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Video Resize & Converter

Resize video to 1080p, 720p, 480p or 360p and convert between MP4 and WebM with adjustable CRF quality.

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About this tool

Video Resize & Converter lets you change the resolution of any video and convert between MP4 and WebM container formats, all within your browser using ffmpeg.wasm. No video is ever uploaded to a server. Choose from original resolution, 1080p, 720p, 480p, or 360p — the tool preserves the original aspect ratio automatically. Use the CRF quality slider to balance file size and visual quality: lower CRF means better quality and larger files; 23 is the ffmpeg default and a good starting point. MP4 output uses H.264 with AAC audio; WebM uses VP9 with Opus audio.

Why use it

  • 100% private — video never leaves your device

  • Convert between MP4 and WebM formats

  • Five resolution presets including original

  • CRF quality slider for precise size vs quality control

  • No watermarks, no signup, free to use

  • Aspect ratio preserved automatically when downscaling

How to use

  1. Upload your video file (MP4, WebM, or MOV).
  2. Choose the output format: MP4 or WebM.
  3. Select a target resolution or keep Original to only convert the format.
  4. Adjust the CRF slider — lower values give better quality at the cost of larger file size.
  5. Click 'Convert Video' and wait for processing to complete.
  6. Preview the result and download.
  7. Choose the output format: MP4 (H.264 + AAC) or WebM (VP9 + Opus).

When it helps

  • Downscaling a 4K recording to 1080p or 720p for easier sharing
  • Converting WebM to MP4 for compatibility with devices or editors
  • Compressing a high-bitrate video to reduce file size for email or upload
  • Preparing video for web delivery at multiple resolutions
  • Producing a smaller proxy file for editing on a slower laptop
  • Re-encoding a phone recording to a more universally playable MP4

Examples

4K to 1080p MP4

Input

5-min 4K MOV, MP4 output, 1080p, CRF 23

Expected result

1080p MP4 around 80–120 MB, plays in any device or editor

WebM to MP4 for compatibility

Input

3-min WebM screencast, MP4 output, original resolution, CRF 23

Expected result

Equivalent MP4 file playable in QuickTime, iOS, Windows Media Player and beyond

Compress for email upload

Input

2-min 1080p MP4 (200 MB), MP4 output, 720p, CRF 28

Expected result

Around 25–35 MB MP4 that fits inside most email size caps

Tips

  • Start with CRF 23 and only adjust if the result looks wrong — most viewers can't tell 22 from 24, but file size doubles around CRF 18.
  • If file size is your priority, set 720p resolution and CRF 26 — typically halves the size of a 1080p original.
  • Choose WebM/VP9 when targeting modern browsers and storage matters; choose MP4/H.264 when your audience may be on older devices or non-browser players.
  • For short demos under 30 seconds, CRF 18–20 produces visibly cleaner motion with only a modest size hit.
  • Re-encoding always loses some quality. If the goal is just trimming, use the Video Trimmer tool instead — it's lossless.
  • Keep the browser tab focused during encode — background tabs run slower because of throttling.

Frequently Asked Questions

Does my video leave my device?
No, processing is 100% local in your browser. Your video is never uploaded to any server.
What does CRF mean?
CRF (Constant Rate Factor) controls encoding quality. Lower values give better quality and larger files. 18 is near-lossless; 23 is the default; 32 is highly compressed.
Will the aspect ratio be preserved when I resize?
Yes. The tool uses scale with force_original_aspect_ratio=decrease so the output dimensions respect your original aspect ratio.
What codec does MP4 output use?
H.264 video with AAC audio — the most widely compatible combination for playback on all devices and browsers.
What codec does WebM output use?
VP9 video with Opus audio — an open standard well-supported in Chrome, Firefox and Edge.
How long does conversion take?
Unlike stream-copy trimming, format conversion requires re-encoding and can take several minutes for long or high-resolution videos on slower machines.
Why does it need SharedArrayBuffer?
ffmpeg.wasm needs SharedArrayBuffer to run multi-threaded encoding. Browsers only expose it on cross-origin-isolated (COOP/COEP) pages, which this site is.
Is there a maximum input file size?
No server cap — but very large files (multi-GB) are constrained by browser memory. Most laptops handle 1–2 GB comfortably; larger files may need 16 GB+ of RAM.

Explore the category

Video Compressor Reduce MP4, WebM and MOV file size with adjustable H.264 CRF compression — runs entirely in your browser. Video to GIF Converter Convert MP4 and WebM video clips into smooth, optimised GIF animations with two-pass palette rendering. GIF to Video Converter Convert animated GIFs to MP4 (H.264) or WebM (VP9) video files. Smaller, smoother and far more efficient than GIF. Browser-only via ffmpeg.wasm. Video Trimmer Cut and trim MP4, WebM, MOV videos to exact start and end times, right in your browser. Video Merger Concatenate multiple MP4, WebM or MOV clips into one continuous video file — no upload required. Video Cropper Crop a rectangular region out of any MP4, WebM or MOV using a draggable selection rectangle.
MP4 and H.264 Explained: How Modern Video Compression Works An MP4 is just a wrapper. The real magic is the codec inside. We unpack containers, codecs, B-frames, and why H.264 is still everywhere desp Mar 2026 · 9 min GIF vs WebP vs MP4: Which Animation Format Wins on the Web? Animated GIFs are nostalgic but heavy. WebP and MP4 deliver the same motion at a fraction of the size. We compare file size, browser support Mar 2026 · 9 min

Glossary

ffmpeg.wasm
WebAssembly build of FFmpeg that performs the full encode pipeline in-browser, with no server-side help.
SharedArrayBuffer
Shared memory primitive enabling multi-threaded WebAssembly. Required by ffmpeg.wasm and only present on cross-origin-isolated pages.
CRF (Constant Rate Factor)
An encoder setting that targets a constant perceived quality. Lower CRF means higher quality and bigger files; common range is 18–28.
Codec
The compression algorithm. H.264 (AVC) is the standard MP4 video codec; VP9 is the standard WebM video codec; AV1 is the newer high-efficiency codec.
Container format
The wrapper file (MP4, WebM, MOV) that holds the encoded video and audio streams along with metadata.
Bitrate
How many kilobits per second the encoder allocates. Higher bitrate means better quality and larger files. CRF mode sets this dynamically per scene.
Two-pass encoding
An encoding mode where the encoder runs twice — first to analyse content, then to produce the final output. Useful for hitting precise size targets.
Aspect ratio
The ratio of width to height (16:9, 4:3). Preserved automatically when downscaling so the picture isn't stretched.