Subtitle Preview Tool

Subtitle Preview Tool pairs a video file from your own computer with SRT text you paste in and plays them together, captions overlaid on the picture exactly as a viewer would see them. You choose a local video, supply the subtitle text, optionally set a timing offset in milliseconds, and press the load and preview control to start playback with live captions.

It closes the gap between a subtitle file that looks correct in a text editor and one that reads well on screen. Timing that seems fine as numbers can land a beat late on a punchline, and a line inside the 42-character rule can still crowd a busy shot. Translators checking a delivered file, YouTubers verifying auto-generated captions, and accessibility reviewers signing off a deliverable all need to watch the result rather than read the numbers.

The browser loads your video from disk rather than transferring it anywhere. The pasted SRT is parsed into cues with start and end times in milliseconds, and during playback the current media time is compared against those windows so each caption appears and clears on cue. The offset is applied to every cue time before that comparison, so a track drifting uniformly early or late can be corrected - a positive offset pushes captions later, a negative one pulls them earlier - without editing the file.

Nothing is uploaded or stored: closing the tab discards both video reference and text. Playback quality depends only on your own machine and the formats your browser decodes, typically MP4 and WebM.

• Local video playback: Pick a video file from your own disk and it plays directly in the page; the file is read locally by the browser and is never transferred to a server.
• Live caption overlay: Pasted SRT cues are rendered over the picture in sync with playback, so you judge the subtitles in context - against the shot, the cut and the dialogue - instead of against a timeline of numbers.
• Offset in milliseconds: The offset control shifts every cue by the amount you enter, letting you test whether a track that feels slightly ahead or behind lines up once nudged, before you commit the change in a shifting tool.
• Non-destructive checking: The preview reads your subtitle text but does not rewrite it, so experimenting with offsets costs nothing and your source file stays exactly as delivered.
• Standard browser formats: Any video your browser can already decode - commonly MP4 with H.264 and WebM - works here, with no plugin, codec pack or conversion step.
• Scrub and re-check: Because playback uses the normal video controls, you can jump to a specific moment and watch a single troublesome cue repeatedly rather than sitting through the whole programme.
• Zero setup: There is no account, no install and no project to create; open the page, pick a file, paste the subtitles and press play.

1. Click the video file control and choose the clip you want to check from your own computer, using a format your browser plays natively such as MP4 or WebM.
2. Open your subtitle file in a text editor, copy the whole SRT, and paste it into the subtitle text box on this page.
3. Leave the offset at zero for the first pass so you can judge the file exactly as it was delivered.
4. Press the load and preview control, then play the video and watch the captions appear over the picture in real time.
5. If every caption lands consistently early or late, enter an offset in milliseconds - a positive value delays the captions, a negative value advances them - and reload the preview to test the correction.
6. Scrub to the moments you are unsure about, such as fast dialogue or a shot change, and replay them to confirm the cue in and out points feel natural.
7. Note the offset that worked, then apply it permanently with a subtitle shifting tool and re-preview the corrected file before you deliver it.

Example 1 - a track that runs half a second late. Captions consistently appear just after the line is spoken. Testing an offset of -500 ms in the preview shows the fix works before you rewrite anything.

Cue as written:  00:00:12,400 --> 00:00:15,100
Offset applied:  -500 ms
Shown at:        00:00:11,900 --> 00:00:14,600

Example 2 - checking a fast cue in context. A dense line looks acceptable on paper but proves unreadable on screen, and watching it is the only way to be sure.

3
00:01:04,000 --> 00:01:05,100
The committee reconvenes tomorrow at nine sharp.

On screen: 47 characters in 1.1 s - gone before it can be read

Previewing turns an abstract reading-speed number into an obvious verdict, and catches problems a text editor simply cannot show.

• Catch sync errors before publishing: Watching the captions play against the picture reveals drift, early cue-ins and late cue-outs that look perfectly fine as timestamps in a text file.
• Fewer redelivery cycles: Reviewing in context means fewer subtitle files bounced back by a client or a platform reviewer, which saves an entire round of corrections and re-approval.
• Confidence for accessibility sign-off: Anyone responsible for caption quality under accessibility policy can see for themselves that captions are readable, correctly timed and present throughout, rather than taking a report on trust.
• Find the right offset cheaply: Trying a shift in the preview costs seconds and changes nothing on disk, so you can converge on the correct correction before touching the real file.
• Total privacy for unreleased material: Your video and subtitles are handled entirely in the browser, so screeners, rough cuts and embargoed content never touch a third-party server.
• Works anywhere with a browser: No editing suite, no upload quota and no software install, so a reviewer on any machine can check a file in minutes.