FLV to MP4 Converter

How to Convert FLV to MP4 Online

1. Upload Your FLV Files: Drag and drop or click "Add Files" to select FLV clips from your computer. Batch upload is supported, and large recordings (multi-GB OBS captures, archived screencasts) are handled on our servers.
2. Pick a Quality Preset: Open Advanced Options and choose a Preset under Constant Quality — defaults to "Very High (Recommended)" which maps to a low CRF for near-source quality. Switch to Constant Bitrate for streaming targets, Variable Bitrate for size efficiency, or Specific file size to land on an exact MB cap. Audio Codec defaults to AAC for MP4 compatibility.
3. Resize or Trim (Optional): Use Preset Resolutions (4320p down to 144p), Resolution Percentage to scale by share of the source, Width x Height for an exact box, or Width / Height with aspect-ratio lock. Trim to a Time Range if you only need a clip from a long recording.
4. Convert and Download: Click Convert. Files are uploaded over an encrypted connection, processed on our servers, and deleted automatically after a few hours — no sign-up, no watermark, never shared. Need the reverse direction? See MP4 to FLV.

Why Convert FLV to MP4?

FLV (Flash Video) was released by Macromedia on September 10, 2003 and was the dominant container for web video for over a decade — early YouTube, Vimeo, Hulu, and Twitch all served FLV through the Adobe Flash Player browser plugin. Adobe ended support for Flash Player on December 31, 2020, and from January 12, 2021 onward Adobe blocked Flash content from running. Every major browser (Chrome, Firefox, Edge, Safari) removed the Flash plugin years ago, which means FLV files now require a third-party player (VLC, MPV, PotPlayer) or conversion to play at all. MP4 (ISO/IEC 14496-14) replaces it as the universal container — supported natively by every modern OS, browser, smart TV, phone, and editing app.

• Make legacy recordings playable again — OBS Studio defaulted to FLV for its safety on crashes; thousands of streamers and lecturers have archives of FLV captures from 2014-2020 that now need MP4 wrappers for Premiere, DaVinci Resolve, Final Cut, and CapCut to import them.
• Edit in modern NLEs — Premiere Pro and Final Cut dropped reliable FLV ingest years ago. Remuxing to MP4 (keeping the H.264 stream untouched) restores instant import.
• Upload to current platforms — YouTube, Instagram, TikTok, X, LinkedIn, and Facebook all accept MP4. Most reject FLV outright or transcode it badly.
• Preserve early YouTube downloads — clips saved by 2005-2015 downloaders (KeepVid, Vdownloader) frequently landed as FLV. Converting preserves them before all major players drop legacy codec support.
• Stream through HTML5 video — <video> elements play MP4 directly; FLV requires JavaScript wrappers like flv.js plus an MSE-capable browser, which is fragile for archival hosting.
• Smaller file with the same picture — if your FLV uses Sorenson Spark or VP6, re-encoding to H.264 inside MP4 typically cuts size 30-50% at matched quality.

FLV vs MP4 — Container Comparison

Remux vs Re-encode — Which Should You Pick?

Most FLV files (especially OBS recordings and YouTube downloads from 2010+) already contain H.264 video and AAC audio — the same codecs MP4 uses. In that case the file can be remuxed: the video and audio streams are copied bit-for-bit into a new MP4 container with no quality loss and almost no processing time. If the FLV contains older codecs (Sorenson Spark, VP6, Nellymoser), MP4 has to re-encode the video — slower, lossy, but the result still plays everywhere.

Choose the default Very High preset if you're not sure — it will remux when streams are compatible and re-encode at near-source quality otherwise.

Frequently Asked Questions

Why won't my FLV file play anymore?

Browsers and OSes no longer ship Flash Player after Adobe's December 31, 2020 end-of-life date, and from January 12, 2021 Adobe actively blocks Flash content. Standalone players like VLC and MPV can still open FLV, but anything you want to share, edit, or upload needs an MP4 wrapper.

Is converting FLV to MP4 lossless?

It can be. If the FLV already holds H.264 video and AAC audio (typical for OBS captures and YouTube clips from roughly 2010 onward), the streams are remuxed directly into MP4 with zero quality loss. If the FLV uses older codecs (Sorenson Spark, On2 VP6, Nellymoser audio), MP4 has to re-encode and you'll see a small generation loss — usually invisible at the default Very High preset.

How do I know which codec my FLV uses?

Open the file in VLC and check Tools → Codec Information, or run ffprobe input.flv. OBS Studio recordings from 2016+ are virtually always H.264 + AAC. Files from early YouTube downloaders and Flash-era web players are more often VP6 or Sorenson Spark.

Will the converted MP4 work on iPhone, Android, smart TVs, and PowerPoint?

Yes — MP4 with H.264 video and AAC audio is the most widely supported video combination in the world. The default preset produces a file that plays natively on iOS, Android, Windows, macOS, Linux, Apple TV, Roku, Fire TV, Chromecast, modern smart TVs, and embeds cleanly in PowerPoint, Keynote, and Google Slides.

Can I keep the original resolution and frame rate?

Yes. Leave Video Resolution on "Keep original" and the source's resolution, frame rate, and aspect ratio carry over unchanged. Only switch to a Preset Resolution if you specifically need to downscale for upload limits or upscaling for editing parity.

What's the file size limit?

XConvert processes files on its servers and deletes them automatically after a few hours. Multi-GB OBS recordings and long screencasts convert fine on modern desktops; on a phone with limited RAM you may want to trim first using Trim FLV.

Should I pick Constant Bitrate, Variable Bitrate, or Constant Quality?

Use Constant Quality (the default) when you want the best picture for the smallest reasonable size — the encoder spends bits where they're needed. Pick Variable Bitrate when you have a target average bitrate (uploading to a platform that recommends one). Pick Constant Bitrate only for legacy streaming or hardware decoders that demand a fixed rate. Specific file size is for "this MP4 must be under 25 MB to email" cases.

Why is my OBS FLV so much smaller than expected after conversion?

OBS often records at high constant-quality CRF values to keep the file safe on crashes. Converting at the same or slightly higher CRF doesn't add detail, so the new MP4 may shrink 5-15% just from container overhead differences. To shrink further, run the output through Compress FLV before conversion, or compress video online afterward, or pick a lower preset.

Can I extract just the audio from an FLV?

Yes — use FLV to MP3 if you want a standalone audio file. The MP4 conversion here always preserves the audio track alongside the video.