What is the WCAG relative luminance formula?

Each sRGB channel is divided by 255, then gamma-linearised: values at or below 0.03928 are divided by 12.92, and higher values use ((c + 0.055) / 1.055) raised to 2.4. The linear channels are weighted 0.2126 for red, 0.7152 for green, and 0.0722 for blue and summed to give relative luminance L.

How is the contrast ratio itself calculated?

Contrast equals (L_lighter + 0.05) divided by (L_darker + 0.05), where L_lighter is the larger of the two luminances. The 0.05 term models ambient flare. The result ranges from 1:1 for identical colours up to 21:1 for black on white.

What thresholds must I meet?

WCAG AA requires 4.5:1 for normal text and 3:1 for large text (roughly 18pt, or 14pt bold). AAA requires 7:1 for normal text and 4.5:1 for large text. The tool reports pass or fail against each of these.

Why does the green channel dominate the luminance?

The weights reflect human eye sensitivity, which peaks in the green part of the spectrum. Green is weighted 0.7152, far above red at 0.2126 and blue at 0.0722, so changes to the green channel move luminance the most.

Does this match the newer APCA algorithm?

No. This implements the WCAG 2.x contrast formula, which is what current WCAG 2.0, 2.1, and 2.2 conformance requires. APCA is a different perceptual model proposed for WCAG 3 and is not used here.

WCAG Contrast Ratio Algorithm Tester

If you are implementing WCAG contrast checking in code, a single wrong constant or a misplaced gamma step silently shifts every result. This tool runs the WCAG 2.1 formula one step at a time and prints every intermediate value, so you can diff your own implementation against the spec channel by channel.

How it works

For each of the two colours the tool walks the relative-luminance computation:

Normalise — divide each sRGB channel by 255 to get a value in the range 0 to 1.
Linearise (gamma correction) — values at or below 0.03928 are divided by 12.92; higher values use ((c + 0.055) / 1.055) raised to the power 2.4.
Weight and sum — multiply the linear channels by 0.2126 (R), 0.7152 (G), and 0.0722 (B) and add them to get the relative luminance L.

With both luminances in hand it computes the contrast ratio:

contrast = (L_lighter + 0.05) / (L_darker + 0.05)

The 0.05 flare term and the lighter-over-darker ordering mean the ratio always runs from 1:1 up to a maximum of 21:1.

Example and thresholds

Test #595959 on #ffffff and you can follow each channel from its 0–255 value, through normalisation, into its linear value, and watch the weighted sum produce the luminance — ending in a ratio of about 7.0:1. The tool then checks that ratio against the conformance thresholds:

Normal text: 4.5:1 for AA, 7:1 for AAA.
Large text (about 18pt, or 14pt bold): 3:1 for AA, 4.5:1 for AAA.

Because every intermediate value is shown, this is the fastest way to confirm a contrast library is producing spec-correct numbers rather than just a final pass or fail.