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Answers about captions, each one leading with the short answer and citing where it comes from: the platforms’ own pages, the standards, the studies. Facts checked on 22 October 2026.
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Caption safe zones for vertical video
Where TikTok, Reels, Shorts and Snapchat cover the picture, in pixels on a 1080 × 1920 frame, from their own guides.
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Vertical video specs, platform by platform
Length, shape, resolution, file size and captions on seven platforms, from their own help pages.
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SRT, VTT or burned-in captions?
What SRT and WebVTT files are, which platforms take them, and when to burn captions into the video instead.
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Do captions increase views?
What studies and platform tests actually found about captions, attention and watch time, and what nobody has shown.
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How accurate are automatic captions?
What word error rate means, what today’s speech recognition scores, and what makes it fail.
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Open vs closed captions: are burned-in captions accessible?
The difference between open, closed captions and subtitles, and what WCAG says about burned-in captions.
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How long should a caption be, and how fast?
Characters per line, lines per caption and reading speed from the BBC, Netflix and DCMP, with vertical video in mind.
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How TikTok’s auto captions work
TikTok’s two caption tools, how to edit them, what viewers see, and what they cannot do.
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How Instagram’s auto captions work on Reels
Instagram’s closed captions, the Captions sticker, the Edits app, and their limits.
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How YouTube’s automatic captions work on Shorts
YouTube’s 67-language automatic captions, how to fix them in Studio, why they fail, and caption files.
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How CapCut’s auto captions work
CapCut’s auto captions on mobile, desktop and web, where the speech is recognised, and their limits.