WCAG 2 vs APCA — which contrast standard should you use in 2026?

Designers keep encountering two contrast numbers that disagree and no clear guidance on which to obey. The short answer is that they answer different questions and you need both. The longer answer explains when to trust which.

What each one measures

WCAG 2 computes a ratio between the relative luminance of two colours, on a scale from 1:1 to 21:1. Thresholds are fixed: 4.5:1 for body text, 3:1 for large text and interface components, 7:1 for AAA.

APCA — the Accessible Perceptual Contrast Algorithm — produces a signed lightness contrast value, Lc, from roughly −108 to +106. The sign tells you the polarity: positive is dark text on a light background, negative is light on dark. Crucially, the threshold is not fixed. It depends on font size and weight.

That difference is the whole story.

Where WCAG 2 breaks down

It cannot see typography. A 12px hairline caption and a 32px bold heading in the same two colours receive an identical score. Anyone who has shipped a technically-passing caption that turned out to be unreadable has met this limitation in person.

Ink on screen is what you actually perceive, and a thin glyph puts far less of it there than a heavy one at the same colour.

It is unreliable in dark mode. The WCAG 2 formula adds a fixed 0.05 constant to both luminance values, intended to model screen flare. At the dark end of the range that constant dominates the calculation, so pairs near black can post a comfortably passing ratio while reading as dim and muddy.

This is not a fringe complaint — it is one of the specific problems APCA was built to correct, and it matters more every year as dark mode becomes the default for a large share of users.

Where APCA is uncomfortable

APCA’s strictness produces results that surprise people:

14px at regular weight cannot pass at any contrast. Not black on white. Not any combination. APCA says text at that size and weight needs at least 16px, or bold. WCAG 2 will pass a 14px caption at 4.5:1 without comment.

16px regular needs Lc 90, which in practice means near-black text on white. A lot of body copy that feels perfectly comfortable does not clear that bar.

Whether you find this rigorous or unreasonable depends on your priors. What is not in dispute is that it models the underlying perception more accurately than a ratio of two luminances can.

The status question, answered honestly

There is a lot of confusion here, so to be precise:

APCA is not a standard. It was included in an early WCAG 3 working draft and then pulled back to exploratory status in July 2023. WCAG 3.0 is not expected to reach Recommendation before 2028 at the earliest, and plausibly later.

WCAG 2.1 and 2.2 AA remain the operative legal benchmark. The ADA in the US, the European Accessibility Act, and equivalent regimes elsewhere all reference WCAG 2.x. If you are asked to demonstrate conformance, the ratio is what you will be asked for.

So anyone telling you to switch to APCA is giving you advice that could fail an audit. And anyone telling you to ignore APCA is telling you to ignore the better readability model.

How to actually use both

Ship to WCAG 2.2 AA. This is non-negotiable if conformance matters to you at all. 4.5:1 body, 3:1 large text and UI components.

Check APCA for anything small, thin, or on a dark background. These are the three cases where WCAG 2 is least trustworthy. If APCA disagrees with a WCAG pass, believe APCA about the reading experience even though you record the WCAG number.

Treat a bare WCAG pass as a warning. Clearing 4.5:1 by 0.02 is not meaningfully different from failing by 0.02. If your APCA figure is also marginal, the pair will break the moment someone sets it a little smaller.

Use APCA to argue for typography changes, not just colour changes. This is where it earns its keep. When APCA says a colour pair cannot work at 14px regular, the fix is often to change the type rather than the colour — and APCA gives you the evidence to make that case.

A worked disagreement

Consider mid-grey text, #6b7280, on white, at 14px regular.

WCAG 2 gives you 4.83:1. That is a pass for body text at AA, and most teams would ship it.

APCA gives you Lc 72 at a size and weight where no contrast passes at all. Its recommendation is not “darken the grey” but “make the text 16px, or make it bold.”

Both are correct within their own model. One of them is describing the reading experience.

The practical rule

Record the WCAG ratio for compliance. Design to the APCA figure for readability. When they disagree, the stricter one is the one your readers will experience.

Our contrast checker shows both side by side at the size and weight you are actually shipping, and says explicitly when they disagree and why.