Subtitles for Reels and TikTok: the safe zone nobody publishes
Two things everyone repeats about captions on short video turn out to be shaky. The first is the safe zone - the precise pixel margins you are told to keep clear so the interface does not cover your text. The second is the reason you are supposed to caption at all. Both deserve a closer look, because what is actually published is more useful than what is passed around.
The mute statistic is from 2016, and it was never confirmed
You have seen it: 85% of Facebook video is watched without sound. It comes from a Digiday piece in May 2016, and the number was assembled from a handful of publishers reporting their own dashboards - one of whom actually reported a range of 50 to 80%. Facebook never confirmed it and has never exposed sound-on data in its analytics. It has been recycled for a decade with the source quietly detached.
Instagram published its own figure in 2022: about one third of video plays on Instagram happen with the sound off. That is Instagram measuring Instagram, and it is a long way from 85%.
TikTok is further away still. Citing its own research, TikTok says 88% of its users consider sound vital to the experience. TikTok is a sound-on platform, and captioning a TikTok because “nobody has the sound on” is an argument built on someone else’s platform from a decade ago.
So caption for the real reasons. People who are deaf or hard of hearing. People watching in a waiting room. Comprehension of accented speech, names, numbers and jargon. And retention - one consumer study of 5,616 US adults found 80% were more likely to watch a video to completion when captions were available, and 80% of caption users were not deaf or hard of hearing. That study is from 2019 and the “in public places” qualifier on its headline number gets dropped constantly, so treat it as directional rather than precise.
One claim to avoid entirely: neither Meta nor TikTok has ever said captions affect reach or ranking. That one is invented.
There is no official safe zone for organic posts
This is the finding that surprised me most. Neither company publishes safe-zone guidance for organic Reels or TikToks. Both publish one only for advertisers, and they publish it in incompatible forms.
Meta gives percentages, in its Ads Guide, and the wording is the same for Reels and Stories:
“Consider leaving roughly 14% of the top, 35% of the bottom, and 6% on each side of your asset free from text, logos, or other key creative elements.”
On a 1080 x 1920 canvas that works out to roughly 269 px clear at the top, 672 px at the bottom, and 65 px each side - leaving a usable box about 950 x 979. Note how much bigger that bottom margin is than the “340 px” figure you will find in older guides. That number is 17.7% and it is Stories-era; against Meta’s current 35% it is simply wrong.
Two more things Meta says that rarely get quoted. On screens taller than 9:16, Meta “may either zoom the creative to better fill the whole screen (which can crop areas outside the safe zone)” or letterbox it - so the safe zone is a guideline, not a guarantee. And for disclaimers specifically, Meta asks for the bottom 40% clear.
TikTok takes the opposite approach and refuses to give numbers at all:
“The safe zone size is determined by the dimension (vertical, horizontal or square), the ad caption length and any interactive add-on usage; the longer the caption, the smaller the safe zone will be.”
Instead of pixel values, TikTok distributes downloadable overlay templates through Ads Manager. That is a deliberate design choice and it is the honest answer: on TikTok the safe zone is a moving target that shrinks as your own caption grows.
Which means every precise pixel figure circulating for TikTok is third-party measurement. Researching this article I found five different sets, all confidently stated, all for 1080 x 1920 - with bottom margins ranging from 120 px to 484 px. That spread is the story. Do not trust a TikTok safe-zone number, including one from us.
The practical rule that survives all of this: keep captions in the middle third of the frame vertically, well inside the sides, and check the result in the app before you publish. Meta’s Ads Manager has a “Safe zone guardrail” overlay that shows you the real thing. Before you get that far, our caption safe zone checker draws Meta’s margins over your own frame and tells you which captions land inside them.
What the built-in captions do and do not give you
Both platforms now generate captions automatically, and both do it by default.
Instagram has had auto-generated captions on by default since 2022, in a published list of 17 languages including English, Spanish, Portuguese, French, German, Italian, Arabic, Turkish, Russian, Japanese, Hindi and Indonesian. That list has not been updated publicly since. Creators find the toggle under More options when posting, or afterwards under Edit and Advanced settings.
TikTok now generates captions on upload as well - worth knowing if your instructions date from the launch, when the creator had to opt in. Set the language under More options before posting, and you can edit or delete captions by tapping them, before or after publishing.
Here is the limitation that matters. Neither platform will give you the caption file. There is no SRT to download, nothing to edit outside the app, nothing to reuse for the YouTube cut or the LinkedIn version of the same video. The one exception is Meta’s paid ads, where you can upload an SRT named clip.en_US.srt and download it back out again - a facility organic posting simply does not have.
So if the same video is going to more than one place, or the captions need to be right rather than approximately right, they have to be made outside the platform.
Readability is the part you actually control
Auto-captions are usually accurate and often exhausting to read, because accuracy and readability are different problems. Broadcast subtitling has known the numbers for decades:
- About 17 characters per second. Above that, viewers stop reading and start skimming.
- Two lines maximum, and on a vertical video one is better.
- Around 42 characters per line as an upper bound.
- At least a second on screen, even for a two-word caption.
TikTok publishes one hard number of its own that lines up neatly with this: it recommends displaying 5 to 10 words per second for text on screen, and says that pacing produced a 2.1x awareness lift compared with other timings in its research. Five to ten words a second is roughly the same territory as 17 characters a second, arrived at independently.
Our subtitle readability checker measures an SRT or VTT against exactly these limits and flags every caption that reads too fast, runs too long or uses too many lines. The four numbers, and where they come from goes through the reasoning.
On font size, both platforms decline to publish a minimum. Meta says only to use “a modern, clean font in a large enough type size and a contrasting hue”. The “48 to 72 px bold” advice you will find everywhere is agency convention, not a specification - useful as a starting point, but do not cite it as a rule.
One more reason to keep text large
Instagram’s head, Adam Mosseri, said in late 2024 that Instagram re-encodes video that is not getting watched: “if something isn’t watched for a long time… we will move to a lower quality video. And then if it’s watched again a lot, then we’ll re-render the higher quality video.”
Low-view Reels get a worse encode. Thin, small caption text is exactly what falls apart first under a lower bitrate - so the videos least able to afford illegible captions are the ones most likely to get them. Large, high-contrast, few words.
The workflow that actually works
- Get an accurate transcript first. Upload the video to Speecho and get an SRT with proper punctuation and speaker labels - the first 15 minutes are free.
- Check it before you style it with the readability checker. Fix anything over 17 characters per second by splitting the caption, not by shortening the time.
- Convert if your editor wants something else. The subtitle converter moves between SRT, VTT and plain text in your browser.
- Burn them in. Caption Studio does it for you: pick a style, lift it into the middle third, and switch on the word-by-word highlight if you want the Shorts look - the timing comes from the transcript, nothing is keyframed. Or do it in your editor, large and high contrast.
- Look at it in the app before you publish - on a phone, not a laptop preview.
The reason to do it this way rather than tapping the platform’s toggle is that you end up owning the file. The same captions go on the YouTube cut, the LinkedIn post and the version you send a client, and none of them start from scratch.
