About this tool
Resizing video changes the pixel dimensions of each frame. Going from 4K (3840x2160) to 1080p (1920x1080) cuts the pixel count by 75% and the file size by ~70-80%. For most viewing contexts (phones, social platforms, email attachments), 720p or 1080p is enough; 4K is overkill.
This tool resizes in your browser using FFmpeg.wasm. Pick a target resolution from presets (4K / 1080p / 720p / 480p / 360p) or set a custom width. Aspect ratio is preserved by default.
How it works
Open your video
Drop an MP4 / MOV / WebM file.
Pick target resolution
4K / 1080p / 720p / 480p / 360p preset, or custom width.
Pick output codec
H.264 (universal) or H.265 / HEVC (smaller, requires modern decoder).
Process and download
Resized video saves to your device.
Use cases
Email attachments
25 MB cap is too small for 4K. Resize to 720p drops a 100 MB file to under 25 MB.
Social platforms
YouTube and TikTok handle 4K. Twitter, Instagram, and most messaging apps cap below 1080p anyway.
Mobile viewing
Phone screens rarely need above 1080p. Resize to save bandwidth.
Storage
A folder of 4K phone videos at ~50 MB / minute compresses to 1080p at ~10 MB / minute.
Format and spec details
| Common resolutions | 4K (3840x2160), 1080p (1920x1080), 720p (1280x720), 480p (854x480), 360p (640x360) |
|---|---|
| Codec options | H.264 (universal), H.265 / HEVC (~50% smaller, modern only), VP9 (web) |
| Source formats | MP4, MOV, WebM, AVI, MKV, FLV |
Tips and best practices
- 1080p is enough for almost everything. 4K is overkill for non-cinema viewing.
- Use H.265 if your audience is on modern devices - 50% file size reduction.
- Resize before uploading to bandwidth-constrained platforms.
How browser-based audio/video tools work
Modern browsers ship with Web Codecs API, MediaRecorder, and Web Audio API - enough to decode, manipulate, and re-encode most media formats client-side. This tool uses those APIs (with FFmpeg.wasm as a fallback for less common codecs).
The processing flow
- File is loaded as a binary Uint8Array.
- The codec is detected from the container (MP4 = MPEG-4, MKV = Matroska, WebM = WebM) and the codec atoms.
- Frames are decoded into raw audio samples (PCM) or video frames (YCbCr / RGB).
- The requested transformation (trim, convert, resize) is applied frame-by-frame.
- Frames are re-encoded into the output codec and packaged into the output container.
Common audio/video formats
| Container | Common codecs | Best for |
|---|---|---|
| MP4 | H.264 / H.265 video, AAC audio | Universal compatibility; default for web video |
| WebM | VP9 / AV1 video, Opus audio | Open-source web standard; smaller than MP4 |
| MKV | Any codec (container only) | High-quality archival; not browser-native |
| MOV | ProRes / H.264, PCM / AAC | Apple ecosystem; ProRes for professional editing |
| MP3 | MP3 audio only | Universal audio; lossy |
| WAV | PCM audio (lossless) | Editing source; CD-quality archival |
| FLAC | Lossless compressed audio | Music archival; ~50% of WAV size, perfect quality |
| AAC / M4A | Advanced Audio Coding | iOS default; better quality than MP3 at same bitrate |
Lossy vs lossless
- Lossy (MP3, AAC, Opus, H.264): discards data the human ear/eye can't notice. 80-90% size reduction. Each re-encode loses more quality (generation loss).
- Lossless (FLAC, WAV, ALAC, FFV1): bit-perfect reproduction. ~50% size of raw. Each re-encode is identical to the source.
Bitrate quick reference
| Use case | Audio bitrate | Video bitrate (1080p) |
|---|---|---|
| Voice (phone, podcast) | 32-64 kbps | n/a |
| Music (mid-quality) | 128 kbps MP3 | n/a |
| Music (transparent) | 256-320 kbps MP3 or 128 kbps Opus | n/a |
| Streaming HD | n/a | 5,000-8,000 kbps |
| Streaming 4K | n/a | 15,000-25,000 kbps |
| Archival | FLAC lossless | ProRes 422 or H.265 CRF 18 |
Privacy and offline operation
Every operation in this tool runs client-side using your browser's built-in APIs (Canvas, Web Audio, WebAssembly). No data is uploaded. After the initial page load you can disconnect from the internet and the tool keeps working.
We use Google Analytics and AdSense for the page itself, but neither sees the content of the files you process.
Frequently asked questions
Will resizing make the video blurry?
Downscaling is essentially lossless visually. Upscaling beyond the source resolution makes the video softer.
Should I pick H.264 or H.265?
H.264 for universal compatibility. H.265 / HEVC for ~50% smaller file size at equivalent quality, requires modern decoder (most 2017+ devices).
Does resizing keep the audio?
Yes - audio is preserved or re-encoded as AAC.
Is my video uploaded anywhere?
No. All processing happens in your browser using Web Codecs / FFmpeg.wasm. Files stay in your tab's memory. Disconnect from the internet after page load and the tool still works.
Why is conversion slow?
Video re-encoding is CPU-intensive. A 1-minute 1080p clip can take 30-90 seconds to encode in the browser - desktop apps with hardware acceleration are 5-10x faster. Use this tool for short clips; for hour-long footage use HandBrake or FFmpeg on your machine.
Will the converted file lose quality?
Yes, slightly, if the source and destination are both lossy formats. Going from H.264 to H.264 at the same bitrate adds a small amount of generation loss. Going from H.264 to a lossless codec preserves the existing quality but doesn't restore what was lost on the first encode.
Can I convert between any two formats?
Most common pairs (MP4 <-> WebM, MP3 <-> AAC, WAV <-> FLAC) work in any modern browser. Exotic codecs (ProRes, FFV1, JPEG 2000) may require FFmpeg.wasm and run slowly.
What's the maximum file size I can process?
Practical limit is your browser's available memory (typically 2-4 GB). 30-minute 1080p videos process fine. 2-hour 4K source files may crash the tab; use a desktop tool for those.
How accurate is the Video Resize?
It applies the standard formula. Accuracy is limited only by your input precision. For decisions with material consequences (taxes, medical, legal, structural), use the result as a starting point and verify with a qualified professional in the relevant field.
Is the Video Resize free to use?
Yes. 100% free, no signup, no payment, no API key. The site is funded by display ads around the tool but not inside the calculation flow.
Are my inputs saved anywhere?
No. All inputs stay in your browser tab. Closing the tab discards them. The site uses Google Analytics for traffic measurement (anonymized) but the analytics never see what you type into the form.
Can I use the Video Resize on my phone?
Yes. The tool is responsive and tested on iOS Safari, Android Chrome, and major desktop browsers. Touch targets meet Apple's 44pt and Google's 48dp minimum.
Does the Video Resize work offline?
Yes. Once the page has loaded, it works without internet. The calculation runs in JavaScript on your device.
How do I report a bug or suggest improvement to the Video Resize?
Email hi@3tej.com with the URL of this page and a description of what you saw vs expected. We typically respond within 72 hours.
Can I share results from the Video Resize?
Take a screenshot or copy the output. The page doesn't generate shareable URLs for specific calculations - inputs stay in your browser only.
Why are the results different from another video resize tool?
Most likely: different formula assumptions, different default values, different rounding rules, or different applicable rates. Check the methodology if both tools document it. Both can be valid for different scenarios.