APCA Contrast Calculator (myndex.com)

Accessible Contrast Relative to Font Size and Weight APCA reports lightness contrast as an Lc value from Lc 0 to Lc 106 for dark text on a light background, and Lc 0 to Lc -108 for light text on a dark background (dark mode). The minus sign merely indicates negative contrast, which means light text on a dark background.

It will be moving to its own URL soon: apcacontrast.com

That URL works now, as a redirect, but will become the host soon.