Colour contrast ratio

A numerical measure of the perceived brightness difference between a foreground colour (typically text) and its background, expressed as a ratio ranging from 1:1 (no contrast, identical colours) to 21:1 (maximum contrast, black on white). WCAG defines minimum contrast ratios to ensure content is readable by people with low vision and colour vision deficiencies: SC 1.4.3 (Level AA) requires at least 4.5:1 for normal text and 3:1 for large text, while SC 1.4.6 (Level AAA) requires 7:1 and 4.5:1 respectively. SC 1.4.11 (Level AA, added in WCAG 2.1) extends contrast requirements to non-text elements such as user interface components and graphical objects, requiring 3:1 against adjacent colours. Contrast ratios are calculated using the relative luminance of each colour, and tools such as the WebAIM Contrast Checker and browser developer tools can evaluate compliance automatically.