What makes a subtitle readable

Contrast is the whole problem

Plain white text is perfectly readable until the shot cuts to snow, a bright sky, a whiteboard, or a pale wall, at which point it disappears. Since you cannot control what is behind the text for the whole video, the text has to carry its own contrast with it. A dark outline around light letters does that: the outline separates the glyph from whatever is behind it, so the same subtitle survives a dark scene and a blown-out one.

Subtitles here are burned in bold white with a black outline and a slight shadow, which is the conventional answer for exactly this reason. A translucent box behind the text is the other common approach; it is more legible still and costs more of the picture.

Weight and size

Video gets compressed, and compression is unkind to thin strokes: a light weight at a small size turns mushy at the bitrates social platforms re-encode to. Bold survives that. Size should be judged relative to the frame rather than in points, because the same video is watched on a phone at arm's length and on a laptop across a desk. If it is comfortable on a phone it is comfortable everywhere.

Position

Bottom centre is the convention because it is the part of the frame least likely to hold the subject of the shot, and because a fixed position means the eye knows where to return. The cost is that the lower third is also where broadcast-style name plates, platform interfaces, and your own graphics tend to live. Leaving that band clear when you edit is easier than fighting it afterwards.

Line length and breaks

A long line forces the eye to travel, and a badly broken one forces it to backtrack. Breaking at a natural grammatical boundary rather than at whatever word hits the edge is most of what separates comfortable subtitles from tiring ones. Subtitle formatting rules covers the specifics, and timing and reading speed covers how long a line needs to stay up.

Capitals do not help

All caps looks emphatic and reads slower. Part of how people recognise a familiar word is its outline, the pattern of ascenders and descenders, and capitalising everything flattens every word into the same rectangle. Reserve it for the occasional word that genuinely needs stress.

Scripts that are not Latin

Direction and line breaking change with the script. Hebrew and Arabic run right to left, and subtitle text is marked for direction so it renders in the correct order rather than reversed. Languages written without spaces between words break by different rules than English does, and a break in the wrong place is more disruptive there than a slightly long line would be. The language pages note what changes for each one.

A four-part check

• Watch it on a phone. That is the smallest screen it will be judged on.
• Watch it muted, which is how most social video is seen.
• Skip to the brightest shot in the video and check the text is still separable from it.
• Find the longest line and read it at normal speed. If you cannot finish before it goes, it is too long or too fast.