You’ve typed a size into your caption editor, and you have no idea if it’s right. 40 looks small. 80 looks enormous. Then you post, and the captions look different again on your feed. The problem is that the number in the size box doesn’t mean the same thing from one app to the next, and it doesn’t tell you how big the words will look on a phone.
The short answer. Size captions against the 1080×1920 frame, not by the number in the size box. As a starting point, aim for capital letters roughly 60 to 80 pixels tall for one or two lines of captions, and 100 pixels or more for one word at a time. Keep the longest line inside about 80% of the frame width, then check it on a phone at arm’s length after you upload.
Why the size number doesn’t help
Caption apps measure text in different units. Some use points, some use pixels at the export resolution, some use a percentage of the frame or their own scale. A size of 60 in one app can be twice the size of 60 in another.
Even within one app, the same size looks different in different fonts. A size setting controls the font’s em box, an invisible square each letter is drawn in, and fonts fill that square very differently. A heavy display font with tall capitals can look a third bigger than a narrow sans serif at the same setting.
So the number is only useful inside one app with one font. To compare anything, measure what ends up in the exported frame.
Measure in frame pixels
Vertical video for TikTok, Reels and Shorts is 1080 pixels wide and 1920 tall. Whatever size your editor uses, the text ends up as some number of those pixels, and that’s the size that matters.
How to measure it
- Export a short test clip with one caption on screen.
- Pause on a frame with a capital letter like “H” or “E” in it and take a screenshot.
- Open the screenshot in any app that shows image dimensions and crop tightly around the capital letter. The height of the crop is your cap height in pixels.
If your screenshot isn’t 1080 pixels wide (a phone screenshot of the playing video often isn’t), scale the result: divide your measurement by the screenshot width and multiply by 1080.
Cap height is the measure to use because it ignores the font’s empty space. Two fonts with the same cap height look roughly the same size, whatever their size settings say.
What a frame pixel looks like on a phone
A 1080-pixel-wide frame filling a phone screen around 6 to 7 centimetres wide puts each frame pixel at roughly 0.06 millimetres. So capitals 70 pixels tall come out around 4 millimetres on the glass. That’s comfortable at arm’s length for a short phrase. Half that and most people would need to bring the phone closer, which nobody does on a feed.
Starting sizes by caption style
These are starting points, not rules. Use them for the first test export, then adjust by eye on a phone.
| Caption style | Cap height on a 1080×1920 frame | Why |
|---|---|---|
| One word at a time | 100 to 160 px | One word has the whole width, and it’s on screen for a fraction of a second |
| One short line, two to four words | 70 to 90 px | Big enough to read in one glance, small enough to fit the line |
| Two lines with word highlight | 55 to 75 px | Smaller per word, because the viewer gets the whole block for longer |
| Title or hook text | 90 px and up | Read once, at the start, before the viewer decides to stay |
Width decides it more than height
On a 1080-pixel frame, size is a trade between height and how many characters fit on a line. Get that wrong and the app wraps a phrase onto a third line, or a single long word runs off the edge.
Keep a margin on both sides
Keep the longest line inside about 864 pixels, which is 80% of the frame width, centred. That leaves room for two things. The right edge of TikTok, Reels and Shorts carries a column of buttons, and depending on the phone and the app, a 9:16 video can be cropped slightly at the sides to fill a taller screen. Text near the edges is the first to be covered or cut.
Do the rough arithmetic
In a heavy caption font, an average letter is somewhere around three quarters as wide as the capitals are tall, give or take, depending on the font. At a 70-pixel cap height, that’s roughly 50 pixels per character, spaces included. An 864-pixel line then holds about 17 characters. “this changed everything” is 23, so at that size it wraps.
That’s why one-word captions can go so much bigger: an eight-letter word at 100 pixels tall still fits with room to spare.
If your lines keep wrapping, you have three options: go down a size, use a narrower font, or show fewer words per caption. The third usually reads best, since short chunks are easier to take in on a phone anyway.
When bigger stops helping
There’s a ceiling. Captions that are too big cause their own problems.
- They cover the subject. On a talking-head video, two lines of large text can sit over the speaker’s mouth or chin. Viewers on mute watch the face as well as the words.
- They force more lines. Bigger text means fewer words per line, so a phrase spreads onto three lines and the block grows taller, not just wider.
- They hide what you’re showing. Tutorials, recipes and product videos need the frame. Large captions over a screen recording can hide the very thing being explained.
If text at the size you want covers too much, a heavier font at a slightly smaller size usually reads just as well. The traits that make a font hold up small are covered in the best font for video captions.
Size and placement together
Size and position depend on each other. A larger caption needs more clear space, and the clear space on vertical video is limited. The bottom third of TikTok, Reels and Shorts holds the username, the post text and, at the time of writing, an audio or music row, and the right edge holds the like and comment buttons. That leaves the centre of the frame and the area just above it.
Each platform covers a slightly different area, so check the guide for wherever you post most, starting with where to add captions to a TikTok video so they stay clear of its interface. In Capster, text size and position on a nine-point grid are both on the free tier, so you can try a larger size and move it up a row in the same pass. The animated caption styles page shows which presets show one word at a time and which show two lines, which is the biggest single factor in how large you can go.
A quick sizing check
- Pick the caption style first: one word, one line or two lines.
- Set the size so capitals land in the range for that style.
- Check the longest caption fits inside 80% of the frame width.
- Export a short test and watch it on your phone, held at arm’s length.
- Post it as a draft or private video and check it again behind the platform’s buttons.