A video refuses to play, or plays with sound but no picture, or takes forever to start. The answer is almost always in the file's header — which codec, which container, how it was laid out. Most tools that read it want you to upload the file first, which for a 4 GB video is both slow and a strange thing to agree to. This one reads the header in your browser and never sends the file anywhere.
Why nothing needs to be uploaded
An MP4 is a tree of boxes. A small one near the start, ftyp, names the format. A larger one called moov holds the metadata for every track: codec, dimensions, duration, sample rates. The actual video and audio sit in mdat, which is essentially the whole file.
Everything worth reporting lives in moov, and moov is usually a few kilobytes. Your browser can read a slice of a file without loading the rest, so this page reads the first few megabytes, parses the boxes there, and stops. A four-gigabyte video is inspected as quickly as a four-megabyte one, because the difference between them is entirely mdat and mdat is never touched.
Why some videos are slow to start
The order of those boxes matters. If moov sits after mdat, a player streaming over a network cannot begin until it has fetched to the end of the file to find the metadata. That is why a video can start instantly from your own disk and stall for ages from a server.
Moving moov to the front is called fast start, and this page reports whether a file has it. Fixing it does not re-encode anything — it rewrites the layout — which is why it takes seconds even on a large file.
Why won't this video play?
Nearly always the codec, not the file extension. .mp4 is a container, and containers hold different things. The common cases:
H.265 / HEVC (hvc1, hev1) — excellent compression, patchy browser support. Safari plays it; Chrome and Firefox depend on the operating system and hardware. A file that plays on an iPhone and not on a Windows laptop is usually this.
H.264 / AVC (avc1) — plays essentially everywhere. If compatibility matters more than file size, this is the safe answer.
AV1 (av01) — better compression than both, supported in recent browsers, but slow to decode without hardware support on older machines.
Apple ProRes (apcn, ap4h) — an editing format, not a delivery one. Huge files, and no browser will play it. If a camera or editor handed you this, it needs converting before it goes anywhere near a web page.
What the bitrate tells you
The average bitrate here is file size divided by duration, which is the honest measure of how much data per second a viewer must receive. Compare it against the connection you expect them to have: a 12 Mbps video is unwatchable on a 6 Mbps line no matter how good the encode is. Our speed test gives you the other half of that comparison.
What this does not read
MP4, M4V and MOV only — all variants of the same ISO base media format. MKV, AVI, WMV and WebM use different container structures and are not parsed here. Nor does this decode the video: frame rate, colour space and encoder settings live inside the codec configuration and would need a full decoder to read reliably, which is exactly the several megabytes of WebAssembly this page avoids shipping.
Privacy note
The file is read with the browser's own file API and never transmitted — not to this site, not to anyone. There is no upload, no temporary storage, and no server involved in the parsing at all. Close the tab and nothing remains. You can confirm this in your browser's network tab: inspecting a file produces no outbound request.
Common questions
Is my video file uploaded anywhere?
No. The browser reads the first few megabytes of the file from your own disk and parses the header there. Nothing is transmitted, to this site or anyone else, and you can confirm it in your browser's network tab — inspecting a file produces no outbound request.
How can it inspect a 4 GB file so quickly?
Everything reported lives in a metadata box called moov, which is usually a few kilobytes. Browsers can read a slice of a file without loading the rest, so only the header is read. The video data itself, which is nearly all of the file, is never touched.
Why won't my MP4 play?
Almost always the codec rather than the extension. MP4 is a container that can hold different codecs. H.265/HEVC has patchy support outside Safari, Apple ProRes will not play in any browser, and AV1 needs a recent browser. H.264/AVC plays essentially everywhere and is the safe choice for compatibility.
What does 'starts without full download' mean?
It reports whether the metadata box sits before the video data. If it does not, a player streaming over a network has to fetch to the end of the file before it can start, which is why some videos stall for a long time online but play instantly from disk. Moving it is called fast start and does not require re-encoding.
Which file formats does this support?
MP4, M4V and MOV, which are all variants of the ISO base media file format. MKV, AVI, WMV and WebM use different container structures and are not parsed.
Why does it not show the frame rate?
Frame rate, colour space and encoder settings live inside the codec configuration rather than the container metadata, and reading them reliably needs a full decoder — several megabytes of WebAssembly. This page deliberately ships none, which is why it loads instantly.
What is a good bitrate for a video?
It depends entirely on the connection you expect viewers to have. The average bitrate shown here is file size divided by duration, which is how much data per second a viewer must receive. A 12 Mbps video is unwatchable on a 6 Mbps connection regardless of how well it was encoded.
Why do the numbers differ slightly from VLC or MediaInfo?
Those tools read the whole file and can compute exact per-track bitrates and frame counts. This reads only the container header, so the bitrate shown is an average across the whole file including audio and container overhead. For codec, resolution, duration and layout the values are the same.
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