APCA Contrast Calculator

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.