Add Subtitles to a Long Video

Three limits that shape a long job

Short clips forgive loose habits. A long recording exposes them: the AI step has a ceiling, the render produces a big file on your own machine, and a review of several hundred lines rarely fits in one sitting. Each source of subtitles behaves differently with length:

AI subtitles

Up to 2 hours of video per run. A longer file is turned away with "AI subtitles work on videos up to 2 hours long." before any audio leaves your computer, so nothing is charged.

Imported or typed subtitles

No length limit of their own. An SRT covering a full conference day loads like any other.

Rendering

No fixed cap on length. Every frame is decoded, drawn on and encoded again on your computer, so the time grows with duration and resolution, and the finished MP4 has to fit into the browser's storage or memory.

Speed depends entirely on the device, so no page can promise you a render time. The progress panel's own estimate, measured on your machine with your file, is the figure to go by.

What an hour of AI subtitles costs

Credits are counted per started minute of video, with 1,000 credits worth one US dollar:

Only one AI job runs per account at a time, so a transcription or dub you started elsewhere on mydubly has to finish first. Registered accounts also receive 10 free credits a day, which add up through the month and reset on the 1st.

How the render handles a big file

When you click Render video, the editor first estimates the size of the finished MP4 from the length, the resolution and the bitrate it chose, and shows that figure in the "Ready to render" summary. Then it decides where to write:

• To disk, when the browser offers private file storage and enough of it is free. The MP4 is streamed out in chunks as it is encoded, so memory stops being the constraint.
• To memory, when disk streaming isn't available or space is short and the file is small enough. If it comes close to what this browser can hold, the summary advises: "This is a large video for this device. Close other tabs before rendering."
• Nowhere, when neither fits. The summary then gives the expected size and suggests trimming the video into shorter parts, or using Chrome or Edge on a computer with free disk space. There is no server to fall back on.

The bitrate is set generously, normally a little above the source's own, so expect the subtitled copy to be about the size of the original or larger, and free at least that much space first. Processing large files in the browser explains the mechanics, and subtitling a large video file focuses on multi-gigabyte sources such as 4K recordings.

Watching the progress panel

The panel lists the stages in order: Preparing video, Preparing subtitles, then Rendering subtitles and Encoding video running together frame by frame, and finally Finalizing MP4. Alongside sit a percentage, the time elapsed and, once the first few seconds have passed, an estimate of the time left.

• Cancel works at any stage. It removes the half-written file and returns you to the editor with "Rendering canceled. Your subtitles are still here."
• Leaving the page mid-render triggers the browser's "leave site?" prompt, a last guard against closing the wrong tab.
• If the device runs out of room, the error says which kind: "Your device ran out of storage space while saving the video", or that the browser ran out of memory. Free space or close other tabs, then render again.
• Keep the computer on power and stop it from sleeping. Other windows can stay open, but heavy work in them competes for the same processor.

Drafts: coming back tomorrow

Reviewing an hour of subtitles usually spans more than one session. The editor saves a draft in this browser a moment after each change: the lines, the style and the settings of the blur band, never the video itself. Each draft is matched to the video by its file name, size and last-modified date, and the five most recent videos keep theirs.

• Open the same file later and the draft comes back on its own, with a note saying when it was saved.
• When a video arrives with subtitles already attached, as it does from the Burn subtitles button on the mydubly home page, the editor asks instead, with "Restore draft" and "Keep new subtitles".
• A renamed, re-exported or trimmed copy counts as a different video, so its draft won't be found. Keep working on the exact same file.
• "Start new video" discards the current video's draft.

Private windows and cleanup settings can wipe browser storage, so finish each session with Export SRT. Importing that file later brings the lines back, though you would set the style again.

Reviewing hundreds of lines without losing your place

• Work in blocks of a few minutes and stop at a natural break, such as a slide change, so the next session starts somewhere recognisable.
• Let the count beside the Subtitles tab guide you. It shows how many lines carry warnings; the Up and Down arrows step through lines and move the playhead with them.
• Zoom the timeline in for fine timing and use Fit to see the whole recording again.
• There is no find-and-replace, so keep a short list of names and terms the AI misheard and correct them as they appear.
• Raise the playback speed through stretches of clear speech and lower it for questions from the audience, crosstalk or music.

For lectures in particular, lecture video captions covers terminology and slides, and fixing auto-generated subtitles suggests an order for checking errors.

Recordings longer than two hours

The AI limit applies per run, so split the transcription while keeping a single full-length video to render:

1. Cut the recording into parts shorter than 2 hours each with a video tool, and note the exact length of part 1 in seconds. Splitting a video file covers free ways to do it.
2. Open part 1 in the editor with AI subtitles on, review the lines and click Export SRT.
3. Open part 2 and let the AI write its subtitles. Type part 1's length into "Shift all by" and click Apply, so every line moves to its place in the full recording, then click Export SRT again.
4. In a plain text editor, paste the second file's contents below the first, leave a blank line between them, and save the result as one .srt file. The numbering can stay as it is, because the editor ignores it on import and renumbers on export.
5. Open the full-length video, load the combined file with Import SRT / VTT, check the lines around the join and render.

If the full recording is too large for your computer to render in one piece, render the parts separately and publish them as a series. Shifting subtitle timing explains offsets in more detail.

Before you press Render on a long file

• The lines have been reviewed at least once, and the warnings that remain are ones you have looked at.
• An exported SRT sits next to the video as a backup and for platforms that accept caption files.
• Free disk space is at least the size the summary estimates, with some to spare.
• The laptop is plugged in, sleep is off, and nothing else heavy is scheduled to run.

If you only need the text, not a subtitled video, lecture transcription and the AI subtitle generator produce transcripts and subtitle files without a render, and transcribing long audio files explains how long recordings are handled for speech recognition.

About the subtitle editor on this page

Opens

MP4, MOV, M4V, WebM, MKV videos your browser can decode

Subtitles from

AI (from the speech, optionally translated into one of 21 languages), an SRT or VTT file, or typing them in

You download

An MP4 with the subtitles drawn into the picture at the original resolution and frame rate, plus SRT and VTT files

Cost

Editing, styling, rendering and SRT/VTT export are free, with no account and no watermark. AI subtitles cost 1 credit per minute (5-credit minimum per video); signing in with Google gives 100 free credits and accounts get 10 free credits a day

Privacy

The video never leaves your device and is rendered in your browser. Only for AI subtitles is the audio sent, over HTTPS, and it is deleted within 30 minutes of delivery

Rendering needs

Chrome or Edge 94+, Safari 16.4+, or Firefox 130+ on a computer. Editing and SRT/VTT export work in any current browser