Convert FLV to MKV — Free & Private

Will my video be re-encoded or just remuxed when converting FLV to MKV?

Often a remux. Matroska is a flexible container that can hold both of the codecs commonly found in FLV files — H.264 (used by most FLVs from 2008 onward) and VP6/Sorenson (older Flash exports). When the codec is supported, Convertlo repackages the video into MKV directly, with no quality loss and almost no processing time.

• H.264-in-FLV → MKV: typically a fast, lossless remux
• VP6/Sorenson-in-FLV → MKV: also remuxed in most cases, since MKV supports these older codecs too
• A remux only repackages the stream — it won't fix playback on devices that can't decode VP6, even inside MKV

Is FLV to MKV a lossless remux, or does it re-encode — what happens to VP6 and H.264 FLV streams?

H.264 FLV (the standard since 2007) converts via remux — the video stream is moved into MKV without re-encoding, so quality is bit-for-bit identical. Older VP6 or Sorenson Spark FLV must be re-encoded to H.264, which adds one round of lossy compression.

• H.264 FLV (post-2007 Flash video): remuxed into MKV — no re-encode, identical quality
• VP6 / Sorenson Spark FLV (pre-2007): re-encoded to H.264 — slight quality loss, more at low bitrates
• Audio (AAC or MP3): carried over without re-encoding in either case

How much will the file size change going from FLV to MKV?

If the conversion is a remux (the common case for H.264 FLV), the file size barely changes — you're repackaging the same video data into a different wrapper. Only if Convertlo needs to re-encode an older VP6/Sorenson stream will the size shift noticeably, usually getting smaller as it moves to a more modern codec.

• H.264 FLV → MKV remux: file size essentially unchanged
• VP6/Sorenson FLV → MKV (re-encoded): often smaller, depending on target codec and bitrate
• MKV's container overhead is negligible compared to the codec's effect on size

Is there any quality difference between the original FLV and the MKV output?

For H.264 FLV: no difference — it's a lossless remux. For VP6 or Sorenson Spark FLV: a small quality loss occurs during re-encoding. In both cases, the MKV output plays in VLC, Plex, and Kodi; the FLV original doesn't.

• H.264 FLV → MKV: bit-for-bit remux — zero quality loss
• VP6 / Sorenson Spark FLV → MKV: one generation of re-encoding — minor loss at moderate bitrates
• Audio: AAC or MP3 from FLV transfers without re-encoding either way