Why does my video say it is 1920 by 1080 when it is a portrait clip?

Because it is stored that way with a 90 degree rotation flag, which is how phones record. The page shows both the stored size and the size it displays at when they differ.

What is fast start and does it matter?

It means the index sits before the media, so a player can begin before the file has downloaded. It matters for a video served from a web page and not at all for one opened from your own disk.

Why is the frame rate 29.97?

That is a real rate, inherited from NTSC broadcast, and equals 30000/1001. Plenty of cameras and screen recorders still use it.

Can it tell me the codec of a WebM file?

Not yet. WebM uses a different container format from MP4 and this reads MP4 atoms. The duration and picture size still come from the player.

Video Metadata Viewer

Reads the file's own description of itself: which codec, how many frames a second, what the audio track is, and whether the picture carries a rotation flag. Only the first few megabytes are read, so it answers immediately.

Choose a video or drop it hereMP4, MOV or WebM. Only the first few megabytes are read.

Runs entirely in your browser. Your files are never uploaded.

How to use Video Metadata Viewer

1Choose a video, or drag it onto the box.
2Read the codec, frame rate and track details.
3Check the rotation if the video is appearing sideways.
4Copy the details out if you need them elsewhere.

Where the numbers come from

Two sources, because neither is complete on its own. The player reports the duration and the size the picture comes out at. The container's own atoms hold the codec, the frame rate, the frame count and the rotation, and nothing in the browser exposes those.

The frame rate is worked out from the table of sample durations, which is why it can come back as 29.97 rather than 30: that is genuinely what a lot of footage runs at, being 30000 divided by 1001.

Rotation, and the sideways video

Phones record in landscape and write a rotation into the track header rather than turning the pixels. A portrait clip is therefore stored as 1920 by 1080 with a 90 degree flag, and plays upright because the player reads the flag.

Software that ignores the flag shows it on its side, which is the usual explanation for a video that looks fine on a phone and wrong once it is edited or uploaded. Both numbers are shown here when they differ.

Why a video stalls before it plays

An MP4 keeps its index in a moov atom, and the index can sit at the front of the file or the end. At the front, a player can start as soon as the first part arrives. At the end, it has to fetch the whole file first, which is why some videos sit there before starting.

This says which arrangement a file uses. Moving it is what the fast start option in encoders does.

Limits

Codec, frame rate and rotation are read from MP4 and MOV. For WebM and Matroska only what the player reports is available.
Only the first four megabytes are read, so a file with its index at the end shows fewer details.
Nothing is changed. This reads a file and reports what it finds.
Variable frame rate footage reports the rate of its first run of frames.

Frequently asked questions

Why does my video say it is 1920 by 1080 when it is a portrait clip?: Because it is stored that way with a 90 degree rotation flag, which is how phones record. The page shows both the stored size and the size it displays at when they differ.
What is fast start and does it matter?: It means the index sits before the media, so a player can begin before the file has downloaded. It matters for a video served from a web page and not at all for one opened from your own disk.
Why is the frame rate 29.97?: That is a real rate, inherited from NTSC broadcast, and equals 30000/1001. Plenty of cameras and screen recorders still use it.
Can it tell me the codec of a WebM file?: Not yet. WebM uses a different container format from MP4 and this reads MP4 atoms. The duration and picture size still come from the player.