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APCA Contrast Checker

Score a colour pair with APCA, the contrast model behind WCAG 3.

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The two colours

Result 
The score appears here

Quick Tool Overview

The APCA Contrast Checker scores a pair with the Accessible Perceptual Contrast Algorithm — the model WCAG 3 is being built on — returning an Lc value whose sign carries the polarity and whose magnitude maps to real font sizes and weights.

What it does
Calculates the APCA Lc contrast score
You give it
A text colour and a background colour
You get back
Lc, the type it supports, and the WCAG 2 ratio alongside
Best for
Dark mode work and judging genuine readability
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About the APCA Contrast Checker

APCA — the Accessible Perceptual Contrast Algorithm — is the contrast model being developed for WCAG 3. Instead of a ratio it returns Lc, a perceptual lightness contrast value from roughly -108 to 106, and the sign carries the polarity of the pair.

That polarity is the substantive difference from WCAG 2. Dark text on a light background and light text on a dark background are not equally readable at the same measured contrast, and APCA scores them separately: positive Lc means dark text on light, negative means light on dark. It also accounts for the way thin, small type needs far more contrast than large, heavy type.

The page reports your Lc with the font sizes and weights it supports, taken from the published APCA readability guidance, and previews the pair at each of them. Lc 75 is the working minimum for body text, Lc 60 for larger content text, and Lc 30 the absolute floor for anything that has to be seen at all, such as a border or a disabled control.

Key features of the APCA Contrast Checker

Polarity aware

Dark text on light and light text on dark are scored separately, because they are not equally readable at the same measured contrast.

Mapped to real type

Each score is matched to the smallest size and lightest weight it supports, from the published APCA readability tables.

Both models side by side

The classic WCAG 2 ratio is shown too, because that is still what an audit tests against today.

The published constants

Implements APCA-W3 0.1.9 exactly, including the soft black clamp and the separate exponents per polarity.

Live previews

The pair rendered at each supported size, so the number and the reality can be compared directly.

How to use the APCA Contrast Checker

  1. 1

    Enter the text and background

    Any notation works. The order matters here — APCA is polarity-aware.

  2. 2

    Read the Lc score

    The sign tells you the polarity; the magnitude tells you how much contrast you have.

  3. 3

    Match it to your type

    The table shows the smallest size and lightest weight that this score supports.

  4. 4

    Compare with WCAG 2

    The classic ratio is shown alongside, because that is still what an audit will test against today.

Frequently asked questions

Both, for now. WCAG 2.2 is what conformance is legally measured against today, so a design still has to pass it. APCA is the better guide to whether text is genuinely readable, and it is where the standard is heading, so it is worth designing to as well.

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