Legibility floors: the smallest caption that still works on a phone

01Is there a standard font size for subtitles?

No, and we ran one for months before our own numbers killed it. The rule was a readability floor expressed as a percentage of frame height, which sounds rigorous and is measuring the wrong property. One card passed at 0.78 percent of frame height and was legible without effort: black ink on pale green, 150 pixels tall. Its sibling was unreadable at the identical 150 pixels and the identical 0.78 percent, because it was dark blue on pale blue.

Same diameter, same percentage, opposite outcomes. Legibility is ink contrast and source glyph resolution. It is not delivered size.

The note that retired our own readability floor

The two thresholds themselves are not ours. They are the W3C's: 4.5 to 1 for normal text and 3 to 1 for large text. What is ours is where we apply them. Full 4.5 at delivery width, relaxed to 3 at 360 pixels, because most of the loss at embed size is a resampling artifact rather than a design fault, and holding the tighter number there would fail things that are genuinely fine.

02Floor one: contrast under the worst pixel, not the average

A mean of 36 and 239 describes no pixel anybody is looking at. That sentence is the whole section. If a word crosses from a dark doorway onto a lit shoulder, the average says the contrast is comfortable and the second half of the word is invisible. Measure under each run of letters, take the worst ground you find there, and judge against that.

One word, four different backgrounds

Tap the numbers

1234

This frame passes, and it passes for reasons that are decisions rather than luck: one word, heavy weight, and a set of grounds that are all dark enough at the point the ink actually sits.

When we re-measured fifteen delivered cards, two patterns accounted for nearly all the failures. The first was pale display type over a photograph with a soft gradient scrim under it: a scrim is a ramp, so it is weakest exactly where the type sits. The second was small letterspaced capitals at 24 to 30 pixels, which look considered on a desktop and stop existing in a tile.

03How long does a caption need to stay on screen?

At least 1.2 seconds, or 0.35 seconds per word, whichever is longer, measured at full opacity. A six-word line therefore needs 2.1 seconds. Fades are excluded from the count, because type at partial alpha is not readable, and excluding them is the difference between a rule and a rule that works.

Will your caption survive a phone?

Type your own line into it

The line you want on screen

How long it holds

Type size

Plate behind the type

Words-

Dwell floor-

Your hold-

Contrast-

The dwell arithmetic here is exactly the gate we run: max(1.2s, 0.35s per word). The contrast readout is a simplification of it. Ours measures the worst local ground under every run of letters rather than one figure for the frame, which is the difference that failed fourteen of our own cards.

Two word rates sit underneath that floor and they are three to four times apart. A word that replaces what was on screen needs roughly 0.75 seconds. A word that joins a string already there needs only 0.2 to 0.4, because by the fifth cut the reader has had a second and a half with the sentence and has one new word to pick up. Confusing the two is how films end up moving faster than their own content.

We watched exactly that happen. One film put eleven screens of paragraph copy across 279 frames, a mean of 0.85 seconds each, and not one of them was readable. The type was beautiful. The pacing was confident. The film moved faster than its own content, which is a sentence I now use as a diagnosis rather than a description.

04Weight beats size, and it is a resolution fact

It sounds like a taste argument and it is arithmetic. An ultralight face at 36 pixels integrates to a stem 0.71 pixels wide. That is narrower than a pixel, so no stroke ever fully covers a pixel column, and only 6 to 7 percent of the letter's ink reaches full opacity. The same face at regular weight has a 3.70 pixel stem and 78 percent of its ink at full.

A related trap, because it is the same mistake wearing a nicer suit: one weight of one face is about ratio, not count. A 134-pixel ultralight headline sitting above a 42-pixel light sub-line reads as two different typefaces to anybody who is not the person who set it.

05A plate is not a failure of nerve

Designers resist putting a solid shape behind type because it looks like a compromise. It is not a compromise, it is the only way to make the ground under the letters a decision instead of an accident. Where the plate is allowed to sit is a separate question, and the safe zones are constraints rather than guidelines. The single card that passed our re-measure was the only one of fifteen that did not put type over a photograph. That is not a coincidence and it is not a small finding: the failure was the house style, not one bad card.

Type dies on a cut. Never on a fade.

We confirmed this to the frame, three times inside a single reference film: the title vanishes at frame 53, the ingredient list at frame 290, a headline at frame 415, and all three of those frames are picture cuts. Nothing fades out. Type appears inside a shot and is killed by the edit, which is also why fade time cannot be counted toward dwell.

06How do I check my own captions?

Export the finished file, not the timeline. Scale it to 360 pixels wide, which is roughly the real width of a feed tile, and look at it there. Do it in the same pass as the rest of the second-by-second build check, because type timing and cut timing are the same timing. Everything below is doable in ten minutes without buying anything, and the first two checks will find most of what is wrong.

Everything in this piece is a floor. Clearing all of it produces a caption that can be read, which is not the same as a caption worth reading, and no gate we own has an opinion about the second thing. But a line nobody can read has no chance at all, and it is the cheapest failure in the entire film to prevent. Fix the type, never the threshold.