SubtitleMedic

Sync Two Subtitles

One subtitle file matches your video and the other comes early or late? Add both (.srt, .vtt, .ass, .sbv or .sub), see how far apart they are, close the gap in one click, and download the corrected file.

Processed in your browserNo sign-up.srt, .vtt, .ass, .sbv, & .sub

How to sync one subtitle file to another

  1. Add the file that is in sync

    Drop the subtitle file that already matches your video into the first box. It is only read: nothing about it is changed or saved.

  2. Add the file to fix

    Drop the file that comes early or late into the second box, or drop both at once: the first one is taken as the file in sync. Both open in your browser; nothing is uploaded.

  3. Close the gap

    The line at the top of the options says how far apart the two files are. When the gap is the same on most lines, press the button under it to shift the file to fix by that amount. You can also choose a direction and type the seconds yourself, or nudge with +0.5s and +1s.

  4. Check, then download

    The preview shows both files side by side with the gap left on each pair of lines; in-sync lines read 0.0 s. Open your video under Preview to watch the fixed file on it. Download it, and it is saved in the format it came in.

Why two subtitle files for the same film are out of sync

They were timed for different releases. A film exists in many versions: with or without a studio logo, with a recap, with a few seconds of black before the first scene. A subtitle file is timed against one of them. Put it next to a file timed for another version and every line is late or early by the same amount, often between half a second and ten seconds. That is a constant gap, and one shift closes it.

One of them was already fixed by hand. Many subtitle files have been moved once to fit somebody’s copy of the video. The translation you found in your own language may carry such a shift while the original-language file does not. The lines still pair up perfectly; they are only offset.

They were made for different frame rates. Film runs at 23.976 or 24 frames per second, European TV copies at 25. A subtitle made for one and played against the other starts close and drifts further away with every minute. This tool recognises that pattern and says so instead of offering a shift that would only help for a few minutes. Fix the rate with Change Frame Rate, then sync.

They are different cuts. A director’s cut or a TV edit adds and removes scenes. The gap then jumps in the middle of the film. Compare the running times in the two file cards: a difference of minutes means no single shift will fit the whole file. Shift Subtitle Timing can move a range of lines at a time for those cases.

How the gap is measured

The tool does not need to know which line of one file belongs to which line of the other, and it does not read the text. Every line of the file in sync “votes” for the distance to the lines near it in the other file, and the distance most lines agree on is the gap. Translators split sentences differently, so many votes are noise; the real gap still stands out, because only it repeats through the whole film.

Gaps under 0.2 seconds count as in sync: that is how far apart two careful translators time the same sentence. The fix button rounds to a tenth of a second, which is what people type and well inside that limit.

When fewer than five lines are available, or when no distance is shared by enough lines, the tool says nothing rather than guess. You can still enter a shift by hand and judge it in the side-by-side preview.

Tips for syncing subtitles

Use the language of the audio as the reference. Subtitles in the language spoken on screen are usually timed most tightly to the voices. Sync your own language to that file and it inherits the good timing.

Check a line near the end. A constant gap looks the same at minute 2 and minute 90. Type a time such as 1:20:00 into “Go to line” and read the gap column there. If it has grown, the files differ in frame rate and need Change Frame Rate first.

Watch it on the video before you download. Open your video under Preview and jump to a scene with fast dialogue. A subtitle that appears as the mouth opens is in sync; half a second late is easy to see.

Keep the original. The download gets “synced” in its name, for example movie.vi.synced.srt, so the file you started with is never overwritten. Rename the fixed file to match your video once you are happy with it, so players load it automatically.

Frequently asked questions

Are my subtitle files uploaded anywhere?

No. Both files are read and compared by JavaScript in this browser tab. Neither leaves your device, and the tool keeps working if you go offline after the page loads. A video you open to check the result plays straight from your disk.

Which file goes where?

Put the file that already matches your video under “File in sync”, and the one that comes early or late under “File to fix”. Only the file to fix is changed and downloaded. If you added them the wrong way round, use the swap button; a shift you entered is cleared, because it was measured for the other file.

Do the two files have to be in the same language?

No. The tool compares when lines start, not what they say, so an English file can be the reference for a Vietnamese one. Two files in the same language work as well, for example a good release and a badly timed copy of it.

Does the download change anything besides the timing?

No. The file keeps its format, its text and its formatting. An .ass file stays .ass with its styles, a .vtt file keeps its cue settings. Every line moves by the same amount, and “synced” is added to the file name so you can tell it from the original.

The tool says the gap grows through the film. What now?

A gap that is small at the start and large at the end means the two files were made for different frame rates, usually 25 against 23.976 fps. No single shift fixes that. Convert the file to fix with Change Frame Rate first, then come back; what is left is a constant gap, which this tool closes.

Neither of my files matches the video. Can I still use this?

This tool lines one file up with another, so it needs one good file. If you have none, open your video in Shift Subtitle Timing and move one file until it fits, then use that file as the reference here.

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