The Web Content Accessibility Guidelines (WCAG) define minimum contrast ratios to ensure text is readable by people with low vision or color blindness. WCAG 2.1 Level AA requires a ratio of at least 4.5:1 for normal text and 3:1 for large text (18pt+ or bold 14pt+). Level AAA requires 7:1 for normal text and 4.5:1 for large text.

Contrast ratio is calculated as (L1 + 0.05) / (L2 + 0.05), where L1 is the relative luminance of the lighter color and L2 is the relative luminance of the darker color. Relative luminance is computed from linearized RGB values using the formula defined in WCAG 2.1. The ratio ranges from 1:1 (no contrast, identical colors) to 21:1 (pure black on white).

Approximately 8% of men and 0.5% of women have some form of color vision deficiency (color blindness), and many more people have low vision conditions. Insufficient contrast makes text difficult or impossible to read for these users, violating accessibility laws in many countries (including ADA in the US and EN 301 549 in the EU).

AA is the baseline standard required by most accessibility regulations and is achievable for most color combinations. AAA is a higher standard that provides maximum readability. AAA compliance is recommended for critical legal, medical, or financial content, but it can be difficult to achieve without significantly limiting design choices.

Yes. WCAG 2.1 also requires a minimum 3:1 contrast ratio for UI components (buttons, form fields, icons) and graphical objects that convey meaning, as defined in Success Criterion 1.4.11 Non-text Contrast. This is a Level AA requirement and applies to the boundaries of interactive elements against their background.