Resizing a video means re-encoding the source frame by frame in your browser, producing a fresh file with new pixel dimensions — not a copy with only the metadata changed. The browser decodes the source frame by frame, draws each one onto a resized canvas at up to 30 frames per second, captures that canvas, and records it using a built-in WebM codec, so processing normally takes about as long as the source plays. Because the output is re-recorded rather than copied, bitrate, frame timing, audio layout, color metadata, and final file size can all differ from what your source looks like today. Before you click the button, the four decisions that matter most are: should the picture be contained inside a bounding box (Fit) or stretched to exact dimensions (Stretch), what target width and height you'll use, whether an even-pixel size is important to you, and what to do if the file is too large or too long for the browser to handle. None of these are obstacles — they're simply the facts the tool works with so the result matches what you actually intended.
A re-encode changes more than the pixel dimensions
The Video Resizer draws decoded frames onto a resized canvas and records that canvas through your browser's MediaRecorder API at up to 30 frames per second. Because every frame is freshly captured, the resulting WebM is not byte-identical to the source: the encoder picks its own bitrate, frame timing is rebuilt rather than copied, and any audio tracks your browser exposes are re-attached to the WebM container. Color metadata, exact bitrate, file size, and audio layout can all shift. It isn't a bug — it's the natural behavior of any real-time re-encode. Knowing this in advance means you won't compare the two files byte-for-byte and assume something is broken when the sizes differ.
This is also why processing takes roughly as long as the source: the browser is decoding and re-recording in real time, not just touching metadata. A five-minute source will take close to five minutes to finish. The output WebM also receives a finite duration through a shared audited Matroska duration writer so the player sees a fixed length rather than an indefinite stream, and unsupported codecs, invalid dimensions, excessive source metadata, or empty recorder output all fail visibly rather than producing a misleading download.
Fit vs Stretch: the choice that determines how the picture looks
Fit and Stretch behave differently and the right one depends on what you want the result to look like.
| Mode | What you enter | How the result is built | Picture effect |
|---|---|---|---|
| Fit | A maximum width and height — the bounding box | Applies the smaller of the two scales to keep the source ratio, never crops | Image sits centered inside the box, with letterbox-style space if the box ratio doesn't match the source |
| Stretch | An exact width and height | Uses the width and height independently | Can intentionally distort the picture if the box ratio doesn't match the source |
Fit mode treats the entered dimensions as a bounding box, applies the smaller width-or-height scale, preserves the source ratio, and never crops. Stretch mode uses the requested width and height independently, so the picture fills the box exactly and can distort when your target ratio doesn't match the source.
A worked example for Fit, step by step:
- Source dimensions: 1280 × 720
- Target bounding box: 500 × 300
- Scale by width: 500 ÷ 1280 = 0.3906
- Scale by height: 300 ÷ 720 = 0.4167
- Smaller scale chosen: 0.3906
- New width: 1280 × 0.3906 = 500
- New height: 720 × 0.3906 = 281.25, rounded down to the nearest even pixel = 280
- Output: 500 × 280
Stretch would have produced 500 × 300 exactly, regardless of the source ratio, which means a 1280 × 720 source would visibly squash horizontally and stretch vertically.
Source limits and dimension rules that decide whether the tool can run
Before you resize, check these constraints because the tool rejects inputs that exceed them rather than silently producing a broken file.
| Constraint | Value |
|---|---|
| Maximum source file size | 500 MiB |
| Maximum source duration | 5 minutes |
| Maximum pixels per side (source) | 4096 |
| Maximum source resolution area | 3840 × 2160 pixels |
| Target width or height | Whole number, 2–1920 pixels |
Output dimensions are also rounded to the nearest even pixel because common browser video encoders are more reliable with codec-safe even sizes. If you enter an odd value like 281, the tool rounds it down to 280 before encoding. Knowing this lets you pick clean numbers (320, 480, 640, 1080, 1280, 1920) so you get exactly the dimensions you planned and don't lose a pixel to rounding.
The tool also doesn't promise to upscale detail, remove black bars, crop a subject, preserve subtitles or multiple audio tracks, bypass DRM, or deliver professional mastering. Only edit media you own or have rights to, and remember that the source itself stays on your device throughout.
Resize a video locally with Video Resizer
- Open the Video Resizer and select one supported local video file (MP4, WebM, MOV, M4V, or Ogg). Wait for the browser to finish loading the source metadata before continuing — entering dimensions before metadata loads can lead to mismatched geometry.
- Enter a maximum width and a maximum height in pixels. Each value must be a whole number from 2 to 1920.
- Choose Fit to preserve the source aspect ratio inside a bounding box, or Stretch to force the exact width and height and accept potential distortion.
- Select Resize video. Keep the tab open while the browser encodes in real time — processing normally takes about as long as the source plays.
- When the recorder finishes, download the WebM result. The download is generated locally and can be revoked when you close the tab.
The full pipeline — decode, canvas resizing, recording, and download generation — runs in the current browser tab, so the file doesn't leave your device. For a closer description of how that local pipeline works end-to-end, see the Video Resizer explained: how local resizing works walkthrough.
When browser resizing is the wrong tool
Local browser resizing is fast and private, but it isn't a substitute for a desktop encoder in every case. Reach for a dedicated tool when you need frame-accurate delivery or exact frame timing — the browser records at up to 30 fps and can't honor arbitrary source frame timings. Long footage is also out of bounds, since sources longer than five minutes exceed the local tool's limits. If you need MP4 output, the recorder produces WebM only because it relies on built-in browser codecs. And if you need exact bitrate control or a color-managed workflow, the encoder picks its own bitrate and color metadata isn't guaranteed to round-trip through the re-encode, so the output size and color behavior can differ from the source.
For everything that fits inside the limits above, the Video Resizer handles the resize locally in your browser tab, produces a WebM you can download, and keeps the source on your device the whole time.
For a deeper look, see What to Review After You Resize a Video: A Checklist.