Dual subtitles: how to show two languages at once

The short answer

To show two languages at once, combine the two subtitle files into one, with each caption holding a line in each language. Open the first file in the subtitle merger, choose Two languages together, add the second file and download the result.

Dual subtitles are popular with language learners, with families who speak different languages, and with translators who want to compare a translation with the original. Most players show only one subtitle track at a time, so one combined file is the easiest way to see both on any player, TV or platform.

Before you combine: line up both files

The merger pairs captions by time, so both files need to be in sync with the same video. Play the video with each file on its own first. If one is early, late or drifting, fix it with the subtitle time shifter before combining.

Both files should also be made for the same release of the video. A file made for a cinema version and another made for a TV version can differ by whole scenes, and no amount of shifting will make them match everywhere. If the files agree at the start but not later, look for subtitles made for your exact release.

It also helps to clean both files. Sound descriptions such as [door creaks], speaker names and repeated captions take up space that two languages need. The subtitle cleaner removes them, though you may want to keep them if the file is for viewers who are deaf or hard of hearing.

How to combine two subtitle files

The whole process runs in your browser. Neither file is uploaded.

• Open the subtitle merger and choose the file whose timing you trust most. This is the first file; the combined captions use its timings.
• Under Second subtitle file, choose the file in the other language.
• Choose Two languages together.
• Under Put the second language, choose Below the first or Above the first.
• Read the summary and look through the preview, then download the file.

How the captions are paired

For each caption in the second file, the merger looks for the caption in the first file that overlaps it in time the most, and adds its text to that caption, below or above the first language. The combined caption keeps the start and end time of the first file's caption; the second file's own times are not used.

If several captions in the second file overlap the same caption in the first, all of them are added in order. A caption in the second file that overlaps nothing in the first stays a caption of its own, with its own times. The summary tells you how many captions were added to captions of the first file and how many stayed on their own.

First file (English)
00:00:01,000 --> 00:00:03,000
Hello.

00:00:03,500 --> 00:00:05,000
How are you?

Second file (Spanish)
00:00:01,200 --> 00:00:02,000
Hola.

00:00:02,100 --> 00:00:04,500
¿Cómo estás?

00:00:09,000 --> 00:00:10,000
Adiós.

Combined
1
00:00:01,000 --> 00:00:03,000
Hello.
Hola.

2
00:00:03,500 --> 00:00:05,000
How are you?
¿Cómo estás?

3
00:00:09,000 --> 00:00:10,000
Adiós.

Reading the example

In the example, Hola fits entirely inside the first English caption, so it joins it. ¿Cómo estás? overlaps both English captions, by 0.9 seconds with the first and by 1 second with the second, so it joins the second. Adiós overlaps no English caption and stays on its own at its own time.

This is why it matters that the files are in sync. If the second file were two seconds late, many lines would attach to the wrong caption. When the summary shows that a lot of captions stayed on their own, the files probably do not line up.

The format of the combined file

The merger saves the result in the format of the first file: an SRT file stays SRT, VTT stays VTT and so on. SSA files are saved as ASS, and SubViewer files as SRT. If you need another format, open the combined file in the converter afterwards.

The two languages look the same in the result, because the merger adds plain lines of text. Italic, bold and underline inside each language are kept where the format supports them. If you want the second language in another color or size, convert the combined file to ASS and change the style of those lines in an ASS editor such as Aegisub.

Keep dual subtitles readable

Two languages double the text on screen, so captions get long quickly. A two-line caption in each language becomes four lines, which covers a lot of the picture. Run the combined file through the subtitle checker: with its default limit of two lines, it lists every caption that grew taller, and it shows the reading speed of each one.

• Prefer files where each caption has one short line, such as subtitles made for small screens.
• Put your stronger language second if you are learning, so your eye lands on the new language first.
• If one language has much longer captions, use it as the first file, so the shorter lines are the ones added.

When the result looks wrong

Most problems with dual subtitles come from the two files, not from the merge itself.

• Many captions stay on their own: the files are out of sync with each other. Fix the timing of one file first.
• Lines attach to the wrong caption: the second file is slightly early or late. Shift it by the gap and combine again.
• Everything matches at the start and then slips: one file was made for a different frame rate or a different cut of the video.
• Letters in the second language look wrong: the second file's encoding is detected automatically. Open it in the converter first, choose the right text encoding there, download it as UTF-8 and use that file instead.
• Some captions have three or four lines: the second file splits sentences differently. That is normal; shorten those captions in a text editor if they cover too much of the picture.

Other ways to show two languages

Some players can show two subtitle tracks at the same time without combining them. The mpv player, for example, has a secondary subtitle option that shows a second track at the top of the screen. Browser extensions for streaming sites offer something similar. These are useful when you want to switch the second language on and off, but they only work in those programs; a combined file works anywhere a normal subtitle file does.

The merger's other mode, One after the other, is for something different: joining the files of a film released in two parts, such as CD1 and CD2, into one file.

Common questions

Can I combine three languages? Yes. Combine two files, download the result, then open it in the merger again as the first file and add the third.

Will dual subtitles work on my TV? Usually, yes. The combined file is an ordinary subtitle file, so any TV or player that shows the format shows both languages.

Does the merger translate subtitles? No. You need a subtitle file in each language. The merger only places them together.