A video resizer on iPhone can resize a short local MP4, WebM, MOV, M4V, or Ogg video to whole-number dimensions up to 1920 pixels per side without uploading the source. The Video Resizer works in the current browser tab. It validates the selected file, decodes it, draws its frames onto a resized canvas, captures that canvas at up to 30 frames per second, and adds any browser-exposed audio tracks. From there, MediaRecorder produces a WebM and the browser creates a local download for the result. You enter a maximum width and height, then select Fit or Stretch. Fit treats those values as a bounding box, preserves the source aspect ratio, and never crops; Stretch uses the exact width and height independently, so the picture can be distorted. Output dimensions are rounded down to positive even values because even pixel sizes are more reliable for common browser encoders. A source can be up to 500 MiB and five minutes, but every source geometry and time limit still applies. Resizing is a real-time re-encode, so keep the iPhone browser tab open until the WebM is ready.
What the iPhone resizing workflow changes
Video Resizer changes the pixel dimensions of a local video. It does not simply attach new resolution information to an unchanged file. The browser decodes the source, processes its frames, and records a new WebM. This distinction matters because the resulting quality, bitrate, frame timing, audio layout, color metadata, and file size can differ from those of the original recording.
- The source remains on the iPhone and is not uploaded to a server.
- Supported source containers are MP4, WebM, MOV, M4V, and Ogg.
- The output is a downloadable WebM rather than an MP4.
- Any available audio must be exposed by the browser to be included.
- The tool does not upscale detail, crop a subject, remove black bars, preserve subtitles, or preserve multiple tracks.
Use media you own or have permission to edit. The tool cannot bypass digital rights management or turn an unsupported source codec into a valid editable stream.
Resize a video on iPhone in your browser
The complete process takes place on the iPhone, but the source must satisfy the tool’s format, size, duration, and geometry constraints. Keep the original file available in the iPhone file picker while completing the workflow.
- Open the tool. In the current browser on your iPhone, open Video Resizer. Processing happens in that tab, and the source remains local. If the browser lacks a required capability, the tool fails visibly instead of producing a misleading download.
- Choose one source video. Use the file picker to select one supported local video. Wait for the browser to load its metadata before continuing. A file with an unsupported codec may not decode even when its container name appears in the accepted list.
- Enter the target dimensions. Type a whole-number maximum width and height from 2 through 1920 pixels. Base them on the source’s actual pixel dimensions. Each final output dimension is made even, so the tool rounds an odd requested value down by one pixel.
- Select the geometry mode. Choose Fit to preserve the source aspect ratio inside the requested box without cropping. Choose Stretch when the width and height should be applied independently, with the understanding that the picture may be distorted.
- Start the resize. Select “Resize video” and keep the browser tab open. This is real-time encoding, not an instant metadata edit; processing normally takes about as long as the video.
- Download the result. When encoding finishes, download the generated WebM from the current tab. The result is WebM with a finite duration, not an MP4 version of the source.
Choose Fit or Stretch for the output
Fit and Stretch solve different resizing tasks. Fit is the safer choice when the original composition should remain visually correct. Stretch is appropriate only when exact width and height values matter more than preserving the source’s proportions.
| Mode | Target dimensions | Output geometry | Resulting trade-off |
|---|---|---|---|
| Fit | Treats width and height as a maximum bounding box | Uses the smaller of the two scale factors, preserves the source aspect ratio, and never crops | One or both output dimensions may remain below the requested maximum, and the tool does not upscale detail |
| Stretch | Applies width and height independently, subject to even-output rounding | Produces the requested geometry without preserving the source ratio | The picture can be intentionally distorted |
For a 2160 × 3840 portrait source placed inside a 1920 × 1080 Fit box, the scale is calculated as the smaller target-to-source ratio. Scale = min(1920 ÷ 2160, 1080 ÷ 3840) = 0.28125. The raw output is 2160 × 0.28125 by 3840 × 0.28125, or 607.5 × 1080. After rounding down to a positive even width, the result is 606 × 1080 pixels. It fits inside the target box without cropping or distortion.
Stretch would use the requested width and height independently. It should not be used merely to force a thumbnail or editing field to display a preferred numeric size. If preserving the picture is important, select Fit first and inspect the downloaded WebM before sharing it.
Check the file, size, and dimension limits
All source and target constraints apply together. A file that passes its container check can still be rejected for exceeding a time, size, or geometry limit. Review these requirements before opening a large iPhone recording:
| Item | Requirement or behavior |
|---|---|
| Source containers | Short MP4, WebM, MOV, M4V, or Ogg video |
| Source size and duration | Up to 500 MiB and no longer than five minutes |
| Source geometry | No more than 4096 pixels on either side and total pixel area no greater than 3840 × 2160 |
| Target dimensions | A whole number from 2 to 1920 pixels for each width and height |
| Dimension adjustment | Output dimensions are rounded down to positive even values |
| Output | A WebM created in the current browser tab |
| Processing time | Real-time encoding that normally takes about the length of the source |
| Frame capture | Decoded frames are drawn to a canvas and captured at up to 30 frames per second |
Having the correct container extension does not guarantee successful decoding. Unsupported codecs, invalid dimensions, excessive source metadata, empty recorder output, and browser capability gaps are reported visibly. This prevents an incomplete processing attempt from looking like a finished resize.
How the local WebM is created
After a valid file and target geometry are established, the browser decodes the source and resizes each frame on a canvas. MDN documents the Canvas captureStream API, which provides the real-time media stream used for that captured canvas. The MediaRecorder API then records the stream using a VP9 or VP8 WebM configuration supported by the browser.
Available audio tracks exposed by the browser are added to the capture. Subtitles and multiple tracks are not preserved, and the resulting audio layout can differ from the source. A shared Matroska duration-writing step gives the generated file a finite duration. The browser then exposes a revocable local download URL, allowing the finished WebM to be saved without placing the original recording on a remote processing server.
Because this is a new encode, do not expect the output file to be a lossless version of the source at an identical bitrate. It is intended for changing supported pixel dimensions in a browser, not for professional mastering or exact media delivery specifications.
When resizing is not the right edit
Use a different workflow when the recording exceeds five minutes, is larger than 500 MiB, has source dimensions outside the supported geometry, or must be delivered as MP4. The tool does not promise professional frame-accurate output, exact bitrate control, color-managed processing, subtitle preservation, or multi-track delivery. A dedicated desktop encoder is more appropriate for those requirements.
Resizing also cannot remove black bars, crop a subject, upscale lost detail, preserve multiple audio or subtitle tracks, or bypass DRM. Fit changes dimensions while preserving the source ratio, but it does not claim that the entire visual file remains unchanged. Stretch should be reserved for situations where geometric distortion is intentional. For a short supported iPhone clip, Fit is the default choice for retaining the original composition, while Stretch is available when independent dimensions are required.
For a deeper look, see Can I Use a Video Cropper Without Uploading a File?.
For a deeper look, see Video Resizer on Laptop: Resize Locally in Your Browser.