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APCA vs WCAG Contrast: What Decides Today
Two ways to answer "can people read this?" — one perceptually sophisticated and still a draft, one blunt and written into law and standards. This guide compares them without selling either.
The position, in one paragraph: APCA (Lc values, roughly −108 to +106) models polarity and font size and often matches human judgement better at the margins. It is also a draft method with no normative status. Every legal and standards reference in force — WCAG 2.1/2.2 AA, EN 301 549, the EAA route — uses the ratio method. So: design with both in view if you like; decide with WCAG 2.1. That is the rule every tool on this site follows.
How each method computes
WCAG ratio. Linearise the sRGB channels, take the weighted luminance (green dominates at 0.7152), and divide (lighter + 0.05) by (darker + 0.05). Symmetric: swapping text and background changes nothing. The output is one number, 1 to 21, compared with 4.5 / 3 / 7 by use. Its known weaknesses are real: it is generous to some dark-ground pairs and harsh to some saturated mid-tones, and it knows nothing about font size beyond the large-text classification you supply.
APCA. Raises the sRGB channels to a power, applies a soft black clamp, and combines the two luminances with polarity-dependent exponents — dark-on-light and light-on-dark are genuinely different calculations (black on white scores about Lc 106; white on black about Lc −101, and that asymmetry is by design). Its guidance then maps Lc to minimum font sizes and weights in a table, rather than to a single pass line.
Where they disagree — and what to do about it
The contested zone is mid-contrast: pairs around 3:1–4.5:1 by ratio, especially light text on saturated mid-tone grounds, where APCA tends to demand more and the ratio tends to wave things through — and some dark-mode pairs, where the ratio is generous and APCA stricter still. If a pairing passes WCAG but scores a weak Lc for its intended size, the professional reading is: legal, and probably hard to read — worth strengthening if the hue allows, especially for small text. If a pairing fails WCAG but scores a comfortable Lc, the reading is the reverse: readable, perhaps — and still a conformance failure, in an audit, in a procurement check, in court. The pair checker shows both numbers side by side with exactly these labels, so the disagreement is visible rather than adjudicated silently.
Why this site will not "switch to APCA"
Because the question a checker answers is not "which method is better science?" but "what will this pairing be judged against?" Today the answer is the ratio — in WCAG 2.1 and 2.2, in EN 301 549, and in the national rules that reference them. A tool that quietly decided by a draft method would be making a legal claim it cannot support. If WCAG 3 lands with APCA (or a successor) inside and the standards chain re-points, the verdicts here will change in the open, with the methodology page recording when and why. Until then: APCA is shown as a labelled second opinion — never as a verdict, and never as "compliance".
APCA questions
Can I use APCA to judge whether my site meets WCAG?
No. WCAG conformance is judged against WCAG criteria, and today that means the 2.x ratio method. APCA results can inform design judgement — spotting pairs the ratio method treats generously, for instance — but an APCA pass is not a WCAG pass, an APCA fail is not a WCAG fail, and presenting either as the other is how audit evidence goes wrong.
Why do the two methods disagree about the same pair?
Because they model different things. The ratio method uses relative luminance only — it is symmetric (A on B equals B on A) and size-blind (you supply the large-text classification). APCA weights spatial frequency and polarity: dark text on a light ground scores differently from the reverse, and its guidance ties required Lc to font size and weight in a lookup table. Disagreement at the margins is the expected behaviour of two different models, not a bug in either.
Will APCA replace the ratio in WCAG 3?
APCA is a candidate method under development for a future WCAG 3, which remains a working draft — timelines and final methods are the W3C’s to settle. When the referenced standard changes, the tools and the law that points at the standard change with it; until then the ratio decides. This site watches the standards quarterly and dates its framing — if APCA’s status changes, the guide changes first, in the open.