro ($9.99 a year or $14.99 once) removes the limits. Nothing is uploaded, which matters for footage of other people.

CapCut: by hand, with masks and keyframes

1. Add the clip, then add it a second time as an overlay on top of itself.
2. Apply a blur or mosaic effect to the overlay only.
3. Add a circle mask to the overlay and place it over the face.
4. Add keyframes along the clip and move the mask so it follows the face. Repeat for every face.

This works, and CapCut is free to start, but each face is manual work and the result drifts whenever the person moves quickly. Some CapCut features run in the cloud, so check what the app says before you use it on sensitive footage.

YouTube Studio: free, after upload

1. Upload the video to YouTube, unlisted or private.
2. In YouTube Studio, open the video, go to Editor, then Blur. Pick Face blur to select from the faces YouTube found, or Custom blur to draw a box that follows an object.
3. Save. YouTube renders the blurred version in the background.

It is free and needs no app, but the unblurred video has to be uploaded to YouTube first, it only helps if YouTube is where the video goes, and you cannot download the blurred file in full quality for use elsewhere.

Trade-offs

• Automatic detection saves time but still needs a look: a face turned away or very small can be missed by any tool.
• Manual masking gives full control and costs minutes per face per minute of video.
• On-device tools keep the footage private; cloud tools need the original uploaded, which may itself be the thing you wanted to avoid.

Related

• Blur faces in CapCut, or use a face blur app?
• YouTube Studio's blur tool: what it can and cannot do
• How to blur a face in a photo on iPhone