A video upload format is more than a file extension. The container (MP4, MOV, WebM, MKV, AVI) holds video, audio and metadata; the codecs (H.264, H.265, VP9, AV1, ProRes for video; AAC, MP3, Opus, PCM for audio) determine how those streams are encoded. An online editor must support both.
The practical starting point: MP4 containing H.264 video and AAC audio is widely compatible. It is a starting point, not a universal rule; always check the documented requirements of the editor you use.
Container versus codec, once and for all
A container is packaging; a codec is compression. Two files ending in .mp4 can contain entirely different codecs and behave entirely differently on upload. That single fact explains most "but it's an MP4!" support tickets.
The containers in short: MP4 for delivery and online playback, MOV for recording and professional workflows (large files, sometimes unsupported codecs), WebM for browser-focused media, MKV for flexible multi-stream storage and not universally accepted, AVI for older workflows and often oversized. These are descriptions, not guarantees that a given editor supports them.
The properties that matter
Resolution (1920 by 1080, 1080 by 1920, 1080 by 1080) is pixel dimensions; higher does not automatically mean better, and it does mean bigger files and slower processing. Aspect ratio is shape; never stretch footage into a different one, crop or reframe instead. Frame rate: preserve the source rate where possible, since unnecessary conversion duplicates or drops frames. Watch for variable frame rate from phones and screen recorders, a common cause of drifting audio and imprecise cuts; converting to constant frame rate often fixes it.
Bitrate governs compression artefacts: blockiness, blurred motion, banding, unreadable screen text. The chain to respect is original, compressed upload, re-encoded working file, compressed export; avoid heavily compressing footage several times before editing. Colour: HDR footage in an SDR workflow can look washed out or oversaturated, so a consistent SDR pipeline is easier for standard social and web work.
When uploads fail
The nine usual causes: unsupported container or codec; a decode failure after a successful upload; missing audio from an unsupported audio codec; sync problems from variable frame rate; wrong orientation from rotation metadata; washed-out colour from HDR conversion; an interrupted transfer; a damaged recording; and processing that never finishes on an unsupported property.
The response order: confirm the documented limits, test the source locally, check browser and connection, re-encode only when needed, and always preserve the original. Do not repeatedly re-upload the same file without first checking the error and the file's technical properties.
Exporting
Match the container, codecs, aspect ratio, resolution, frame rate and audio to the destination, and check the frame: nothing stretched, nothing important cropped, captions not clipped. Then remember why an export can look worse than the preview: a weak source, repeated re-encoding, too low a bitrate, an unnecessary resolution change, colour conversion, or platform compression after upload. A larger export file is not automatically better.
Where ReadyForm fits
ReadyForm accepts MP4, MOV and M4V, up to 500 MB per file and 20 files per video, and renders server-side so no local export settings are involved. Those limits are shown in the uploader itself, which is where the current values always live; product requirements change faster than any guide. See how the edit is made or the supported files help article.