FLV to MP4
In your browser, no upload
MP4
Flash Video → H.264 · re-encoded
Video
FLV → MP4Specification
7 rows| Item | Value | Notes |
|---|---|---|
| Input | FLV | Or any video FFmpeg reads |
| Output | MP4, H.264 + AAC | Plays on phones, browsers and editors |
| Picture | Re-encoded as H.264 | x264, veryfast preset, CRF 23 |
| Sound | AAC | Copied when it is already AAC or MP3, else AAC 128 kbps |
| Time | Measured before it starts | A 2-second test encode on this device |
| Engine | This browser’s own encoder, or FFmpeg (16.8 MB) | FFmpeg is downloaded only when the browser cannot do the job, and only when you press the button that says so |
| Processed in | This browser | Not sent to a server |
What happens
5What happens, and why
FLV is Flash Video, the format YouTube and many other video sites used in the Flash years, and the one many old downloads and live-stream recordings are still in. Inside is usually H.264 with AAC sound, sometimes the older Sorenson Spark or On2 VP6 picture with MP3 sound. Adobe ended Flash Player at the close of 2020, and players and phones stopped reading FLV with it.
This page writes the same video as an MP4. The picture is re-encoded as H.264. The sound, when it is AAC or MP3, is copied into the MP4 without re-encoding; otherwise it is re-encoded as AAC at 128 kbps. An FLV that already holds H.264 would not strictly need its picture re-encoded, but the tool on this site that copies instead (MOV to MP4) reads only MP4-family files, so here it is re-encoded, and the badge says so.
First the page asks the browser itself. Browsers carry the video encoders they use for calls and screen recording — H.264, VP9, AAC, Opus, often on the graphics chip — and when this browser can read your file and write the result with them, that is what happens: nothing is downloaded, the time is measured here first, and the line under the badge says the browser’s encoder did the work. MP4, MOV, WebM and MKV files usually qualify. AVI, WMV and FLV do not, and neither does an MP3, so those go the way below.
The work is done by FFmpeg, the program most video software is built on, compiled to WebAssembly so that it runs inside this page. It is large — 16.8 MB — and it is not downloaded until you press the button that says so. Then it reads your file, encodes 2 seconds from the middle of it as a test, and tells you how long the whole file will take on this device, before anything starts. That figure is measured here rather than taken from a table: the same engine and settings, your processor, your video. It is still an estimate, and it is labelled as one.
It is slower than a program installed on a computer. Here FFmpeg runs on one processor core, because a browser only lets a page use more when the site is served with particular security headers, and it cannot use the graphics chip’s video encoder at all. A long high-resolution video can take many minutes, and longer on a phone. The estimate says so before you commit to it, the progress line shows the time left as it goes, and Cancel stops it at once. When the browser’s own encoder can do the job, it is far quicker, and nothing is downloaded.
FAQ
4Does it lose quality
A little, as any re-encode does, and the badge says so. The picture is encoded with x264 at CRF 23, a constant-quality setting that spends bits where the picture needs them; at normal viewing most people cannot tell it from the source. What is lost is not recovered by converting back.
Why does it take so long
Every frame is decoded and encoded again, on one core of your processor, inside the browser. Installed programs use every core and usually the graphics chip’s encoder; a web page gets neither by default. The page measures how long it will be on your device before you start, so the wait is never a surprise.
What if my phone runs out of memory
The page stops and says so, rather than handing back half a file. The video you drop is read from disk in pieces, never copied whole into memory, but the new file is built in memory until it is finished, so a long, high-resolution video can be too much for a phone. A shorter clip, a smaller size, or a computer will get through.
Is the video uploaded
No. The engine is downloaded to your browser, and your file is read and written there. There is no upload endpoint on this site, so there is nowhere for the video to go.