APCA Contrast Checker

1. Enter the text color as Foreground and the surface as Background. Order matters in APCA, so keep text in the Foreground field.
2. Read the Lc value and the readability tier it falls into.
3. Compare it with the WCAG 2.1 ratio and pass or fail rows shown for the same pair.
4. Look at the live preview at the size and weight you plan to ship, since APCA's requirements change with both.

APCA, the Accessible Perceptual Contrast Algorithm developed by Andrew Somers (Myndex), reports lightness contrast as Lc on a scale of roughly −108 to +106. Zero means no usable contrast. Where the WCAG 2 ratio gives the same answer whichever color is the text, APCA is polarity-aware.

Dark text on a light background gives a positive Lc; light text on a dark background gives a negative one. Black on white sits at the top of the positive end and white on black at the bottom of the negative end. For thresholds, read the magnitude: Lc −75 and Lc 75 meet the same bar.

• Lc 90: preferred for body text.
• Lc 75: minimum for columns of body text.
• Lc 60: minimum for other content text that is not body copy.
• Lc 45: minimum for large or bold headlines and for non-text elements like icons.
• Lc 30: minimum for placeholder and disabled-style text.
• Lc 15: close to invisible for most readers.

Treat these as starting points. Thin or small type needs a higher Lc, while large or heavy type can work with less.

The WCAG 2 ratio is a plain ratio of two luminances that ignores polarity. APCA models how perceived contrast shifts across the lightness range and with text size, so the two methods can disagree. They diverge most often on mid-tone colors paired with white text and on dark color pairs, where the WCAG 2 ratio tends to overstate contrast. APCA vs WCAG compares the two in detail.

Legal status

APCA is not a legal conformance requirement. It was proposed as the contrast method for WCAG 3, but WCAG 3 is still an early Working Draft and its contrast method is not final. The DOJ's 2024 ADA rule for state and local governments, Section 508 and EN 301 549 all reference WCAG 2.x AA, so you still need 4.5:1 for normal text and 3:1 for large text.

Using both results together

Treat WCAG 2 AA as the requirement and APCA as a readability check. If a pair passes AA but its Lc is low for the text size you use, adjust lightness until both agree; a darker step from the shades generator is often enough. The WCAG contrast checker page covers the ratio rules.