CONVERT
H264 → FLV
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Fast, secure H264 to FLV conversion. No registration required.
H264 is a video container, so playback depends on the codec inside as well as the wrapper itself. Reaching a FLV from there is one hop. If you need a FLV version of a H264 clip for a social platform, a stock site or a CMS upload widget, this tool handles the job without re-rendering anything when it does not have to. The output is the same pixel data in a container the destination actually accepts. In practice H264 is a video container, so playback depends on the codec inside as well as the wrapper itself. On the other end, FLV is the Adobe Flash Video container, now deprecated but still lingering in archives.
H.264 Raw Stream
Source formatH.264 raw stream is an elementary bitstream containing only the video data encoded with the H.264/AVC codec without any container. It is commonly used as an intermediate format in video processing pipelines and for hardware encoder output.
Flash Video
Target formatFLV was the dominant web video format during the Flash era. While Flash is now deprecated, many legacy video files still exist in FLV format and need conversion to modern formats.
Why convert H264 to FLV
Sending H264 to someone on a non-matching operating system frequently leads to "file cannot be opened". FLV avoids that by sitting in the middle of everyone's compatibility list. The repackage runs quickly and without generational loss when codecs already align.
HOW TO CONVERT
H264 → FLV
Drop the video file
Select a H264 file. We read the container and stream descriptors to plan the conversion.
FFmpeg handles the repackage
When codecs align, FFmpeg rewraps the existing streams into a FLV container — no quality loss, near-instant finish.
Retrieve the FLV
The FLV download is ready in seconds for stream-copy jobs, minutes for full transcodes.
Common Use Cases
Share across platforms
Send FLV files to anyone without worrying about whether they have the right software for H264.
Embed in documents
Drop FLV output into Word, Google Docs, PowerPoint, Notion or a website without conversion warnings.
Optimize size
FLV often produces smaller files than H264 for web, email and storage.
Archive & future-proof
Store in a widely-supported format that will still open on future operating systems without legacy plugins.
H264 vs FLV — Strengths and limitations
What each format does best, and where it falls short.
H264 Strengths
- Universal hardware decode on every device since ~2010.
- 40-50% smaller than MPEG-2 at equal quality.
- Mature ecosystem with dozens of encoders (x264 is the open-source gold standard).
- Every browser, phone, TV, and car infotainment supports H.264.
- Supports everything from 144p vertical phone video to 8K HDR masters.
Limitations
- Patent-encumbered — encoding royalties apply for commercial use.
- 30-50% larger than H.265/AV1 at equivalent quality.
- Raw .h264 bytestreams have no timecode — containers (MP4/MKV) add that.
FLV Strengths
- Low overhead — the container is extremely compact.
- Designed for streaming — progressive download and seeking work well.
- Decoded natively by Flash Player on every OS for 20 years.
Limitations
- Flash Player is dead — no modern browser can play FLV without conversion.
- Legacy codecs (Sorenson, VP6) are poorly supported in modern tooling.
- Hardware video decoders never added FLV support.
H264 vs FLV — Technical specifications
Side-by-side comparison of the technical details.
H264
- MIME type
- video/h264
- Extensions
- .h264, .264, .avc (raw bytestream)
- Standard
- ITU-T Rec. H.264 / ISO/IEC 14496-10 (AVC)
- Typical containers
- MP4, MKV, MOV, TS, FLV
- Profiles
- Baseline, Main, High, High 10, High 4:2:2, High 4:4:4
FLV
- MIME type
- video/x-flv
- Extensions
- .flv, .f4v
- Video codecs
- Sorenson Spark, VP6, H.264 (F4V)
- Audio codecs
- MP3, Nellymoser, AAC
- Status
- Deprecated since December 31, 2020
| Specification | H264 | FLV |
|---|---|---|
| MIME type | video/h264 | video/x-flv |
| Extensions | .h264, .264, .avc (raw bytestream) | .flv, .f4v |
| Standard | ITU-T Rec. H.264 / ISO/IEC 14496-10 (AVC) | — |
| Typical containers | MP4, MKV, MOV, TS, FLV | — |
| Profiles | Baseline, Main, High, High 10, High 4:2:2, High 4:4:4 | — |
| Video codecs | — | Sorenson Spark, VP6, H.264 (F4V) |
| Audio codecs | — | MP3, Nellymoser, AAC |
| Status | — | Deprecated since December 31, 2020 |
H264 vs FLV — Typical file sizes
Approximate file sizes for common scenarios.
H264
- 1080p 30fps @ 5 Mbps (1 min) ~37 MB
- 4K 60fps @ 35 Mbps (1 min) ~260 MB
- HD streaming (1 hour, 6 Mbps) ~2.7 GB
FLV
- 10-min YouTube 2008-era video 40-80 MB
- 45-min TV show (FLV H.264) 200-500 MB
Quality & Compatibility
Resolution, frame rate and colour space are preserved end-to-end. If the FLV container does not support some H264 features (chapters, multiple subtitle tracks, DRM-protected streams), those are flattened or dropped with a warning. Hard-coded subtitles in the video frames always survive.
Tips for Best Results
- Embedded subtitle tracks convert between H264 and FLV when both containers support the same subtitle codec; otherwise burn the subtitles into the video first.
- For portrait (9:16) clips, make sure the FLV encoder preserves the correct display aspect ratio — some players default to 16:9 if SAR is ambiguous.
- Long recordings (over an hour) benefit from chapter metadata; FLV may not preserve H264 chapters — check before relying on them.
Frequently Asked Questions
Only when it has to. If the codecs inside H264 (usually H.264 or H.265 for video, AAC for audio) are accepted by FLV, we stream-copy — the bytes are repackaged into the new container with zero re-encoding and no quality loss. When the source uses a codec the target does not support, we transcode at a matching bitrate to keep the visual quality close to the original.
With stream copy, expect the job to finish in seconds to tens of seconds regardless of video length — the work is mostly rewriting the container. Transcoding is slower (roughly real-time: a ten-minute clip takes about ten minutes) because every frame must be decoded and re-encoded. The progress bar shows which mode applies.
Yes. Resolution, frame rate, colour space and bit depth are preserved by default; stream copy is literally bit-identical on these parameters. If you explicitly pick a lower bitrate or a different codec in Advanced, the output is rebuilt to those settings, but the default is always "match the source".
Related comparisons
See these formats side by side to understand which fits your use case best.
Related Guides
H.264/AVC Video Codec: Complete Technical Guide
Complete technical guide to H.264/AVC video codec: motion compensation, intra prediction, profiles (Baseline/Main/High), levels, CRF quality, x264 encoder settings, and container formats.
Read guideFLV Files: Flash Video Format History, Codecs, and Conversion Guide
Complete guide to FLV (Flash Video) files — Sorenson/VP6/H.264 codecs, file structure, YouTube history, FFmpeg conversion to MP4/WebM, batch processing, and digital preservation.
Read guideHEVC / H.265 vs H.264: Differences, Compatibility, and When to Use Each
What is HEVC H.265, how much better is the compression vs H.264, which devices support it, and when does it make sense to use H.265 instead of H.264.
Read guideSecure & Private Conversion
Your files are encrypted during transfer, processed in isolated containers, and automatically deleted within 60 minutes. We never read, share, or store your data.