Subtitles and captions: styles and file formats

By James Jones, founder of Granary Digital · Updated 5 November 2026

A subtitle file is a text file. Each block holds the words, a start time and an end time, and the picture and the sound stay as they are. Most of the work is in agreeing the rules before anyone types a word, then timing the words against a picture that will not change.

What the rules say

The Ofcom summary of the TV access services code sets targets for television channels. It does not set them for every video. Targets start about a year after a channel launches and rise gradually, and the full levels apply after ten years. At full level they are subtitling on 80% of programmes, rising to 90% for ITV 1 and Channel 4, and 100% for BBC channels. Audio description is 10% of programmes and signing is 5%.

Channels with less than 0.05% of total UK TV viewing do not have to provide access services. Where meeting the full subtitling target would cost more than 1% of a channel's turnover, lower targets apply (one-third and two-thirds of the usual levels), and if those still exceed 1%, no access services are required on that channel.

If your film is going to a streaming service, these figures do not apply directly. Read the streaming access code page and ask the platform for its own specification.

Choose the format before you time anything

Get the format in writing before you start. SRT is widely used and can be checked in any text editor. WebVTT is common on web players. Broadcasters often give their own template, or ask for the captions to be built inside the editing system and exported with the programme.

Ask the platform or broadcaster for these details as well:

• The file format, and whether the captions should be a separate file or burnt into the picture.
• The maximum number of characters per line and the number of lines per caption.
• The reading speed the platform expects.
• Whether speaker names and sound effects should be included.

Limits differ between platforms, so use the figures the platform gives you, not a number from memory.

How to make a subtitle file

1. Take the script or transcribe from the audio, but use the locked picture as the reference, not a rough cut.
2. Set the project frame rate before timing. A caption in a 25 frames per second project changes on frame numbers, not on tenths of a second.
3. Break each caption at a natural phrase. Keep each speaker on a clear line, and mark speaker changes if the platform asks for them.
4. Check each caption stays on screen long enough to read, and leaves the screen when the speaker stops.
5. Export the file, then load it into a player and watch the first, middle and last minute.
6. Play the finished version on a phone and on a television. Small screens show errors that look fine in the timeline.

Worked example

An interview is locked on a Friday. On Monday the editor trims the second half to fix a pause, and the captions after the trim now sit early on the picture. The fix is to re-time from the trim point using the locked file, not to nudge each caption by eye. Re-timing once takes less time than correcting thirty captions one by one.

Common mistakes

• Timing against a rough cut, then changing the picture afterwards.
• Captions that stay on screen after the speaker has finished.
• Too many words in one caption, so it flashes past before anyone can read it.
• Delivering only the burnt-in version when the platform wants a clean picture with a separate file.
• Missing speaker names when two people speak in the same shot.
• Typing the words from memory instead of from the final script.

For the wider delivery picture, see the delivery specs cheat sheet and the edit workflow page.

Sources

• Ofcom, Plain English Summary: TV Access Services Code, read 5 November 2026

Related guides

• Edit workflow: proxies, conform, delivery
• Delivery specs cheat sheet for broadcast and online
• Colour test patterns and bars
• Lower thirds and title-safe templates
• Credits order worksheet (UK)
• All guides and tools

The picture at the top of this page is an illustration, not a photograph of a client or a real job.