What this ratio measures

The checker converts sRGB channels to relative luminance and compares the lighter and darker colors. Equal colors have a ratio of 1:1; black against white has 21:1. The displayed ratio is rounded for readability, but pass decisions use the unrounded value. A value just below a threshold does not pass merely because it rounds up on screen.

Understand normal and large text

For WCAG 2.x Level AA text contrast, the usual threshold is 4.5:1 for normal text and 3:1 for large text. Large means at least 18 points, or at least 14 points when bold. The corresponding enhanced Level AAA thresholds are 7:1 and 4.5:1. This checker labels those four comparisons separately. Font size and weight must be established in the real page, not inferred from the size of this sample.

A color pair is only one accessibility check

The result assumes two opaque solid colors. It does not evaluate images, gradients, transparency, focus indicators, component boundaries, text resizing, keyboard access, or screen-reader behavior. Passing a text contrast test is not a certification that a website meets all of WCAG. Inspect the weakest-contrast part of a real design, and use the official W3C guidance to understand exceptions and context.

Worked examples

Foreground	Background	Contrast ratio
#000000	#FFFFFF	21:1
#FFFFFF	#FFFFFF	1:1

Good questions.

Does a rounded 4.50 always pass?

No. The pass uses the full value. For example, a value slightly below 4.5 can round to 4.50 and still fail.

Does this certify accessibility?

No. It evaluates a solid color pair for selected text contrast thresholds only.

Can I test a transparent background?

Not directly. First determine the final composited colors in the actual design.

References & further reading

Updated 5 November 2026 · Methods & corrections

Color contrast checker