They're careful approximations, not published specifications — no platform documents where its own buttons sit, and all of them move things between releases. Treat the margins as guidance with a little room to spare rather than a guarantee, and check a real post if a caption is sitting right on the line.
Why is the bottom margin so deep?
Because the caption, the handle, the audio strip and the progress bar all stack up down there, and Reels stacks them deepest — about a quarter of the frame. That's why the safe margin shown is the worst case across the platforms you've ticked rather than an average: something safe on average is still covered somewhere.
Do I need this if I use your captions?
No. The built-in caption styles already sit inside the safe area for all of these, and the editor draws the same guides while you drag. This tool is for footage you're cutting somewhere else.
What about landscape or square posts?
Not covered here — this is 9:16 only, because that's where the platform furniture actually eats your frame. A 1:1 or 16:9 post on the same platforms has very little chrome over it by comparison.
What happens to my video?
Nothing leaves the page. If you give it a video, the browser decodes a single frame locally and the file is discarded when you close the tab — there is no upload in this tool at all. Formats your browser can't play, like ProRes, simply won't open, and it says so rather than pretending.
Our caption styles are already positioned inside the safe area for every platform here, and the editor draws the same guides while you drag. Five dollars a month.