Why won't my WebM screen recording play on an iPhone?

Safari's WebM support has historically been inconsistent across iOS and macOS versions. Converting to MP4 with H.264 video sidesteps the issue entirely, since H.264 decoders are essentially universal on Apple hardware.

Is this conversion lossless like some others on this site?

No, this is a genuine re-encode. VP8/VP9 and H.264 are fundamentally different, incompatible codecs, so FFmpeg has to fully decode the video and recompress it — there's no way to simply relabel or remux one as the other.

Will I notice a quality difference after converting?

At CRF 23, generally no for typical viewing — this setting targets visually consistent quality with the source rather than an arbitrary fixed bitrate, so the difference is usually not perceptible without close inspection.

Why won't my video editor import the WebM file directly?

A lot of video editing software, including older versions of some mainstream editors, either doesn't support WebM at all or handles it unreliably compared to their long-established MP4/H.264 import pipelines. Converting first avoids that friction.

Does this work with recordings from Loom, OBS, or a browser extension?

Yes, any standard WebM file works the same way regardless of which tool produced it, as long as it uses standard VP8/VP9 video and Opus/Vorbis audio.

What does the faststart flag actually change?

It moves the MP4's metadata index to the beginning of the file instead of the end, letting browsers and players start playback while the rest of the file is still downloading — useful if the converted video will be embedded or previewed on the web.

Why does this take longer than a stream-copy conversion like MP4 to MKV?

Because VP8/VP9 and H.264 are incompatible codecs, every frame has to be decoded and recompressed rather than simply copied between containers, which is inherently more CPU-intensive.

Can I convert a WebM with VP8 video, not just VP9?

Yes, both VP8 and VP9 are decoded and re-encoded to H.264 the same way; the tool doesn't require you to know which one your source file uses.

Is there a resolution or length limit?

No artificial limit from the tool itself; larger or longer files simply take proportionally longer to re-encode since everything processes locally in your browser.

Will the audio quality hold up after converting from Opus to AAC?

For typical spoken-word and general recording content, the difference is generally not noticeable — AAC at the bitrate used here is a mature, well-regarded codec even though Opus is technically more efficient at very low bitrates.

WEBM to MP4 Converter — Free Online, No Upload

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What is WEBM to MP4?

WebM to MP4 re-encodes VP8 or VP9 video and Opus or Vorbis audio into H.264 video and AAC audio, wrapped in an MP4 container — essentially the reverse trade of MP4 to WebM. Where converting to WebM optimizes for file size and web delivery, converting back to MP4 optimizes for the one thing WebM still can't match: sheer breadth of device and software compatibility.

WebM shows up on people's computers almost exclusively through browser-based tooling — the MediaRecorder API that powers Loom and countless screen-recording extensions, OBS's browser source captures, downloads from platforms that serve VP9 by default, and recordings pulled from WebRTC-based meeting tools. That origin story is exactly why WebM often needs converting before it can go anywhere else: Safari's WebM support has historically been inconsistent, meaning a WebM screen recording can simply fail to play for anyone on an iPhone or a Mac using Safari. Video editing software has a similar gap — a lot of professional and semi-professional editors, including older versions of Premiere Pro and various consumer editing apps, either don't import WebM at all or handle it unreliably compared to their mature, long-established MP4 pipelines. Windows Media Player and many embedded or smart-TV media players fall into the same category: built and tested against MP4/H.264 for years, with WebM support ranging from absent to flaky.

This conversion is a genuine re-encode, not a remux, because H.264 and VP8/VP9 are fundamentally different, incompatible video codecs — there's no way to just relabel a VP9 stream as H.264, the actual compressed bitstream has to be decoded frame by frame and recompressed from scratch. The encode targets CRF 23, FFmpeg's own default constant-rate-factor setting that aims for visually consistent quality rather than a fixed file size, which in practice means the output looks close to indistinguishable from the source for normal viewing even though it's a lossy-to-lossy transcode (VP9 is already lossy, so you're re-compressing already-compressed video, same as most conversions between different lossy codecs).

The -movflags +faststart flag places the MP4's metadata index at the start of the file rather than the end, which specifically helps when the converted video is going to be streamed or previewed in a browser — a faststart file can begin playing before the whole download finishes, while a file with the index at the end has to be fully retrieved first. Everything processes locally through FFmpeg WebAssembly, so a screen recording or downloaded clip you need in MP4 form never has to be uploaded to a conversion service just to get there.

How It Works

FFmpeg fully decodes the WebM's VP8 or VP9 video frames and re-encodes them as H.264 at CRF 23, a quality-targeted setting rather than a fixed bitrate, while Opus or Vorbis audio is re-encoded to AAC. The -movflags +faststart flag then places the MP4's metadata at the front of the file so it can begin streaming before the download completes. Because the video and audio codecs are being genuinely transcoded rather than copied, processing time scales with the source's resolution and length rather than finishing instantly.

How to Use

Upload your WebM file

Drop a screen recording, browser download, or Loom export onto the upload zone.

Confirm the file

Check the filename and size to make sure it's the correct source video before converting.

Click Convert to MP4

FFmpeg decodes the VP8/VP9 video and re-encodes it as H.264 with AAC audio, targeting visually consistent quality at CRF 23.

Wait for the re-encode

Because this is a genuine transcode between incompatible codecs, processing time depends on the video's resolution and length.

Preview if needed

Confirm the converted file plays correctly and looks right before deleting the original WebM.

Download the MP4

Save the finished file — it keeps your original filename with a .mp4 extension, ready to faststart-stream immediately.

Share or import anywhere

The MP4 is now safe to upload to social platforms, open in Safari or iOS, or import into virtually any video editor.

Who Uses This Tool

📱

iPhone & Safari Compatibility

WebM playback on Safari and iOS has historically been inconsistent — convert to MP4 so screen recordings and downloads play reliably.

💻

Sharing Screen Recordings

Browser recordings from Loom, OBS, or extensions default to WebM — convert to MP4 before sending to recipients on mixed devices.

🎬

Social Media Uploads

YouTube, Instagram, and TikTok all expect MP4 with H.264 — converting a WebM download first avoids upload issues.

✂️

Video Editor Import

Some video editing software handles WebM unreliably or not at all — convert to MP4/H.264 for a smoother import into your editing timeline.

📺

Smart TV & Set-Top Playback

Many smart TVs and set-top boxes have mature MP4/H.264 support but patchy or absent WebM decoding.

📧

Email & Messaging Attachments

MP4 attachments open more reliably across email clients and messaging apps than WebM on non-browser recipients' devices.

Why Choose This Tool

Fixes Real Playback Gaps

Re-encodes VP8/VP9 WebM into H.264 MP4, solving actual compatibility issues on Safari, iOS, and many editors rather than just relabeling the file.

CRF 23 Visual Quality Target

Uses a quality-targeted encode rather than an arbitrary fixed bitrate, keeping output close to indistinguishable from the source for normal viewing.

Faststart for Instant Playback

Metadata is placed at the start of the file so browsers and players can begin playback before the download finishes.

Handles Both VP8 and VP9

Converts whichever VP codec the source WebM uses into universally-supported H.264.

Fully Local Processing

FFmpeg WebAssembly re-encodes your video inside the browser tab — nothing is transmitted to a server during conversion.

No Size Cap, No Watermark

Free for files under 40 MB with no account; sign up free for 80 MB or go Pro for 250 MB — output is never watermarked.

Key Benefits

Fixes WebM playback gaps on Safari, iOS, and many smart TVs

CRF 23 targets visually consistent quality, not a lossy fixed-bitrate guess

Faststart flag lets the MP4 begin playing before it finishes downloading

Works with Loom, OBS, and other browser-recorded WebM sources

MP4/H.264 imports reliably into far more video editors than WebM

Browser-based — your source WebM file is never uploaded

No file size limits imposed by a server-side cap

Free with no watermark added to the finished video

Frequently Asked Questions

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WEBM to MP4

What is WEBM to MP4?

How It Works

How to Use

Who Uses This Tool

Why Choose This Tool

Key Benefits

Frequently Asked Questions

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