Six concrete details decide the result when you resize a video: the supported source formats, the source file size and resolution caps, the integer pixel range of the requested dimensions, the choice between Fit and Stretch geometry, the automatic even-pixel rounding, and the WebM file your browser produces instead of your source format. Each detail gates a step in the local pipeline. The format gate decides whether the browser can decode the source at all. The size and resolution gates decide whether the file stays inside the tool's processing budget. The integer range decides the scale of the output. Fit and Stretch decide whether the picture keeps its source ratio or fills the requested box exactly, including by distorting. The even-pixel rule decides the final codec-safe frame size. The WebM output is the artifact you actually download, and it is a re-encoded file rather than a metadata-only dimension change. Knowing each of these details before you start means you can pick a target size that the tool can actually deliver, choose the geometry mode that fits the platform you are posting to, and predict what the downloaded file will and will not preserve from the source.
Source Format, Size, and Resolution Limits That Gate the Resize
Before anything else, the resize tool needs to decode your file. The Video Resizer accepts MP4, WebM, MOV, M4V, and Ogg videos that are stored locally on your device, and it decodes them in the same browser tab that you are using to run the tool. Files outside that format list are not opened, and there is no server step that would rescue an unsupported container. This means the format gate is the first detail to verify.
The second detail is the source file size. The tool accepts files up to 500 MiB and up to five minutes long, which is a hard ceiling rather than a guideline. Videos that exceed either limit are rejected before the resize runs, so checking the file size and duration of your source first saves a wasted step.
The third detail is the source resolution. The tool does not open files that have more than 4096 pixels on any single side, or whose total pixel area exceeds 3840 × 2160 pixels. A typical 4K recording sits at that boundary, while unusually wide panoramic captures or vertically stacked high-resolution clips can exceed the per-side limit and will be refused by the decoder step. If your source sits outside any of these three gates, the resize cannot start.
Fit vs Stretch: The Geometry Decision That Matters Most
Two geometry modes are available, and the choice between them is the single most consequential detail when you resize a video.
Fit treats the width and height you enter as a bounding box. The tool applies the smaller of the two requested scales, which keeps the source aspect ratio intact, and it never crops the picture. The result is a video whose width and height both fit inside the requested numbers while the ratio stays exactly what the source used. Use Fit when the destination platform expects a specific shape (such as 9:16, 1:1, or 4:5) and your source already matches the ratio roughly, or when you want black bars to fill the leftover space inside the box.
Stretch treats the width and height independently. The output uses the exact numbers you entered, even when those numbers change the shape of the picture. Because the tool does not resample intelligently, Stretch can make faces look wider, taller, or otherwise distorted compared to the source. Use Stretch only when the destination requires exact dimensions (such as a header banner, a player that locks to fixed pixels, or a frame that you will composite against a known background), or when the source ratio already matches the requested box.
| Detail | Fit mode | Stretch mode |
|---|---|---|
| Treats entered dimensions as | A bounding box | The exact output size |
| Preserves source aspect ratio | Yes | No |
| Crops the picture | Never | No |
| Can distort faces or shapes | No | Yes, by design |
| Scales by | The smaller of width or height scale | Width and height scales independently |
| Best when | Posting to a platform with a known aspect ratio | Needing an exact pixel size for a fixed container |
The Even-Integer Rule and the Dimension Ceiling
Every requested dimension must be a whole number from 2 to 1920 pixels. Values outside that range are not accepted, and decimal numbers such as 1280.5 are not accepted either. The tool does not expose a slider for fractions of a pixel because the downstream browser encoder cannot deliver a stable output for odd dimensions.
Common browser video encoders are most reliable when the frame width and frame height are even numbers. To keep the encoder stable, the Video Resizer rounds odd pixel values down by one, so a request for 1279 × 719 becomes 1278 × 718 in the actual file you download. This rounding is small (at most one pixel per side) and is rarely visible, but it does mean that the dimensions you read on the source screen may be one pixel smaller than what you typed.
The 1920-pixel upper bound is also a detail to keep in mind. It matches the width of 1080p video, which is the largest standard size most social platforms accept without re-encoding. If your target is 4K, the local browser pipeline is not the right tool, and a desktop encoder becomes the better fit.
| Input or output | Allowed range or value |
|---|---|
| Supported source containers | MP4, WebM, MOV, M4V, Ogg |
| Maximum source file size | 500 MiB |
| Maximum source duration | 5 minutes |
| Maximum pixels per source side | 4096 px |
| Maximum source pixel area | 3840 × 2160 px |
| Requested width range | 2 to 1920 px (whole numbers) |
| Requested height range | 2 to 1920 px (whole numbers) |
| Odd values behaviour | Rounded down by 1 px before encoding |
| Output container | WebM (VP9 or VP8) |
| Output frame rate cap | Up to 30 fps |
How to Resize a Video Locally in Your Browser
Resizing a video locally is a four-step workflow once the source passes the input gates. The whole pipeline runs in the current browser tab, and the file never leaves your device. The Video Resizer walks through each of these steps in the same order.
- Choose one supported local video and wait for the browser to read its metadata. The tool opens the file picker, you select a single MP4, WebM, MOV, M4V, or Ogg clip that is under 500 MiB and under five minutes long, and you wait until the dimensions, duration, and file size populate on screen. If the metadata does not load, the file is not in a supported container or the source resolution exceeds the limits, and the resize cannot start.
- Enter the maximum width and the maximum height that you want for the output. Both values must be whole numbers between 2 and 1920. Write the width and height in the boxes before you choose a geometry mode so the tool can compute the correct scale.
- Choose Fit to keep the source aspect ratio inside the requested box, or choose Stretch to force the exact width and height and accept any distortion. Fit is the right default for social platforms; Stretch is the right choice when an exact pixel size is required.
- Select Resize video and keep the current tab open during the real-time encoding. The browser decodes the source, draws frames onto a resized canvas, captures that canvas at up to 30 fps, attaches any browser-exposed audio tracks, and produces a WebM file using a VP9 or VP8 codec. When the encoding finishes, the download link for the resulting WebM appears. The encode runs at roughly real-time speed, so a three-minute source takes about three minutes to process, and closing the tab cancels the run.
Output Details to Expect From a Local Resize
What comes out is not a metadata-only dimension tweak. The Video Resizer decodes each frame, redraws it on a canvas at the new size, captures the canvas as a stream, and records that stream with the browser's MediaRecorder. The file you download is a fresh WebM, not a repackaged MP4, and several details about the output are worth knowing before you plan your workflow.
The container is WebM. The codec is whichever VP9 or VP8 configuration the current browser exposes through MediaRecorder. The frame rate is capped at 30 fps, so a 60 fps source is re-encoded at 30 fps. The audio layout follows whatever audio tracks the browser can read from the source, which usually means one stereo track and no separate language tracks.
Quality, bitrate, frame timing, audio layout, color metadata, and final file size can all differ from the source. The tool does not expose bitrate or codec controls, and it does not preserve subtitle tracks, multiple audio tracks, or color-managed metadata. If you need any of those to survive unchanged, the local browser pipeline is not the right place to do the resize.
The duration of the output is set by an audited Matroska duration writer rather than by the recording start time, which is why the downloaded WebM plays to the end of the source instead of stopping at the moment the MediaRecorder stream was first captured. This is a small detail, but it is the reason a re-encoded WebM from this pipeline is not truncated.
When a Browser Resizer Is Not the Right Choice
The browser pipeline is intentionally narrow, and the same details that keep it safe also rule it out for some jobs. Use a desktop encoder instead of the Video Resizer when any of the following details matter for the final file:
- The deliverable must be MP4, MOV, or another non-WebM container, because the tool only writes WebM.
- The source is longer than five minutes or larger than 500 MiB, because the input gates refuse the file.
- The output must be larger than 1920 × 1920, because the dimension ceiling stops at 1080p-class sizes.
- The output must keep bitrate, color metadata, subtitle tracks, or multi-track audio exactly as the source provided them, because the re-encode rewrites each of those.
- The workflow requires frame-accurate delivery, batch processing, or scripted automation, because the tool is interactive and runs in a single browser tab.
For any of those jobs, a dedicated desktop encoder with bitrate controls, container selection, and frame-accurate options is the correct tool. The Video Resizer is the right fit when the source passes the input gates, the destination accepts WebM, and the priority is keeping the file on the local device rather than uploading it to a server. If the tool reports a visible failure instead of producing a download, the troubleshooting walkthrough for the Video Resizer covers each error state and the matching fix.
Related reading: Best Way to Resize Video: Pick the Right Method.