Captions for TikTok, Reels and Shorts: Line Length, Reading Speed and Safe Areas

Captions on short-form vertical video do a different job from subtitles on a film. On TikTok, Instagram Reels and YouTube Shorts, many people scroll with the sound off, the screen is narrow, the app covers parts of the frame with buttons and text, and you have a second or two to hold attention. Captions here are part accessibility, part design, part hook.

This guide covers the practical rules: how much text per caption, how fast it can change, where to put it and how to style it.

Why vertical captions need their own rules

Traditional subtitle guidelines were written for a wide, landscape screen viewed from a distance. A common rule there is up to about 42 characters per line, two lines per subtitle. Put 42 characters across a 9:16 phone frame at a readable size and the text either wraps into three or four lines or becomes too small.

Vertical video changes three things:

• Width. The frame is narrow, so lines must be short.
• Overlays. The app places its own UI over your video.
• Attention. Viewers decide in moments whether to keep watching, so captions carry more of the message.

Line length: fewer words, bigger text

For vertical video, think in words, not characters.

• 1–4 words per caption works well for punchy, fast-cut content. The word-by-word or phrase-by-phrase style keeps the eye moving.
• Up to two short lines (roughly 15–25 characters each) works for talking-head explainers and educational content where you want full sentences.
• Never three lines. If you need three, split into two captions.

Break at natural phrase boundaries, exactly as with longer-form subtitles. “Here’s the / trick” reads worse than “Here’s the trick” on one line.

Too much:   The biggest mistake people make with captions
            is putting way too many words on screen at once

Better:     The biggest mistake
            with captions?
            → next caption →
            Too many words
            at once.

Reading speed: how long each caption stays up

Reading speed is measured in characters per second (CPS) — the number of characters in a caption divided by how long it’s on screen. Professional streaming and broadcast style guides commonly cap adult subtitles at around 17–20 CPS.

For short-form content, aim lower than that maximum:

• People are often watching while scrolling or multitasking.
• Large, bold, animated captions take a little longer to parse than plain subtitles.
• Viewers are reading and watching your visuals.

Practical rules of thumb:

• Keep each caption on screen for at least about 0.7–1 second, even if it’s one word.
• If a caption feels like it flashes past, merge it with the next one rather than slowing your speech.
• Match the caption to the words being spoken. Captions that run ahead of the voice spoil punchlines; captions that lag feel broken.

A quick self-test: watch the video once with the sound off. If you have to pause to read anything, that caption is too long or too short-lived.

Safe areas: keep captions clear of the app UI

Every short-form app overlays UI on the video. Exact positions differ between apps and change with app updates, but the pattern is consistent:

• Bottom area: the account name, caption/description text, music or audio label and sometimes a progress bar.
• Right side: a vertical column of buttons — like, comment, share, save, profile.
• Top area: tabs, search and status bar.

The original video also gets cropped or letterboxed differently on different phones, so content near the edges is risky.

A safe placement rule of thumb for a 1080×1920 frame:

• Place captions in the middle to lower-middle of the frame, comfortably above the bottom fifth or so of the screen.
• Keep a generous margin from the right edge where the buttons sit; centring the caption with narrow line widths handles this naturally.
• Keep clear of the top status/tabs area.

Because app layouts change, the most reliable check is practical: export a test, upload it as a private or draft post, and look at it in the actual app on a real phone. Platforms also publish their own safe-zone guidance for ads, which is worth checking for current layouts.

If you’re using ASS to style burned-in captions, the MarginV value in your style and PlayResY: 1920 make it easy to lift captions to a consistent height above the bottom UI. See our guide to subtitle formats for how ASS styles work.

Styling for small screens

Captions on vertical video should be readable at arm’s length, over any background, in bright sunlight.

Font

• Use a bold, sans-serif typeface with clear letterforms.
• Avoid thin weights, scripts and condensed fonts at small sizes.
• Pick one font and stick to it across your videos — it becomes part of your look.

Size

For a 1080×1920 frame, captions are typically somewhere around 60–90 px tall in practice, depending on font and words per line. Test on a phone rather than judging on a desktop monitor, where everything looks bigger.

Contrast

Text has to be readable over bright skies, white walls and busy footage. Options, in rough order of robustness:

1. Solid or semi-opaque box behind the text — the most readable on any background.
2. Thick dark outline (stroke) around light text.
3. Drop shadow — the weakest alone; combine with an outline.

Highlighting

Highlighting the current word in an accent colour is popular because it guides the eye and adds energy. Use one highlight colour consistently, and make sure it still contrasts with the background. Don’t highlight every other word — emphasis only works if it’s rare.

Case

All-caps captions look punchy but are slightly slower to read and can feel like shouting. Sentence case is more readable for longer captions. Many creators use all-caps for very short, one-to-three-word captions and sentence case for full lines.

Accessibility still matters

Stylised captions are still captions. Viewers who are deaf or hard of hearing rely on them, so:

• Caption all speech, not just the highlights.
• Indicate meaningful sounds like [laughs] or [phone buzzes] when they matter.
• Identify speakers when more than one person is talking and it isn’t obvious.
• Don’t let animation hurt legibility. Bouncing, spinning or rapidly flashing words are harder to read and can be uncomfortable for some viewers.

Burned-in or platform captions?

Most apps offer built-in auto-captions that viewers can toggle. They’re easy and accessible, but you have limited control over style and position.

Burning captions into the video gives you full control over font, size, colour and placement, and they look the same on every platform you cross-post to. The trade-off is that viewers can’t turn them off and you can’t fix a typo without re-exporting. Many creators burn in styled captions and also enable platform captions where available. Our guide to burning subtitles into a video walks through the process.

Quick checklist

• 1–4 words per caption for fast content; at most two short lines for explainers.
• Each caption on screen for at least about a second.
• Middle to lower-middle placement, clear of bottom text and right-side buttons.
• Bold sans-serif, large, with a box or thick outline.
• One highlight colour, used sparingly.
• Test on a real phone in the real app before posting.

CutMySub is being designed with vertical video in mind: caption styles that sit inside platform safe areas and render directly into your 9:16 exports. It’s launching soon.