Check what codec your video actually uses

Every video file states what is inside it, in a header a few kilobytes long at the front of the file. Reading that header needs no upload and no install. Drop your video onto the converter on the home page and it reads the header on your own machine, then shows you the container, the video codec, the resolution, the frame rate, the length, the audio codec and channel layout, the file size and the overall bitrate.

Those facts are the whole diagnosis. They tell you whether your problem is the container, the video codec or the audio track — and therefore whether the fix is a three-second repackage or a full re-encode.

The extension is not the codec

Nearly every "my video will not play" thread goes wrong in the same place: someone reads .mp4 off the file name and concludes the file is fine. The extension names the container — the box. The codec is what the picture inside was compressed with, and it is a separate choice the box barely constrains.

An MP4 can legally hold H.264, H.265 (HEVC), AV1 and more. An MKV holds almost anything at all. So two files that both end in .mp4 can behave completely differently: one plays everywhere, the other refuses on a work laptop, a smart TV or an editing timeline. Nothing in the name separates them. The codec line does.

Once you know the container and the codec separately, the symptom stops being mysterious. A file that plays in VLC and nowhere else is nearly always a codec problem. A file one application refuses to import while everything else plays it is nearly always a container problem — which is why Premiere Pro rejects MKV files that play perfectly in any media player.

What the reading shows you, line by line

The tool also warns you when the file carries subtitle tracks or extra audio tracks, because a conversion keeps only the first video and first audio stream. Better to know before you convert.

Is it H.264 or H.265? Why that one line decides everything

If you read one field, read the video codec. The two names look almost identical and behave nothing alike.

H.264 (also written AVC, and tagged avc1 inside an MP4) is the closest thing video has to a universal format. Browsers, phones, TVs, editors, car head units, projectors and conferencing tools all decode it. If the check says H.264, playback is almost never your problem and you should look at the container or the audio track instead.

H.265, also called HEVC, is what an iPhone records by default on the High Efficiency setting, and what most modern cameras offer as an option. It halves the file size at similar quality, and it is patent-encumbered, which is why support is uneven rather than universal. Apple hardware handles it well. Older and stripped-down Windows installs often show a black screen or an error instead of the picture, plenty of TVs and editors refuse it, and web upload forms reject it more often than they admit. If the check reads H.265 (HEVC) and something will not play the file, you have found your answer, and converting HEVC to H.264 is the fix.

One second-order detail catches people out. HEVC inside an MP4 can be tagged either hvc1 or hev1. Both are legal, but Apple's players want hvc1, so a file tagged hev1 can fail on Apple software while an otherwise identical file plays. That is a tagging problem, not a codec problem, and rewriting the file fixes it.

Reading the audio line when the picture is fine

Silence is a separate diagnosis and the audio codec settles it in one glance. AAC is the format MP4 was designed around, and everything worth naming decodes it. The codecs that go quiet are the licensed ones — AC-3 (Dolby Digital), E-AC-3 and DTS — which arrive with MKV rips, Blu-ray and DVD sources and broadcast TS or MTS recordings. Browsers and most phones ship without those decoders, so they play the video and output nothing.

Two other readings are worth recognising. Opus or Vorbis means a WebM source, and both travel badly outside WebM. PCM is uncompressed audio from a camera or screen recorder: fine for quality, but it inflates the file enormously.

In every one of those cases the repair is the same and it is cheap: re-encode the audio to AAC and copy the video across untouched, which is what the no-sound repair does. The picture stays bit-for-bit identical because it is never re-encoded.

Frame rate, and the number the header cannot tell you

The frame-rate line reports what the header declares. It cannot tell you whether the file keeps to that rate. Phones and screen recorders write variable frame rate footage: the rate drops when the scene is static and rises when it is not, while the header still says something tidy like 30 fps.

Players do not care. Editors care enormously — VFR is the usual cause of audio that starts in sync and drifts steadily out of it over a long clip. A frame rate shown as an odd fraction, or a screen recording that will not stay in sync on a timeline, is the signal to convert the file to a constant frame rate before you edit it.

Turning the reading into the right fix

Knowing the codec tells you how expensive the repair is. When the codec in your file is already legal in the container you want, the streams are copied rather than re-encoded — seconds of work, every pixel identical to the source. When it is not legal, the picture has to be re-encoded, which takes real time and costs a generation of quality.

The converter states which of those it is about to do, in words, before you press anything. That is worth reading: a copy is the reason a MOV that Windows refuses can become a working MP4 in a few seconds, and it is the reason you can move an MKV into an MP4 with no quality loss at all. Anything that is genuinely re-encoded is not lossless, no matter how high the quality setting goes.

Other ways to check, and what each one costs you

• VLC — Tools, then Codec Information, with the file open. Accurate and free, but it means installing VLC and reading a dense panel.
• MediaInfo — the reference desktop tool, far more detailed than this page and worth installing if you inspect files often.
• QuickTime Player on a Mac — the Movie Inspector window gives the format and dimensions in one line, though not much audio detail.
• Windows File Explorer — the Details tab of a file's Properties shows frame width, height, data rate and frame rate, but it is unreliable about naming the codec, and often blank for a container Windows does not index.
• Most online codec checkers — these upload your file to a server to read a header that is only a few kilobytes long. That is an odd trade for a diagnostic step, and it is the same trade every server-side converter asks you to make. A file that never leaves your machine cannot be retained, scanned or leaked.

The phone apps do not show codecs at all: the iPhone and Android versions list only the file name and size, because they always re-encode and never need to make the copy-or-re-encode decision. Checking a codec is a browser job. A long 4K re-encode is a job for the app, which uses the phone's hardware encoder and has no size ceiling.