FLV (Flash Video) and MP4 (MPEG-4 Part 14) are built for different jobs. FLV is typically used for old web videos, some streaming tools, while MP4 is best for sharing, social media, streaming. Converting lets you open, share or upload your files wherever MP4 is expected.
How to use the FLV to MP4 converter
- Add your FLV filesClick “Choose Files” or drag and drop them into the box. You can add many files at once — or paste a file with Ctrl+V.
- Choose MP4 and adjust settingsMP4 is selected automatically. Optionally open Settings to change resolution, quality or frame rate.
- Convert and downloadPress “Convert”. Your MP4 files appear instantly — download them one by one or all together as a ZIP.
FLV vs MP4
FLV Flash Video
FLV was the format of Flash-era online video (early YouTube). Flash is gone, so FLV files usually need converting to play on modern devices.
- Compression
- Lossy (Sorenson/H.264)
- Features
- Video + audio
- Origin
- 2003, Adobe
- Best for
- Old web videos, some streaming tools
MP4 MPEG-4 Part 14
MP4 is the most compatible video format in the world. It plays on iPhones, Android, Windows, Mac, smart TVs and every browser, and is accepted by YouTube, Instagram, TikTok and WhatsApp.
- Compression
- Lossy (H.264/H.265)
- Features
- Video, audio, subtitles
- Origin
- 2001, MPEG
- Best for
- Sharing, social media, streaming
Why use ConvertMathly?
Files are processed on your device and never uploaded. Close the tab and they are gone.
No account, no daily cap, no queue and no watermark — convert as much as you want.
The same engine used by professional video tools, running in your browser via WebAssembly.
Windows, Mac, Linux, Chromebook, iPhone and Android — any modern browser, nothing to install.
Frequently asked questions
How do I convert FLV to MP4?
Add your FLV files above, make sure MP4 is selected and press Convert. The MP4 files are ready to download a few seconds later — no installation or account required.
Why does the first conversion take a little longer?
The video engine (FFmpeg compiled to WebAssembly, about 30 MB) is downloaded once and then cached by your browser. After that, conversions start immediately — even offline.
Is there a file size limit?
There is no artificial limit. Because the conversion runs in your browser, the practical limit is your device memory — files up to around 1–2 GB work on most computers. Long videos take longer on phones.
Are my FLV files uploaded to a server?
No. ConvertMathly converts files entirely inside your web browser using JavaScript and WebAssembly. Your files never leave your computer or phone, which makes it safe for private photos and documents.
Can I convert multiple FLV files at once?
Yes — add as many files as you like. They are converted one after another and you can download everything in a single ZIP.