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A video that won't play is usually the wrong codec, not a broken file.

The file extension is just the box. Inside are a video codec and an audio codec, and each device only opens some combinations. Pick the kind of file you have to see what's inside, where it plays, and the one tool that fixes it.

phone_iphoneFrom an iPhoneIMG_2231.MOV

downloadA downloaded filmfilm.2024.1080p.mkv

screen_recordA screen recordingmeeting.webm

videocamAn old camcorder clipMOV00041.AVI

gif_boxA big GIFreaction.gif

inside IMG_2231.MOV

container · QuickTime MOV

movieHEVC (H.265)Apple’s efficient codecmodern

graphic_eqAAC stereoplays everywhereuniversal

size182 MBlength1:24picture4K · 30 fps

phone_iphonecheck_circleiPhoneplaysphone_androidchange_circleAndroidnewer onlydesktop_windowschange_circleWindowsneeds add-onlaptop_maccheck_circleMacplaystvchange_circleSmart TVmodel-dependentlanguagechange_circleWeb browserSafari onlychatcheck_circleWhatsAppauto-convertsslideshowchange_circlePowerPointMac only

buildthe fixThe HEVC video inside is the problem, not the .mov box. Converting to MP4 with H.264 makes it play on every Windows PC, browser and TV.MOV to MP4IMG_2231.MOV 182 MB → .mp4 H.264 · 96 MBarrow_forward

~/open-the-box

See what's really inside your video, and whether this browser can play it.

Drop in an MP4, MOV, MKV, WebM or AVI. Your browser reads the container's own structure: MP4 boxes, Matroska elements or AVI chunks. From that it gets the exact codecs, where the index sits, and the codecs string a player asks about. It then asks this browser three separate questions about playback. Everything happens on your device. Or record a short clip with one of your browser's own encoders and inspect that.

video_fileDrop a video or click to chooseOnly the headers and index are read, so even multi-gigabyte files take moments.

or record 2 seconds with this browser's encoder

WebM · VP8WebM · VP9WebM · AV1MP4 · H.264MP4

Greyed-out formats have no built-in encoder in this browser. Clips are 640 × 360, video only, at 1.5 Mbit/s.

recording-webm-vp8.webm276.7 KB · EBML DocType “webm” · written by ChromeWebMfirst bytes 1A 45 DF A3 9F 42 86 81 01 42 F7 81

durationnot storedsize276.7 KBoverall bitrate-top-level parts4

file map · byte 0 on the left

EBML header36 B · @0Info30 B · @48Tracks67 B · @78Clusters276.5 KB · @145

header, index of tracksaudio and video dataseek indexother

tracks

movieVP8CodecID V_VP8640 × 360 · variable frame ratevp8

No tracks listed for this file.

asked this browser

canPlayType()check_circleprobablyAsked about video/webm; codecs="vp8"

MediaSourcecheck_circleyesWhether a streaming player could feed this to the decoder in chunks.

actual loadcheck_circleloadedDecoded 640 × 360, and the player reports a duration of Infinity, so it can’t show a seek bar.

what we foundwarningNo Duration in the Info element. Players show the length as 0:00 or Infinity until they’ve read the whole file. This is typical of recordings made in a browser.warningNo Cues seek index. Seeking means scanning clusters one by one, which is slow, and some players refuse to seek at all. Remuxing writes a proper index.next stepWebM to MP4: adds a seek indexarrow_forward

~/the-trade-off

Smaller files or plays everywhere: pick where you need to be.

Newer codecs squeeze the same picture into fewer megabytes, but older devices can't decode them. Each dot is a common format for one minute of 1080p video. Click one for the details and the converters that go to and from it.

MP4 · H.264MP4 · HEVCMOVWebM · VP9AV1MKVAVIGIF

plays on more devices →smaller file for the same quality →

plays almost anywheresome devicesneeds specific apps

MP4 · H.264≈ 60 MB / minThe safe default since 2010. Every phone, computer, TV, browser and editing app plays it. If you don’t know what to pick, pick this.

plays oneffectively everythingbest forsharing, uploading, editing

converters for MP4

MOV to MP4arrow_forward MKV to MP4arrow_forward WebM to MP4arrow_forward AVI to MP4arrow_forward GIF to MP4arrow_forward MP4 to MOVarrow_forward MP4 to WebMarrow_forward MP4 to MKVarrow_forward MP4 to AVIarrow_forward MP4 to MP3arrow_forward

~/every-video-tool

Beyond changing the format.

Tools that cut, shrink, or pull something out of a video, with a typical result for each.

movie_editVideo editorTrim, cut, join and add text, right in the browser.12:40arrow_forward1:30 compressVideo compressorShrink for email, WhatsApp or Discord limits.480 MBarrow_forward24 MB gif_boxVideo to GIFTurn a few seconds into a looping animation.4 s cliparrow_forwardloop.gif music_noteExtract audioKeep just the soundtrack as an MP3.talk.mp4arrow_forward.mp3

Every format converter14 pages · each explains what changes

swap_horizMOV to MP4 swap_horizMKV to MP4 swap_horizWebM to MP4 swap_horizAVI to MP4 gif_boxGIF to MP4 swap_horizMP4 to MOV swap_horizMP4 to WebM swap_horizMP4 to MKV swap_horizMP4 to AVI gif_boxMOV to GIF music_noteMP4 to MP3 music_noteMKV to MP3 music_noteWebM to MP3 music_noteMOV to MP3

~/fit-under-a-limit

File size is bitrate × duration. Everything else follows.

To hit a size limit, a compressor has to work backwards: limit ÷ length gives the total bitrate, and the audio takes its share first. What's left, spread over every pixel of every frame, decides how good the video can look. You can then record a few seconds at that bitrate with this browser's own encoder and see how close it lands.

LENGTH2:30

SIZE LIMIT

Discord free · 10 MB8 MBGmail · 25 MB100 MB

RESOLUTION

480p720p1080p1440p4K

FRAME RATE

24 fps30 fps60 fps

VIDEO CODEC

H.264HEVCVP9AV1

AUDIO

none64 kbit/s96 kbit/s128 kbit/s192 kbit/s

video bitrate to aim for426 kbit/s

■ video 8.0 MB■ audio 1.8 MB■ container overhead 212.5 KB

total bitrate523 kbit/s10 MB ÷ 2:30, less 2% overheadbits per pixel0.0071920 × 1080 × 30 fpsH.264-equivalent0.007baseline codec

warningToo few bits for 1080p: expect blocky motion and smeared detail. Even 480p is starved; shorten the clip. (Rule of thumb: about 0.07 bits per pixel in H.264 terms looks clean for typical footage.)

# two-pass H.264ffmpeg -y -i input.mov -vf scale=-2:1080,fps=30 -c:v libx264 -preset slow -b:v 426k -pass 1 -an -f null /dev/nullffmpeg -i input.mov -vf scale=-2:1080,fps=30 -c:v libx264 -preset slow -b:v 426k -pass 2 -c:a aac -b:a 96k -movflags +faststart out.mp4

Check it in this browserrecords 4 s of animation at 1920 × 1080 with video/mp4;codecs=avc1radio_button_checkedRecord 4 s

The browser’s live encoder is asked for exactly this bitrate. Your settings above change what gets recorded.

~/reference

Nine video codecs, tested in the browser you're using.

The last three columns are asked live. canPlayType answers “probably”, “maybe” or nothing. Media Source support is what streaming players need. Recording uses MediaRecorder. Efficiency is the rough bitrate needed for the same quality as H.264, and varies a lot with content and encoder.

codecstandard · yearcodecs stringvs H.264plays hereMSErecords

H.264 / AVCITU-T H.264 · ISO/IEC 14496-10 · 2003avc1.6400281×probablyyesyes

HEVC / H.265ITU-T H.265 · ISO/IEC 23008-2 · 2013hvc1.1.6.L120.90≈ 0.5-0.6×nonono

VVC / H.266ITU-T H.266 · ISO/IEC 23090-3 · 2020vvc1≈ 0.35-0.45×nonono

VP8RFC 6386 · Google · 2010vp8≈ 1×probablyyesyes

VP9VP9 bitstream spec · Google · 2013vp09.00.40.08≈ 0.6-0.7×probablyyesyes

AV1AOMedia AV1 · 2018av01.0.08M.08≈ 0.45-0.55×probablyyesyes

MPEG-4 Part 2ISO/IEC 14496-2 · 1999mp4v.20.9≈ 1.4-1.6×nonono

TheoraXiph.Org · 2004theora≈ 1.5×nonono

ProRes 422 HQApple · SMPTE RDD 36 · 2007apch≈ 15-25× (editing)nonono

Containers, and how to recognise them

containerMIMEmagic bytesstandardwhere the index lives

MP4video/mp4.. .. .. .. 66 74 79 70 (“ftyp”) + brandISO/IEC 14496-12 and -14moov box: at the front with faststart, otherwise at the end.

MOVvideo/quicktime.. .. .. .. 66 74 79 70 71 74 20 20 (“ftypqt ”)Apple QuickTime File Formatmoov atom, the same structure MP4 was based on.

MKVvideo/x-matroska1A 45 DF A3 + DocType “matroska”RFC 9559 (2024)Cues element, located through the SeekHead.

WebMvideo/webm1A 45 DF A3 + DocType “webm”WebM project (Matroska subset)Cues element; often missing in browser recordings.

AVIvideo/x-msvideo52 49 46 46 .. .. .. .. 41 56 49 20 (“RIFF…AVI ”)Microsoft RIFF, 1992idx1 chunk after the media, or OpenDML indx.

MPEG-TSvideo/mp2t47 every 188 bytesISO/IEC 13818-1None: built for broadcast, players read it as it arrives.

FLVvideo/x-flv46 4C 56 01 (“FLV”)Adobe FLV specOptional keyframes list in onMetaData.

WMV / ASFvideo/x-ms-asf30 26 B2 75 8E 66 CF 11Microsoft ASFSimple Index object at the end.

3GPvideo/3gpp.. .. .. .. 66 74 79 70 33 67 70 (“ftyp3gp”)3GPP TS 26.244moov box, as in MP4.

From QuickTime to AV1

1991QuickTime 1.0Apple’s atom-based container: the structure MP4 later grew from.

1992AVIMicrosoft builds Video for Windows on its RIFF chunk format.

2002MatroskaAn open container begins as a fork of the Multimedia Container Format.

2003H.264 and MP4H.264 is approved, and ISO/IEC 14496-14 defines the MP4 file.

2010WebM and VP8Google open-sources VP8 inside a Matroska subset for the web.

2013HEVC and VP9Two successors to H.264 arrive within months of each other.

2018AV1 1.0The Alliance for Open Media releases a royalty-free codec.

2024Matroska RFCRFC 9559 finally makes Matroska an IETF standard.

Sources

[1]ISO base media file formatISO/IEC 14496-12: boxes, moov, mdat and sample tables.ISO ↗[2]Matroska, RFC 9559EBML elements, Cues, SeekHead and CodecIDs.IETF ↗[3]The codecs parameter, RFC 6381How strings like avc1.640028 are built.IETF ↗[4]ITU-T H.264The Advanced Video Coding standard.ITU-T ↗[5]AV1 bitstream and ISOBMFF bindingThe av1C box and the av01 codecs string.AOMedia ↗[6]MediaRecorderThe recording API behind the samples and the bitrate check.W3C ↗

~/other-toolboxes

Not a video?

The other toolboxes, with a typical job for each.

graphic_eqAudio toolsConvert, clip and compress sound.memo.m4aarrow_forward.mp3 imageImage toolsConvert, resize, crop and compress images.IMG_4402.HEICarrow_forward.jpg picture_as_pdfPDF toolsMerge, sign, protect and convert PDFs.contract.docxarrow_forwardsigned.pdf calculateCalculatorsPercentages, dates, loans and savings.15% of 240arrow_forward36