Contrast ratio is a numerical representation of the relative luminance difference between foreground (text or icons) and background colors. It ranges from 1:1 (no difference) to 21:1 (black and white).

WCAG (Web Content Accessibility Guidelines) 2.1 requires a contrast ratio of at least 4.5:1 for normal text and 3:1 for large text (18pt or larger, or 14pt bold or larger). Accessibility design guides cover these standards in detail.

Contrast ratio calculation uses relative luminance of colors. Tools like Chrome DevTools and axe make it easy to check compliance.

In relation to character counting, low contrast ratio text is difficult to read, preventing users from accurately recognizing characters. Color design fundamentals books provide additional insights.