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APCA Contrast Calculator

Enter a text and background hex color to compute the APCA Lc contrast.

How it is calculated

Each colour is converted to relative luminance Y = 0.2126729 R2.4 + 0.7151522 G2.4 + 0.0721750 B2.4 with R, G, B scaled to 0 to 1; very dark values are softly clamped. If the background is lighter than the text (dark text), Lc = ((Ybg0.56 − Ytxt0.57) × 1.14 − 0.027) × 100. If the text is lighter, it uses exponents 0.65 and 0.62 and the result is negative. Colours may be given as 3 or 6 hex digits, with or without #. The tool prints the signed Lc and its absolute value. It returns one score only: it does not look up minimum Lc values for font size or weight, so you compare the number with an APCA table yourself. Differences below a tiny luminance gap return 0. Pairs too close to read, about 7 Lc or less, show 0, as the APCA method sets them.

Worked example

Take mid-grey text #767676 on a white background. Each channel is 118/255 = 0.4627, raised to 2.4, so the luminance is Y = 0.1573; white is 1.0. Dark text on light: (10.56 − 0.15730.57) × 1.14 = (1 − 0.3485) × 1.14 = 0.7427. Subtract the 0.027 offset: 0.7157, times 100 gives 71.6 Lc. Swapping the colours (white on #767676) gives −77.0 Lc, a larger magnitude, because APCA treats light-on-dark differently.

Try this example →

Common mistakes

  • Comparing an Lc value with the WCAG 2 ratio thresholds such as 4.5:1: they are different scales, and 71.6 Lc is not a ratio.
  • Ignoring the sign: a negative Lc just means light text on a dark background; judge it by its absolute value.
  • Assuming swapping text and background gives the same score: the two polarities are calculated differently, so the numbers differ.
  • Using one Lc pass mark for all text: small or thin text needs a much higher Lc than large bold text, and this tool does not give those thresholds.
  • Typing a colour name such as red or an rgb() value: only hex codes work, and anything else returns no result.

About this calculator

The APCA (Accessible Perceptual Contrast Algorithm) is a next-generation contrast metric being developed for WCAG 3, designed to replace the older WCAG 2.x contrast ratio formula, which is known to over- and under-estimate perceived readability for many color combinations. Instead of a simple luminance ratio, APCA accounts for text polarity (light-on-dark vs. dark-on-light), font weight, and font size, producing an Lc (Lightness Contrast) value roughly on a 0–106 scale that better matches how legible text actually looks to the human eye.

Because required Lc thresholds shift with font size and weight — small thin text needs a much higher Lc than large bold text to stay readable — APCA is paired with a font-size/weight lookup table rather than one fixed pass/fail number like WCAG 2's 4.5:1. Web accessibility auditors, UI designers, and design-system teams use APCA scores to choose text and background color pairs that will remain legible under the upcoming WCAG 3 guidelines, especially for typography sizes and weights the older ratio handled poorly.

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  • Editorial team

    Two anchor values help you judge a result. Black text on white scores 106.0 Lc, and white on black scores -107.9, so anything near those is the practical ceiling of the scale. The minus sign only marks light text on a dark background. The tool returns one score and leaves the font size and weight lookup to you, so check the same pair against your smallest text, not just your headings. Which APCA lookup table do you use?

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