I have heard about APCA for measuring contrast. Should I use it instead of the WCAG 2 ratio this tool calculates?

What the tool calculates, exactly

It calculates the contrast ratio as WCAG 2 defines it: each colour becomes a relative luminance, then (lighter + 0.05) is divided by (darker + 0.05). The result is a number between 1:1 and 21:1, compared with the limits of Success Criteria 1.4.3, 1.4.6 and 1.4.11 in WCAG 2.2: 4.5:1, 3:1 and 7:1. Every formula is printed under 'Formulas used' at the bottom of the tool, so you can verify it by hand.

A reference result: #0F6B5F on white gives 6.39:1, passing AA normal text and failing AAA normal text. The tool also shows the relative luminance of each colour, so you can see where the number comes from.

How APCA differs

• It does not give a ratio like 4.5:1 but a different kind of value, so the two numbers cannot be compared directly.
• It treats dark text on light differently from light text on dark, while the WCAG 2 ratio is symmetrical: press 'Swap colors' in the tool and #FFFFFF on #0F6B5F also gives 6.39:1.
• It ties its guidance to font size and weight in more detail than the WCAG 2 split between normal and large text.

So the two measures agree on most colours and disagree on some, especially mid-tones and light text on dark backgrounds. That disagreement is expected and is not an error in either one.

Which one to use for your store

If a client, an organisation or a project requirement asks you to meet WCAG 2 at level AA, what is asked for is the ratio and its limits, not APCA, and that is what the tool checks. APCA is a method discussed for future versions of accessibility guidance and is not a published threshold in WCAG 2, so do not base a pass decision on it when WCAG 2 is the requirement.

If you want to use it, treat it as a second check with another tool, particularly in dark mode, where most of the discussion about it happens. Keep the WCAG 2 ratio as the figure you record, and never say a colour passes WCAG 2 because its APCA value looks good.

A detail that makes the result trustworthy

Pass and fail use the exact, unrounded ratio. The figure shown with two decimals is never rounded up to a limit it did not reach: 4.499 is shown as 4.49 and fails. That is why grey #767676 on white shows 4.54:1 and passes, while its neighbour #777777 shows 4.48:1 and fails.

Open the Color contrast checker and try your own colours. And remember that any contrast measure, WCAG 2 or APCA, measures colour only; on its own it does not make a site fully accessible or compliant with any law.