Colour Contrast Checker
Checks the contrast ratio between two colours against WCAG AA and AAA accessibility thresholds.
Contrast checker
WCAG 2.1 contrast. AA needs 4.5:1 (3:1 large text); AAA needs 7:1 (4.5:1 large). Guidance only.
How to use this calculator
- Choose a text colour and a background colour.
- Read the contrast ratio and which WCAG levels pass.
How it works
Uses the WCAG 2.1 relative-luminance formula. AA needs 4.5:1 (3:1 for large text); AAA needs 7:1 (4.5:1 large).
Worked example
Black on white. = 21:1 — passes every level.
Tips
- Runs entirely in your browser. Your input is not uploaded or stored.
Frequently asked questions
What counts as large text?
About 18.66px bold or 24px regular and up.
Is this official?
It implements the WCAG maths, but always test real designs too.
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Cite this tool
Zerdly. (2026). Colour Contrast Checker. Retrieved from https://www.zerdly.com/tools/colour-contrast-checker/
Tip: Enter any known values to calculate the remaining results.
Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.