OneTaskBy Tim King · Updated
Video Caption Preview
Check SRT or VTT Subtitles on Your Video
OneTask is free, with no sign-up, and built by one person.
If this tool saved you time, a coffee helps keep new ones coming.
How the Video Caption Preview works
This video caption preview plays your video with its .srt or .vtt subtitle file on top, so you can check that every line appears on time and is easy to read. Shift all cues with the Offset slider, then download the corrected VTT. Both files stay in your browser.
A subtitle file can look perfect in a text editor and still fail on screen. Lines appear half a second late, a long sentence flashes past too fast to read, or a cue covers someone's face. The only real test is watching it with the picture, and most players make loading a separate caption file awkward.
How the video caption preview works
The tool reads every cue in your file: start time, end time and text. It understands SRT, with
comma decimals such as 00:01:02,500, and WebVTT, with dots. Formatting tags like <i> and
SSA codes like {\an8} are stripped. As the video plays, the cue covering the current time is
drawn over the picture in the size, style and position you choose.
It also checks the whole file against the common readability rules used by broadcasters and streaming services:
| Check | Limit | Why |
|---|---|---|
| Line length | 42 characters | Longer lines are hard to read at a glance |
| Reading speed | 17 characters/second | Faster cues vanish before most people finish |
| Overlaps | None | Two cues at once stack or flicker |
Fixing timing with Offset
If every caption is early or late by the same amount, the file was timed against a version of the video with a different start, such as one with an intro or a countdown. Drag Offset until speech and text line up. Positive values make captions appear later; negative values make them earlier. As soon as the offset is not zero, a shifted VTT file is ready to download.
Tips and pitfalls
Find a sharp sound. Line up the offset on a hard consonant, a clap or a door slam at the start of a cue. It is far easier to judge than the middle of a sentence.
Drift is different. If captions are right at the start but grow later through the video, the file was made for a different frame rate, usually 25 against 23.976. A single offset cannot fix that.
Text looks wrong? Save the subtitle file as UTF-8. Older encodings show accented letters as symbols.
How to read the result
The stats show the number of cues, when the first one appears, how many lines are over 42 characters, how many cues are over 17 characters a second, and any overlaps. A warning appears if the last cue ends after the video does.
FAQ
How do I preview subtitles on a video?
Select the video and its .srt or .vtt file together, or drop both onto the page. Press play and the captions appear over the picture. Change the size, style or position at any time to see how they will look.
How do I fix subtitles that are out of sync?
If every line is early or late by the same amount, drag Offset until the text matches the speech, then press Download to save a corrected VTT file. Use the SRT and VTT converter if you need it back in SRT format.
What does characters per second mean?
It is the cue's text length divided by how long it stays on screen. Above about 17 characters a second, most viewers cannot finish reading before the line disappears. Split or shorten those cues, or give them more time.
Are my video and subtitles uploaded?
No. The video plays from your own device and the subtitle file is read by your browser. Neither is sent to a server, so unreleased films and confidential transcripts never leave your computer.
See also: all Video & Screen tools, Images, Audio, PDF, SVG, Simulations & Screensavers, Generative Art, Chart Makers, 3D Model Viewers, Drawing Pads, Mockups & 3D Making, Diagram Makers, 3D Science, Tracker Music.
Related tools
Subtitle Burner
Hard-code subtitles from an SRT or VTT file into the picture.
SRT / VTT Converter
Convert SRT subtitles to WebVTT and back, with an optional time shift.
Video Trimmer
Cut a video down to the part between two timestamps.
Video A-B Loop
Loop one passage of a video between two points you set.
Frame By Frame Viewer
Step through a video frame by frame and save any frame.