Free Color Contrast calculator with clear step-by-step results.
Applies the WCAG contrast formula to two relative luminance values, adding the 0.05 flare constant that models ambient reflection off a screen. The pass threshold then depends on whether the text qualifies as large - 18 pt normal or 14 pt bold - which relaxes the requirement from 4.5:1 to 3:1.
Contrast ratio
Ratio = (lighter luminance + 0.05) / (darker luminance + 0.05)
Large text rule
Text counts as large at 18 pt or above, or 14 pt and above when bold
Linearise each sRGB channel, then combine as 0.2126 R + 0.7152 G + 0.0722 B. Browser dev tools and most contrast checkers report it directly.
Pure white has luminance 1 and pure black 0, so the formula caps at (1 + 0.05) / (0 + 0.05) = 21.