For captioning, MP4 and MOV work the same way: both upload, both get the same caption draft, and both come back as an MP4 with the captions burned in. The format only matters at two points. First, the upload gate, where a free account takes files up to 500 MB and every video must be 10 minutes or shorter. Second, the preview, where an iPhone MOV recorded in HEVC may not play back in some browsers even though the captions are generated normally.
So the practical question is not "MP4 or MOV" but "H.264 or HEVC, and how big is the file". This guide explains the two codecs in plain terms, the iPhone setting that decides which one you record in, how to get a file under the 500 MB free limit, and what stays the same in the captioned export.
A container is not a codec
MP4 and MOV are containers. A container is the box: it holds a video stream, an audio stream and some metadata such as the duration and the frame rate. MP4 is the open standard most phones, cameras and websites use. MOV is Apple's QuickTime container, which is what an iPhone or a Mac writes by default.
The codec is what is inside the box. It is the method used to compress the picture. The two you will meet on a phone are:
- H.264, also called AVC. Older, and decoded by almost every device and browser made in the last fifteen years.
- HEVC, also called H.265. Newer, and roughly half the file size for the same picture quality, but not every browser can play it.
Either codec can sit in either container. An iPhone MOV usually holds HEVC. An MP4 exported from a desktop editor usually holds H.264. A MOV from an older iPhone, or from one set to "Most Compatible", holds H.264 too. The file extension tells you the box, not what is in it.
The iPhone setting that decides your codec
Apple's support page "Using HEIF or HEVC media on Apple devices", published 5 December 2025, describes the setting under Settings > Camera > Formats. It has two options:
- High Efficiency records video in HEVC (H.265) and photos in HEIF. Apple says these formats compress better than H.264 and JPEG, so they take up less space on the device and in iCloud Photos without lowering visual quality.
- Most Compatible records video in H.264 and photos in JPEG, which Apple describes as more broadly compatible with other devices and operating systems.
The same page notes that 1080p at 60 fps or lower is more broadly compatible than higher settings, and that sharing a clip by AirDrop, Messages or email may convert it to H.264 if the receiving device cannot handle HEVC. That last point explains a common surprise: a clip that was HEVC on the phone can arrive on a laptop as H.264, with a bigger file than you expected.
For captioning you do not need to change this setting. Both codecs upload. The reason to care is file size and browser preview, covered next.
What happens to each file at upload
| File | Uploads? | Browser preview | Caption draft | Export |
|---|---|---|---|---|
| MP4, H.264 | Yes | Plays in every common browser | Same | MP4, captions burned in |
| MP4, HEVC | Yes | May not play in Android Chrome | Same | MP4, captions burned in |
| MOV, H.264 | Yes | Plays in every common browser | Same | MP4, captions burned in |
| MOV, HEVC (iPhone default) | Yes | May not play in Android Chrome | Same | MP4, captions burned in |
| M4V or WebM | Yes, by extension | Depends on codec | Same | MP4, captions burned in |
Three things to take from the table.
The file type check is by extension. The uploader accepts .mp4, .mov, .m4v and .webm. If a file has a different extension it is turned away with a message asking for an MP4, MOV or WebM video, even if the content inside is ordinary H.264. Renaming an unsupported container does not help, because the server checks the file itself afterwards.
Preview is separate from captioning. The browser preview uses the browser's own video decoder. Android Chrome generally cannot decode HEVC, so an iPhone MOV shows a "preview unavailable" message there. The message also tells you that your captions will still be generated, and that is literally true: the file is transcribed on the server, which does not depend on your browser. Desktop Chrome, Safari and most iPhones play HEVC fine. If you want to see the preview on an Android phone, use the Most Compatible setting for that recording or convert the clip to H.264 first.
Export is always MP4. Whatever you upload, the captioned file is an MP4 with the captions drawn into the picture. There is no MOV export and no separate caption file. The MP4 subtitle generator and MOV subtitle generator pages describe the same workflow from each starting point.
The 500 MB free limit, in minutes
A free account uploads files up to 500 MB. Paid packs raise that to 10 GB. The 10-minute cap on video length applies to everyone.
500 MB spread over 10 minutes is about 6.7 megabits per second of combined video and audio, which is in the range of a phone recording 1080p at 30 fps. The same 500 MB over a 3-minute clip is about 22 megabits per second, which comfortably covers 1080p at 60 fps and some 4K recordings. In other words, short clips rarely hit the limit; long 4K recordings do.
HEVC helps here. Because it compresses better than H.264, a High Efficiency recording of the same scene is smaller, so an iPhone MOV at its default setting is often the file that fits when the H.264 version would not. This is the one place where "MOV" is the easier upload.
How to shrink a file that is over the limit
Try these in order. Each one is lossless for your captions, because the speech is what gets transcribed and none of these steps touch the audio in a way that matters.
- Trim before you upload. Cut the dead air at the start and end on your phone. Every second removed is bitrate you do not have to pay for in the file size, and the 10-minute cap is easier to meet.
- Record or export at 1080p. Most captioned videos end up on a phone screen, and the Starter pack exports at 1080p anyway. A 4K source gives you nothing extra on Starter. Creator and Pro export up to 4K if you need it.
- Keep High Efficiency on. If your phone offers it, HEVC is the smaller file. The preview caveat on Android Chrome does not stop the upload.
- Lower the frame rate. 60 fps roughly doubles the data of 30 fps for the same picture. Talking-head video does not need 60 fps.
- Re-encode on a computer. A desktop video converter can re-save the clip as MP4 with H.264 at a chosen bitrate. Set 1080p and a bitrate between 5 and 8 megabits per second for a talking head. This also fixes the Android preview problem in one step.
- Split a long recording. If a lecture or a webinar runs past 10 minutes, cut it into parts and caption each part. The split also gives you natural chapter breaks.
What you should not do is drop the audio quality to save space. Audio is a small share of the file, and clear speech is what the caption draft depends on.
What stays the same in the export
Whether you started from MP4 or MOV, the export is one MP4 file with the captions burned into the picture. The resolution is 1080p on the Starter pack and up to 4K on Creator and Pro. The aspect ratio and frame rate follow your source. Because the captions are part of the image, they survive every platform re-encode: a Reel, a Short, a WhatsApp forward and a LinkedIn upload all show the same text in the same place. The trade-off is that viewers cannot switch them off, and there is no SRT or VTT file to attach separately. If you need a caption track rather than open captions, the platform's own caption tools are the route, and burned-in captions vs closed captions explains when each one fits.
Every word, timing and style is editable in the browser before you export. The caption draft is the same regardless of container, so checking names, numbers and line breaks is the step that improves the result, not the file format.
Quick decisions
- iPhone user, posting to Instagram or TikTok: leave High Efficiency on, upload the MOV as it is, ignore the preview warning if you are on Android, export the MP4.
- Android user who wants to see the preview: upload MP4 (your phone's default), or convert an iPhone MOV to H.264 first.
- Over 500 MB on a free account: trim, drop to 1080p, or re-encode at 5 to 8 megabits per second. Or buy a pack, which raises the cap to 10 GB.
- Over 10 minutes: split the video. The limit applies to every account.
- Need the export as MOV: not available. The export is MP4, which every platform accepts.
What to do next
Pick the clip you want captioned, check that it is under 10 minutes and, on a free account, under 500 MB, then upload it through add captions to video. A new account has 3 free minutes to start, so you can test an iPhone MOV and an Android MP4 side by side and see that the draft is the same. One-time packs start from $4 (₹49 in India, €4 in the euro area), raise the file cap to 10 GB, and watermark-free export comes with every paid pack. Details are on the pricing page.
