LightBox

Online video converter

WebM to MP4 — Convert to a Video That Plays Anywhere

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MP4, MOV, WebM, GIF, JPG, PNG, WebP, HEIC, ZIP

Up to 100 MB free · 1 GB with Pro

HEIC, animated WebP input, PNG sequences and frame sequence export require Pro.

WebM is a brilliant format for the browser and a headache almost everywhere else. The moment you take a .webm somewhere the web isn't — a video editor, your phone's camera roll, a PowerPoint or Keynote slide, a client's inbox, a smart TV, or a social upload flow — it's the file that won't open. MP4 with H.264 is the exact opposite: three decades of devices, apps, and platforms treat it as the default video. Converting WebM to MP4 turns "the file that won't play" into "the file that plays everywhere."

LightBox re-encodes your WebM to H.264 with FFmpeg on the server, wrapped in a fast-start MP4 that begins playing before it has fully downloaded and uses the broadly compatible color format that phones and editors expect. Your audio comes across as AAC, or you can drop it for a silent clip. Nothing to install, and never a watermark on the result.

You set the terms before you export: choose a quality level, set the frame rate, trim to the seconds that matter, crop off the bars, and resize to the width your destination needs. Drop in a WebM and download a clean, universally playable MP4 in seconds.

Why convert with LightBox

Plays on every device and app

H.264 in an MP4 is the closest thing there is to a universal video. Phones, TVs, editors like Premiere and Final Cut, PowerPoint and Keynote, messaging apps, and every major social platform accept it without a second thought — the one thing WebM can't promise. After converting, the clip simply opens wherever you send it.

Keeps the audio, holds the quality

Your WebM's sound is re-encoded to AAC — the audio track MP4 players expect — so voiceover, music, or dialogue survives intact, or you can switch it off for a silent clip. The video is encoded with x264 in constant-quality mode, so motion and detail stay clean; the quality slider lets you lean toward the source or toward a smaller file.

Trim and size it before you export

A clip headed for a phone doesn't need the same settings as one going into an edit. Set the frame rate and quality, trim to the exact in and out points, crop to the frame you want, and resize to your target width. It runs server-side on real FFmpeg — a properly muxed MP4, not a renamed file that breaks on upload — and never carries a watermark.

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LimitFreeLifetime Pro
File size100 MB1 GB
Per day4 filesUnlimited
Files kept1 hourForever
Batch1 file20 files
QueueStandardPriority
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Frequently asked questions

Compatibility. WebM is efficient and web-native, but plenty of things outside the browser still won't touch it — video editors, phone camera rolls, presentation software, some messaging apps, older devices, and a number of social upload flows. MP4 with H.264 plays in essentially all of them, so convert when the clip needs to leave the browser and just work.

Yes, by default — the soundtrack is re-encoded to AAC, the audio format MP4 players expect, so voiceover, music, or dialogue comes across intact. If you want a silent clip — for a muted autoplay background, say — switch audio off before you export to shed a little size.

Not visibly, at sensible settings. The video is re-encoded with x264 in constant-quality mode, which spends bits where your eye actually looks, so at the default quality the result looks clean. You control the quality slider, so you can push closer to the source or lean toward a smaller file — and re-export in seconds if a result looks off.

Sometimes, though it's often close. WebM's VP9 codec is a little more efficient byte-for-byte than the H.264 most MP4s use, so the same clip can come out slightly larger — the trade you're making is universal compatibility for a small size difference. Lowering the quality, dropping the frame rate, trimming, or resizing all bring the size back down.

Just about anything. Drop it into a video editor, add it to a phone's camera roll, embed it in a slide deck, attach it to an email, upload it to a social platform, or play it on a TV — H.264 MP4 is the format all of those assume. It's the one to reach for whenever a clip has to work outside a web page.

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