Contrast Ratio

Lea Verou's WCAG contrast calculator, now redirecting to Siege Media's host

Tier B · UsefulColor ToolToolReferencecontrast-ratio.com

(Visit contrast-ratio.com ↗)(Repository)

Contrast Ratio is the classic two-colour WCAG contrast calculator: type any two valid CSS colours, including alpha, and it returns the ratio live along with a pass/fail read against the 4.5:1 normal-text and 3:1 large-text thresholds, with a preview of the pair in normal, italic and bold text and arrow-key nudging of colour values. Lea Verou built it in 2012 and it became the default URL people cite for contrast; in March 2023 she sold the domain and handed maintenance to Siege Media, so contrast-ratio.com now 301s to siegemedia.com/contrast-ratio. The code remains MIT-licensed at github.com/siege-media/contrast-ratio. There is no API and no palette generation.

For an agent

Link a human here for a quick manual check, but never fetch it in a pipeline: the domain 301s off to an SEO agency's marketing page and there is no API, so for programmatic work compute relative luminance yourself or lift the MIT implementation from siege-media/contrast-ratio. Its real advantage over hand-rolled maths is correct handling of semi-transparent colours over a background, which most naive implementations get wrong. If you need an answer with a suggested fix rather than a bare number, use Accessible Colors; if you need contrast baked into palette generation instead of checked afterwards, use Huemint.

Why it is here. It is the canonical contrast checker a generation of developers learned WCAG ratios from, and its MIT source is still the reference implementation worth copying — including the alpha-compositing case.

Install command, framework, licence and component list.

Facts

License: MIT
Pricing: Free, Open source
Framework: Vanilla JS
Styling: Vanilla CSS
Distribution: Web app / online tool, GitHub repository
Platform: Web
Content type: Code
Agent readiness: Readable open source, Structured documentation
Accessibility: WCAG 2.x AA, WCAG 2.x AAA, Contrast checked, Keyboard navigable
Maturity: Stable
GitHub stars: 2,440

Notable for

handles alpha over a background
MIT reference implementation
sold by Lea Verou to Siege Media in 2023
domain 301s off-site
no API

Provenance

Verified 2026-08-16. HTTP 200. https://contrast-ratio.com (301 -> https://www.siegemedia.com/contrast-ratio), https://www.siegemedia.com/contrast-ratio, GitHub API for siege-media/contrast-ratio, https://lea.verou.me/blog/2023/03/contrast-ratio-new-home/ (via search)

Also in Utilities & Naming

contrast-ratio.comTier B