Should you export a screen recording as WebM or MP4?

Which format should you pick?

Choose WebM when the video is going somewhere on the web — an upload, a chat message, a ticket attachment. Choose MP4 when it is going into software that is fussy about formats: a video editor, a presentation, a phone camera roll, or a colleague on an older machine.

WebM is what Screencast records natively, so exporting it is effectively instant regardless of how long the recording is. Nothing is re-encoded; the file you already have is handed to your browser's downloads.

MP4 requires a conversion step, which takes time proportional to the length of the recording and the capability of your device. The upside is compatibility: MP4 is the format almost everything will accept without complaint.

When you are unsure, export WebM first. It costs you nothing, gives you a file immediately, and you can always come back and export MP4 from the same recording afterwards — the recording stays in the review panel until you clear it.

What actually accepts WebM

Every major browser, YouTube, Google Drive, Slack, Discord, Notion, and most modern ticketing and documentation tools play WebM without any conversion.

If the destination is a web page or a web application, WebM is almost always fine and often preferable — the files are typically smaller than an equivalent-quality MP4, which matters for upload time and attachment limits.

Where WebM tends to fail is outside the browser. Older versions of Windows Media Player, QuickTime on macOS, PowerPoint and Keynote, the iOS and Android photo libraries, and several video-editing applications will either refuse it or import it without audio.

The practical test is who receives it. A file going into a shared document, a pull request, or a support ticket can be WebM. A file going to someone who will double-click it on their desktop, or into an editing timeline, should be MP4.

How the MP4 conversion works, and why it is sometimes slow

The conversion happens on your device, using WebCodecs when the browser exposes a compatible encoder and an in-page FFmpeg fallback when it does not.

Screencast first tries WebCodecs and asks the browser to prefer hardware acceleration. The browser may still choose a software implementation, and support depends on the input dimensions, codec, browser, and device.

If that path is unavailable or fails, Screencast falls back to FFmpeg running in the page. Conversion time can vary substantially with duration, dimensions, device performance, and available memory. Screencast identifies the active path and shows progress while it runs.

Neither path uploads anything. The message shown during conversion — that your video is being processed only on this device — is a literal description of what is happening, not a reassurance about a server-side process.

Keep the tab open while it works. The conversion is running in that page; navigating away or closing the tab cancels it, and you will have to start again from the recording, which is still saved.

Does converting to MP4 reduce quality?

Slightly, in principle — it is a re-encode rather than a repackage — but the difference is rarely visible in screen-recording content at normal viewing sizes.

Screen recordings are unusually forgiving here. They are mostly static regions, flat colour, and text, which compresses efficiently and shows re-encoding artefacts far less readily than camera footage of a moving scene.

Where you will see it is in fine text at small sizes and in fast-scrolling content. If the recording is a code walkthrough that people will pause and read, exporting WebM and converting elsewhere with settings you control will preserve more detail than any single-button conversion.

The bigger quality lever is upstream of export entirely. Recording at your display's native resolution, leaving the frame on Original, and avoiding unnecessary browser zoom does more for legibility than anything you choose at export time.

Managing file size

Length and visual activity drive size far more than the format does. A still slide deck records tiny; a scrolling page or a video playing inside your recording records large.

If a file is too big for where it needs to go, the effective options are recording a shorter take, recording a smaller surface, or trimming afterwards. Choosing a different export format will not change the order of magnitude.

Recording a single window or browser tab instead of a full multi-monitor desktop is the largest single saving available, and it usually improves the recording anyway by removing everything the viewer did not need to see.

For trimming, Screencast links out to a separate free browser tool rather than building an editor into the recorder. It opens in a new tab and works on the file you have already downloaded.

When MP4 export fails

Export WebM instead — you will still have your recording — then convert separately if you need MP4.

Screencast reports a specific reason when the encoder cannot complete: no compatible hardware encoder for that recording, an encoder that produced no output, or a compatibility encoder that could not finish. In every case the underlying WebM recording is untouched and still exportable.

The most common practical causes are a very long recording on a machine short of memory, or a browser that claimed a hardware encoder it could not actually use for this particular video. Trying again after closing other tabs sometimes succeeds; trying again in Chrome or Edge more often does.

If you have already downloaded the WebM and simply need MP4, any local conversion tool will do it. That keeps the file on your machine, which is the same guarantee Screencast makes.

Use Download MP4 again if the conversion completed but your browser did not save the file. Screencast keeps the converted result in the page until you leave it, so a blocked or missed download does not mean re-running the conversion.