Color Contrast Checker

The contrast ratio compares relative luminance, not the raw sRGB numbers. Each channel is first divided by 255, then linearised: values at or below 0.03928 are divided by 12.92, and higher values are transformed by ((c + 0.055) / 1.055) raised to the power 2.4, undoing the sRGB transfer curve. The linear channels are then weighted 0.2126 for red, 0.7152 for green and 0.0722 for blue, coefficients drawn from the ITU-R BT.709 primaries and reflecting the eye's strong sensitivity to green. The ratio is (L1 + 0.05) / (L2 + 0.05), with L1 the lighter colour. That 0.05 term models ambient light reflecting off the screen, which is what bounds the scale at 21:1 for black on white and stops the ratio diverging as luminance approaches zero. WCAG 2 thresholds are specific: 4.5:1 for normal text at level AA, 3:1 for large text defined as at least 18 point or 14 point bold, 7:1 and 4.5:1 respectively at AAA, and 3:1 for user interface components and meaningful graphics under criterion 1.4.11. Two limitations are well documented. The formula is not perceptually uniform, so it is unreliable for dark-mode pairs, where it can pass combinations that read poorly and fail some that read well. It also ignores font weight and size beyond the coarse large-text cutoff. Contrast alone is not sufficient either: colour must never be the only means of conveying information.

The first W3C guidance took a different form. WCAG 1.0, published in May 1999, asked authors to ensure adequate contrast but relied on a pair of brightness-difference and colour-difference formulas that proved hard to apply consistently. The luminance-ratio approach now in use was introduced with WCAG 2.0, which became a W3C Recommendation in December 2008 and set the 4.5:1 and 7:1 thresholds in Success Criteria 1.4.3 and 1.4.6. WCAG 2.1 followed in June 2018 and added criterion 1.4.11 Non-text Contrast, extending the requirement beyond text to controls, focus indicators and informative graphics. Regulation gave the numbers force. The United States Access Board's Section 508 refresh, published in January 2017, adopted WCAG 2.0 Level AA by reference for federal technology, and the European standard EN 301 549 does much the same for public sector procurement, while a growing body of civil litigation cites the same criteria. Critique of the underlying model has continued in parallel. The ratio's poor handling of light text on dark backgrounds prompted work on a perceptually based replacement, the Accessible Perceptual Contrast Algorithm developed by Andrew Somers, which scores polarity separately and accounts for font weight and size. It has been explored in connection with the in-progress WCAG 3 draft rather than adopted into WCAG 2.