APCA vs WCAG 2.1 Contrast: Which Should You Use in 2025?

The headline

WCAG 2.1 uses a single contrast ratio (1:1 to 21:1) computed from luminance. APCA returns an Lc value (-108 to +106) that takes polarity, font weight and font size into account. Both are valid; they answer different questions.

Where WCAG 2.1 is great

Required by law in most jurisdictions (ADA, EAA, AODA).

One number, easy to communicate.

Universally tooled - every checker on the planet supports it.

Where WCAG 2.1 falls short

Treats white-on-black and black-on-white identically, even though humans see them very differently.

Ignores font weight - 200-weight at 4.5:1 is still a "pass" but is barely readable.

Penalizes mid-tone designs that are objectively legible.

What APCA changes

APCA models how the human eye actually perceives contrast: it weighs background polarity, applies a non-linear lightness curve, and ties thresholds to font weight x size. Apple, Adobe and many design systems already ship with APCA-aware tokens.

How to use both

Compliance baseline. Pass WCAG 2.1 AA on every text token.

Design judgement. Use APCA Lc to tune body text vs headlines vs UI states. A heading at -75 Lc is rich; body at -90 Lc is comfortable.

Document both. When you propose a token color, ship it with the WCAG ratio AND the APCA Lc.

Check both in one screen at our Contrast Checker, or fix failing pairs automatically in Fix A11y.