MKV to MP4: Convert Existing Video Without Generating It Again

Local FFmpeg commands

Install FFmpeg from its official website first. Run these in a terminal with your own file paths; use a new output filename. The first command copies compatible streams; the second encodes H.264 video and AAC audio.

ffmpeg -i input.mkv -map 0:v:0 -map "0:a:0?" -c copy -movflags +faststart output.mp4

ffmpeg -i input.mkv -map 0:v:0 -map "0:a:0?" -c:v libx264 -crf 20 -pix_fmt yuv420p -c:a aac -b:a 192k -movflags +faststart output-h264.mp4

Try remuxing first

If your source contains codecs supported by the destination MP4 player, stream copy can move the video and audio into a new container without encoding them again. Keep the original file and write a separate output. A fast remux preserves the encoded picture quality, but cannot make an unsupported codec compatible.

Convert when the MP4 will not play

For common browser delivery, H.264 video with AAC audio is a practical option. Encoding takes longer and may change quality. Check your source dimensions before encoding: some inputs need scaling or padding to even dimensions. The example selects the first video and optional first audio track; it intentionally leaves subtitle and attachment handling to a separate review.

Check the delivery copy

Play the beginning, middle and end. Confirm duration, aspect ratio, lip sync and the language of the selected audio track. MKV can contain multiple audio tracks, subtitles and attachments that an MP4 copy may omit. Keep a subtitle sidecar or burn approved captions into a separate delivery version if your player cannot display them.