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Color contrast ratio measures how easily text stands out from its background, calculated from the relative brightness of the two colors using the formula in the Web Content Accessibility Guidelines (WCAG). Low contrast text is hard to read for anyone in bright sunlight or on a low quality screen, and it is a hard blocker for users with low vision or color blindness. This tool takes any two hex colors, calculates the exact ratio, and checks it against the WCAG AA and AAA thresholds.
WCAG defines contrast ratio using relative luminance, a measure of how bright a color appears based on its red, green and blue channels. Each channel is converted from its 0 to 255 value into a linear value, weighted (green contributes the most to perceived brightness, blue the least) and summed into a single luminance number between 0 (black) and 1 (white). The contrast ratio is then the lighter luminance plus 0.05, divided by the darker luminance plus 0.05. Pure black on pure white produces the maximum possible ratio of 21 to 1.
WCAG sets different minimums depending on text size. Normal text needs at least 4.5 to 1 for AA compliance and 7 to 1 for the stricter AAA level. Large text, defined as 18 point (24px) or bigger, or 14 point (18.5px) bold or bigger, gets a lower bar because bigger strokes are easier to read at lower contrast: 3 to 1 for AA and 4.5 to 1 for AAA. Most teams target AA as the practical baseline, since AAA is difficult to hit with brand colors and is not required for most content.
Contrast is not a direct Google ranking factor, but it shows up in accessibility audits inside tools like Lighthouse, and poor contrast increases the chance a visitor bounces before reading your content, which does show up in engagement signals. It is also a legal exposure point: businesses in several countries have faced accessibility complaints and lawsuits over sites that fail basic contrast and screen reader requirements. Checking your body text, buttons and links against AA is a cheap way to reduce that risk while making the page easier to use for everyone.
Accessible design and search visibility usually point the same direction: a page that is easy to read is a page people stay on. If you want a full technical and content review, not just contrast, request a free SEO audit and we will show you what else is worth fixing first.
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