Getting started with resizing a video starts with three quick decisions: confirm the exact pixel dimensions you need, decide whether the new file should keep the original's proportions, and pick a tool that can produce a usable file without uploading your source. A first-time resize usually means opening a supported local clip, entering the maximum width and height you want, and choosing between two output modes: aspect-preserving Fit, which scales inside a bounding box and never crops, or exact Stretch, which uses your requested width and height independently and can distort the picture. The browser then decodes the source, redraws each frame onto a resized canvas at up to 30 frames per second, attaches any audio tracks the browser exposes, and records the result as a downloadable WebM (a Matroska-container format defined by the IETF) using built-in VP9 or VP8 codecs. Nothing leaves your computer during this flow, and the resulting file is a real re-encode rather than a metadata-only dimension change, so processing typically takes about as long as the clip itself.
Decide What "Resized" Should Look Like
Before opening a tool, the practical starting point is to agree on what "resized" should mean for this specific job. Two clips at the same target dimensions can be very different files depending on whether you preserve the original aspect ratio or force exact pixels, so the goal you write down first dictates every later setting. Common target dimensions people reach for include 1920x1080 for full-HD desktops, 1280x720 for HD sharing, 1080x1080 for square social posts, and 1080x1920 for vertical reels. Knowing which one matches your destination is the fastest way to avoid a re-do.
Also worth deciding early: are you downscaling, upscaling, or re-framing? The local browser method is at its strongest on controlled downscaling, because shrinking a frame generally keeps visible detail usable. Upscaling rarely adds new detail, so starting with a low-resolution source will not become sharper just because you typed a larger number into a box. If you want a subject to fill a wider or taller container without distortion, fit-style scaling inside a bounding box is almost always the safer default than exact stretch.
Fit vs Stretch: Pick the Right Mode Before You Start
This choice is the single biggest "get started" lever in a resize workflow, because each mode treats the numbers you enter differently and they are not interchangeable once the file is recorded. Fit and Stretch also map onto different real-world platforms: a square destination usually accommodates letterboxing, while a strict 1080x1920 reel slot demands the exact rectangle.
| Mode | How it treats your numbers | Best starting point |
|---|---|---|
| Fit (aspect-preserving) | Uses width and height as a bounding box, applies the smaller of the two scales, keeps the source ratio, and never crops. | Sharing to a destination that can absorb letterboxing or pillarboxing and you want the picture to look natural. |
| Stretch (exact dimensions) | Uses the requested width and height independently and produces that exact rectangle, which can distort, squish, or stretch the picture. | A platform that literally requires a specific pixel rectangle and you are comfortable trading ratio fidelity for shape compliance. |
If you are unsure for a first attempt, Fit is the gentler default because it never crops and never distorts — it only repositions empty space inside the bounding box. Stretch is the right choice only when the receiving surface will reject anything that is not the exact pixel rectangle you typed. Odd dimensions you enter get rounded down by one pixel to the nearest even number, since even pixel sizes are more reliable with common browser video encoders.
Resize a Video Locally in Your Browser
Once your target dimensions and mode are settled, the actual sequence on the Video Resizer page is short and linear. Treat it as a checklist you can repeat on the next file.
- Open Video Resizer in your browser tab — this is the single entry point for the whole flow.
- Choose one supported local video in MP4, WebM, MOV, M4V, or Ogg format and wait for the browser metadata to finish loading before touching any controls.
- Enter the maximum width and the maximum height you want for the output. Each value must be a whole number from 2 to 1920 pixels.
- Choose a mode: Fit to preserve the source aspect ratio inside the bounding box, or Stretch to produce exactly those dimensions.
- Select Resize video and keep the tab open while the tool re-encodes in real time at up to 30 frames per second. Closing the tab cancels the run.
- When the encoder finishes, download the resulting WebM from the same tab. The link is generated locally and points only at your own output.
For a more deliberate walkthrough of the differences between modes, the Compare Approaches to Resize Video: Fit vs Stretch guide is a useful next read once you have a first file in hand.
What the Tool Actually Does (and What Changes)
For a reader who is just starting out, it helps to know what the controls are triggering behind the page. The browser decodes your source file, draws each frame onto a canvas at the target size, captures that canvas as a media stream via the HTMLCanvasElement.captureStream API documented on MDN, records the stream with MediaRecorder also documented on MDN in a VP9 or VP8 WebM profile, and attaches any audio tracks the browser happens to expose from the source. The recorder then writes a finite Matroska duration into the container so the playable length is correct.
Because this is a real re-encode rather than a metadata-only edit, several properties can shift in the output: visual quality depends on the chosen browser codec configuration, bitrate is bounded rather than matched to the source, frame timing is captured at up to 30 fps so high-frame-rate sources may be downsampled, audio layout and channel mix follow what the browser exposes from the source, color metadata is not guaranteed to carry over, and the final file size can differ noticeably from the input. The tool does not upscale detail, remove black bars, crop a subject, preserve subtitles or multiple audio tracks, bypass DRM, or promise professional mastering, and unsupported codecs, invalid dimensions, or empty recorder output fail visibly rather than producing a misleading download.
Check These Limits Before You Start
A short list of hard limits decides whether your first attempt will run or fail. Knowing them in advance saves a wasted retry.
- Supported input formats: MP4, WebM, MOV, M4V, and Ogg.
- Source size: up to 500 MiB and up to five minutes in duration.
- Source dimensions: no side longer than 4096 pixels and no total area larger than 3840x2160.
- Output dimensions: each whole number from 2 to 1920 pixels, with odd values rounded down to the nearest even pixel.
- Output container: always WebM (VP9 or VP8) — there is no MP4 option and no upload step.
If your job needs frame-accurate delivery, footage longer than five minutes, an MP4 container rather than WebM, color-managed grading, or exact bitrate control, treat local browser resizing as a quick first pass and plan to finish the file in a dedicated desktop encoder afterwards.
When Local Browser Resizing Is the Right Starting Point
The cleanest case for this approach is a short clip you own or are allowed to edit, a target dimension you can name in pixels, and a destination that either accepts WebM outright or is happy with a re-encoded copy. It is also a strong fit any time the upload policy of a different tool is a concern, because decode, canvas resize, capture, recording, and download all run in the current tab without sending the source to a server. For longer jobs, controlled bitrate, or MP4 delivery, plan a second pass with a desktop encoder and use the browser result as a clean proof of concept you can judge before committing to a longer render.