Resizing a video produces a new file with changed pixel dimensions, a re-encoded WebM container, and no added detail. The expected result is a smaller or differently shaped video that runs through the same length of footage as the source, but at the resolution requested, with VP9 or VP8 video and any exposed audio track stitched back in. Quality, bitrate, file size, color metadata, and frame timing can all shift from the original, because the operation decodes, redraws, and re-records frames in real time instead of swapping metadata. Expect the picture to keep the source shape (in Fit mode) or stretch to fill the box (in Stretch mode), expect odd dimensions to come back rounded down to even pixel counts, and expect the encode to take roughly as long as the clip itself — because the browser is re-encoding every frame as it plays. The output is always a downloadable WebM created in the current tab, never an MP4, never a metadata-only change, and never a re-upload of the source.
What the Resize Operation Actually Changes
A resize is not a metadata tweak. Video Resizer reads the chosen file in the browser, draws every decoded frame onto a canvas sized to the target geometry, captures that canvas at up to 30 frames per second, and records the result through MediaRecorder into a WebM container. According to MDN's MediaRecorder reference, this approach produces a real, playable file because the recording is driven by an actual media stream rather than by container rewriting. The picture therefore gets re-sampled: pixel data is interpolated to fit the new width and height, the resulting file is rebuilt from scratch, and the original bytes of the source file are never modified. That is why the output looks like a fresh export rather than the same file with new header values.
The Output Is Always a WebM File
Every result produced by the tool is a WebM, not an MP4, MOV, or M4V. The reason is practical: the in-browser encoder uses VP9 or VP8 for video (whichever the browser exposes through MediaRecorder) and the WebM container is the format that pairs with those codecs without requiring a heavy media dependency or a server-side transcode. The output extension is fixed, the audio track is whatever the browser can decode from the source and expose to the canvas capture pipeline, and the duration is patched in through a shared audited Matroska duration writer so the resulting file plays as a finite clip rather than as an open-ended stream. If you need an MP4 for a specific workflow, plan on converting the WebM afterward with a dedicated tool.
Fit vs Stretch: Two Different Visual Results
The two modes in Video Resizer produce visibly different results and the table below summarizes what each one does. Fit treats the entered width and height as a bounding box, picks the smaller of the two scale factors, and preserves the source aspect ratio without cropping. Stretch uses the requested width and height independently, so the output is exactly the size you asked for but the picture can look stretched or squashed.
| Aspect | Fit mode | Stretch mode |
|---|---|---|
| Source ratio preserved | Yes | No |
| Output dimensions | Fits inside the requested box | Exactly the requested values |
| Cropping | Never crops the source | Can distort the picture |
| Use case | Sharing without warping the image | Filling an exact pixel target |
| Black bars | Possible inside the bounding box | None, the frame is filled |
If the source is already the same ratio as the box, both modes produce identical geometry; the difference only shows up when the requested box and the source ratio disagree.
What Happens to Quality, File Size, and Frame Timing
The expected result is not a perfect clone of the source. Because every frame is drawn on a canvas, re-sampled, and re-encoded, the new file picks up the encoding choices the browser makes: a VP9 or VP8 video stream with a bitrate chosen by MediaRecorder, and a frame rate capped at 30 fps even if the source plays higher. Downscaling usually looks sharper per pixel than the source because each output pixel represents an average of source pixels, but upscaling does not add detail and can introduce softness, ringing, or jagged edges. File size depends on the new dimensions, the chosen codec, the bitrate the browser lands on, and the duration of the clip — there is no manual bitrate slider. Frame timing can shift slightly because the canvas is captured in real time, and color metadata, multiple audio tracks, and embedded subtitles are not carried over by the browser pipeline.
How to Resize a Video Locally With Video Resizer
- Open Video Resizer in a desktop browser and choose one supported local file — short MP4, WebM, MOV, M4V, or Ogg — then wait for the browser to read the metadata.
- Enter a maximum width and a maximum height as whole numbers between 2 and 1920 pixels. Odd numbers are rounded down to the nearest even pixel, because even dimensions are more reliable with common browser encoders.
- Pick Fit to preserve the source aspect ratio inside the bounding box, or Stretch to force the exact width and height regardless of distortion.
- Select Resize video and keep the tab open and in the foreground while the browser decodes the source, draws frames onto the resized canvas, captures audio, and records the WebM in real time.
- When the recorder stops, download the resulting WebM from the same tab. If something fails (unsupported codec, invalid dimensions, empty recorder output), the tool fails visibly rather than producing a misleading download.
Limits That Shape the Result
Several hard boundaries decide whether the tool can produce a result at all. The table below lists the ones stated directly in the product contract, so you can check your source before you start.
| Limit | Value |
|---|---|
| Source file size | Up to 500 MiB |
| Source duration | Up to five minutes |
| Source side length | Up to 4096 pixels per side |
| Source total area | Up to 3840 × 2160 pixels |
| Target width or height | Whole number from 2 to 1920 pixels |
| Output container | WebM (VP9 or VP8 video) |
| Capture frame rate | Up to 30 fps |
Anything outside these bounds is rejected up front rather than producing a broken file. For longer footage, frame-accurate delivery, MP4 output, color-managed workflows, or exact bitrate control, the product contract points at a dedicated desktop encoder instead.
When a Browser Result Is Not Enough
The expected behavior is honest: the result is a quick, local WebM re-encode, not a professional master. The tool does not upscale detail, does not remove black bars by cropping, does not preserve subtitles or multi-track audio layouts, does not bypass DRM, and does not promise a specific bitrate. Use the result for quick sharing, previews, sizing a clip to a platform's recommended box, or trimming visual heft before upload. For frame-accurate delivery, hour-long footage, mandated MP4 containers, broadcast color spaces, or any workflow where exact bitrate or timing matters, plan on a desktop encoder with the controls this browser pipeline intentionally does not expose.
For a deeper look, see Trim Video Output: What Format and Quality to Expect.
For a deeper look, see What to Know Before You Resize a Video.