Will your captions read on every shot?
Pick the caption's text colour, the outline or box behind it, and the lightest and darkest colours in your footage. The checker works out what the letters actually sit against and scores the worst case with WCAG's contrast ratio.
How this tool works
- 01
Sample your footage
Pick the lightest and darkest colours that appear behind the caption, such as a sky and a shadow. The worst of the two decides.
- 02
Add what the caption has
Turn on the outline or the box if your style uses them, and set their colours. A translucent box is judged over both footage colours.
- 03
Fix what fails
Below 3:1, change the text colour, add an outline, or move to a solid box until the result passes.
Practical guide
Reading the result
What the letters actually sit against
A caption's contrast is decided by whatever touches the edge of its letters. With a box, that is the box. Without a box, a visible outline borders every letter, so the outline decides. With neither, the text sits directly on the video and has to survive the brightest and darkest parts of it. The checker follows that order and reports which one it measured.
The thresholds
WCAG asks for at least 4.5:1 for normal text and 3:1 for large text, with 7:1 for its stricter AAA level. Captions are usually large and bold, so 3:1 is the floor and 4.5:1 is a safer target. The thresholds are written for text and images of text on screens; they are the common yardstick for caption colours, not a rule written for video.
Why the worst frame matters
Footage changes under a caption that stays the same colour. A white caption scores well against a dark interior and fails the moment the camera turns to a window. Sample the brightest and darkest moments of the clip, not an average frame.
What the checker cannot see
It compares colours, not pictures. Busy texture, motion, thin fonts and small text all make captions harder to read at the same ratio. Treat a pass as necessary, then check the real export at phone size.
Questions about this tool
What contrast ratio do subtitles need?+
Aim for at least 4.5:1 between the text and what it sits against, and never below 3:1. Those are WCAG's thresholds for normal and large text.
Does an outline count towards contrast?+
Yes. When there is no box, a visible outline borders every letter, so the text is measured against the outline colour.
How is a translucent box measured?+
The box colour is blended over both the lightest and darkest footage colours, and the lower of the two results is reported.
Are my colours sent anywhere?+
No. The checker runs entirely in your browser and records only that a control was used, never the colours you chose.