APCA and WCAG Contrast: Why the Numbers Differ
Compare the two calculations with exact color pairs, understand APCA polarity, and keep WCAG 2.2 decisions separate from supplementary Lc measurements.
Updated : Corrected APCA math and status claims; added signed reference pairs and interpretation limits.
Color Match shows a WCAG contrast ratio and a signed APCA Lc value. They measure color contrast differently and their numbers cannot be substituted for one another. If your task is checking a WCAG 2.2 text requirement, make that decision with the WCAG ratio. Use APCA as additional information when investigating typography.
A controlled black-and-white comparison
A reproducible text and background example.
Same pair, larger text.
#000000 / #FFFFFF
Black text on white: a reference pair for both calculations.
21.000000:1 WCAG contrast; 106.0407 Lc APCA.
Normal-text AA: Pass. Large-text AA: Pass. Labels apply to text contrast only.
Reproduce this pair and try a fixA reproducible text and background example.
Same pair, larger text.
#FFFFFF / #000000
The reversed pair: the WCAG ratio stays the same, while APCA changes sign and magnitude.
21.000000:1 WCAG contrast; -107.8847 Lc APCA.
Normal-text AA: Pass. Large-text AA: Pass. Labels apply to text contrast only.
Reproduce this pair and try a fixThe two WCAG results match because its ratio puts the lighter color in the numerator regardless of which color is text. APCA distinguishes dark-on-light from light-on-dark text. In the implementation used here, positive Lc indicates dark text on a lighter background; negative Lc indicates the reverse. The sign does not mean pass or fail.
Do not translate 4.5:1 into one universal Lc target
A ratio threshold and an Lc value belong to different calculations. There is no fixed multiplier that converts every WCAG ratio to APCA, and an Lc recommendation must be interpreted with typography. The same pair can feel different at small, thin body text and at a large, heavy display heading.
A reproducible text and background example.
Same pair, larger text.
#777777 / #FFFFFF
A near-threshold WCAG pair with its independently calculated APCA value.
4.478089:1 WCAG contrast; 71.1111 Lc APCA.
Normal-text AA: Fail. Large-text AA: Pass. Labels apply to text contrast only.
Reproduce this pair and try a fixTry the pair in the checker, swap the colors and compare both outputs. This is a useful diagnostic experiment: the WCAG value should remain unchanged, while the APCA output can change in both sign and size. If only the label changes, the implementation may be taking an absolute value too early.
Which implementation is documented here?
The calculation follows the published APCA 0.0.98G sRGB constants and transfer behavior. It applies the 2.4 power transform, a single low-luminance clamp, and the reference low-contrast cutoffs. Reference checks cover black/white, reversed polarity, middle gray and saturated red. A version label matters because an algorithm or recommendation table can evolve.
A practical decision record
- •Write down the exact text color, background, typeface, size and weight.
- •Record the WCAG ratio and required text category for the conformance task.
- •If using APCA, preserve foreground/background order and the signed Lc output.
- •Review the actual typography instead of treating either number as a complete readability score.
APCA is not presented on this site as a WCAG 2.2 pass criterion or as proof of WCAG 3 conformance. The checker deliberately keeps its supplementary result separate from AA and AAA labels.
See our calculation methods for formulas, reference values and the limitations of opaque sRGB inputs.