Relative luminance sRGB. For official audits use certified tooling.
Hex colors are parsed as sRGB (#RGB expands to #RRGGBB). Relative luminance follows WCAG: linearize each channel, combine with coefficients 0.2126 / 0.7152 / 0.0722, then contrast ratio = (Lmax+0.05)/(Lmin+0.05). Hints map roughly to AA/AAA thresholds for normal/large text (simplified labels).
This calculator uses plain decimal numbers. If a result looks wrong, check whether you used a comma instead of a dot (or vice versa) where the field expects one.