WCAG Contrast Explained: AA, AAA, and Why APCA Is Coming
WCAG (Web Content Accessibility Guidelines) defines minimum contrast ratios for text on backgrounds. Hit them or your site fails an accessibility audit. This article explains the three thresholds, when each applies, and what is changing with APCA.
The three contrast levels
- AA Large (3.0:1) - large text only (18pt+ regular, 14pt+ bold). The minimum for headlines.
- AA (4.5:1) - body text. The legal minimum in most jurisdictions (ADA, EAA, AODA).
- AAA (7.0:1) - the gold standard, recommended for users with low vision.
Contrast ratio is computed as a function of relative luminance: (L1 + 0.05) / (L2 + 0.05) where L1 is the lighter color. Pure white on pure black scores 21:1, the maximum.
How to test contrast in practice
Three rules cover 95% of cases:
- Body text on its primary background: needs 4.5:1.
- Headlines (24px+) on their background: need 3.0:1.
- Icons and form borders that convey meaning: need 3.0:1 against adjacent colors.
Test with our /contrast tool, browser DevTools (Chrome and Firefox both ship contrast scoring), or Stark/Polypane during design.
What WCAG gets wrong
WCAG 2.1 contrast was designed in the 1990s based on CRT monitor research. It systematically over-rates dark color combinations and under-rates light ones. The result: a designer can write WCAG-AA-passing dark mode text that is genuinely hard to read, and fail valid light-mode designs.
This is why APCA (Accessible Perceptual Contrast Algorithm) was developed - and why WCAG 3.0 (still draft) plans to adopt it.
APCA: the next standard
APCA scores from 0 to 108 (positive for dark text on light, negative for light on dark). The thresholds depend on font weight and size:
- Lc 75 - body text minimum (replaces WCAG 4.5:1).
- Lc 60 - headline minimum.
- Lc 45 - large or bold text.
- Lc 90+ - body text gold standard.
Major design systems (GitHub Primer, Atlassian, BBC) already use APCA internally. The /contrast tool reports both WCAG and APCA so you can ship to today’s requirements while futureproofing.
Try the related tools
Frequently asked questions
Does WCAG apply to my site legally?
In the US (ADA), EU (EAA, in force June 2025), UK, Canada (AODA) and Australia, public-facing sites are expected to meet WCAG 2.1 AA. Many private sites adopt it voluntarily to reduce litigation risk.
What is the difference between WCAG 2.1 and 2.2?
WCAG 2.2 (Oct 2023) adds nine new success criteria - mostly around focus, target size, and authentication - but does not change the contrast formulas. AA remains 4.5:1 for body text.
Can I use APCA today?
Yes for design decisions, no as your sole compliance test. Most accessibility audits still require WCAG ratios. Test both, ship to WCAG, design toward APCA.
What about color blindness?
Contrast ratios assume sufficient lightness difference, which helps color-blind users automatically. Additionally, never rely on color alone to convey meaning - pair with icon, label, or pattern.