Converting iPhone WebM video files to MP4

The easiest way is to convert that VP8/VP9 video & OPUS audio .webm to H.264/H.265 & AAC .mp4 with Handbrake. Its default preset is quite good.

https://handbrake.fr/

Or, use ffmpeg to encode H.265 & AAC .mp4 via the Terminal with something like:

ffmpeg -i input.webm -c:v libx265 -crf 28 -preset medium -color_range limited -pix_fmt yuv420p10le -profile:v main10 -x265-params colorprim=1:transfer=1:colormatrix=1:range=limited -timecode 00:00:00:00 -tag:v hvc1 -c:a aac_at -b:a 128k output.mp4

The libx265 encode above uses CPU. If you want much faster encode with slightly lower quality output with he same file size, use Apple's VideoToolbox hardware-accelerated encode (tweak "-b:v 1000K" to set the desired output size):

ffmpeg -i input.webm -vf setsar=sar=1/1,setparams=range=limited:color_primaries=1:color_trc=1:colorspace=1 -c:v hevc_videotoolbox -b:v 1000K -color_range limited -pix_fmt p010le -profile:v main10 -timecode 00:00:00:00 -tag:v hvc1 -c:a aac_at -b:a 128k output.mp4

Oh, if you want to play .webm on an iPhone, download VLC. It can play .webm from Files.app folders.

HandBrake is a great app, and Matti Haveri's suggestion is right on, but it's not an iPhone app. I think that you really need a computer to do the conversions.

Do your WebM files play in the Files app?

Matti Haveri wrote: Oh, if you want to play .webm on an iPhone, download VLC. It can play .webm from Files.app folders.

Right! VLC is so versatile. And there's nothing wrong with keeping some videos in Files rather than putting them in Photos.