Readable subtitles on vertical video
Placement decides whether the apps cover a subtitle. Readability decides whether anyone reads it. There's no official standard for subtitles on vertical social video yet, so this page starts from broadcast guidelines and marks the parts you'll have to test.
There's no vertical standard yet
Subtitle guidelines from broadcasters and streaming services were written for landscape screens. Translation-industry reviews of vertical video note that there's still no agreed rule for things like characters per line on a 9:16 frame. What follows are well-established broadcast rules and why they need adjusting for a narrow frame.
Lines and line length
- Two lines at most. The BBC's subtitle guidelines recommend a maximum of two lines. On vertical video, two short lines are usually easier to read than one long line running into the right-hand buttons.
- Shorter lines than on TV. The BBC guideline allows about 37 characters per line on a landscape screen. A 9:16 frame is much narrower. The area clear of all three apps' recorded regions is 890 px wide on a 1080 × 1920 frame (see one layout for all three), so at a readable size you'll fit fewer characters. Test your own font rather than trusting a fixed count.
- Break at natural pauses. Split lines between phrases, not in the middle of one. That matters more when lines are short.
Reading speed
The BBC's guidelines give a comfortable pace of about 160–180 words per minute for English subtitles, and they were written for an audience that includes older viewers and people who find reading harder. Fast-talking short videos often go well past that. If a subtitle flashes by, cut words or split it into more captions rather than shrinking the text.
Size
No platform publishes a required subtitle size. Creator guides commonly suggest font sizes around 48–80 px on a 1080 × 1920 frame, and the right size depends on the font, the line length and how busy the footage is. A bigger font also makes the subtitle block taller and wider, so it has to fit inside the clear area. In the tool, set the Subtitle box to your real text block (width and height) and check it's still Safe on all three.
Contrast
Video changes behind the text, so plain white text can disappear on a bright sky or a white shirt. The usual fixes are an outline or shadow, or a semi-transparent box behind the text. As a reference point, the web accessibility guideline WCAG 2.2 asks for a contrast ratio of at least 4.5:1 for normal text on web pages. Video isn't a web page, but the idea transfers: check the subtitle over the brightest and busiest parts of your footage, not just one frame.
Burned-in subtitles or app captions
- Burned-in subtitles are part of the video picture. Everyone sees them, they look the same in every app, and they travel with the video when it's reposted. You also have to place them yourself, which is what this tool checks.
- App captions (auto-generated or uploaded, depending on the app) are drawn by the app, and viewers can usually turn them on or off. The app decides where they go, so this tool doesn't check them.
Many creators do both: burned-in subtitles for everyone, plus app captions where the app supports them, because captions the viewer can switch on are useful for accessibility.
A quick checklist
- One or two lines, broken at natural pauses.
- Text readable on a phone at arm's length.
- Outline, shadow or box so it survives bright and busy frames.
- Every subtitle stays on screen long enough to read.
- The subtitle block is Safe on Shorts, Reels and TikTok in the tool.
- A final look on a phone in each app before posting.
Sources
- BBC Subtitle Guidelines (line count, line length, reading speed).
- Nimdzi, "Subtitling Vertical Videos: Guidelines, Where Art Thou?" (no vertical standard yet).
- W3C, Understanding WCAG 2.2: Contrast (Minimum).