Color Contrast Checker

They are WCAG's floor for readability: 4.5:1 for normal text and 3:1 for large text at level AA — the level laws and contracts cite — and 7:1 / 4.5:1 at the stricter AAA. Large means at least 24px, or 18.5px bold. The ratio itself runs from 1:1 (identical) to 21:1 (black on white).

Because screens and eyes disagree: luminance is computed after undoing gamma, so mid-greys carry far less contrast than they appear to on a bright display. The famous boundary is #767676 on white — the lightest grey that passes AA. Anything lighter reads fine on your monitor and fails on someone's phone in the sun.

The tool walks your text color toward black or white — whichever direction gains contrast on this background — in one-percent steps and stops at the first shade that reaches the target. It keeps your hue's character as long as possible; it refuses honestly when even the extreme cannot pass, which happens on mid-toned backgrounds.

Yes, with its own rule: WCAG requires 3:1 for graphical objects and interface components — borders of inputs, icons that carry meaning, focus indicators. The AA-large verdict doubles as that check, since the threshold is the same 3:1.