Contrast ratio

A contrast ratio measures how different two colours are in brightness, on a scale from 1:1 (identical colours) to 21:1 (black on white). Accessibility standards use it to judge whether text and controls stand out enough from their background - for example, WCAG requires body text to have a contrast ratio of at least 4.5:1 against its background.

Large text gets a lower bar: WCAG's Level AA requires only 3:1 for text at 18pt (or 14pt bold) and above, since bigger letterforms remain legible at lower contrast. Level AAA raises both thresholds, to 7:1 for normal text and 4.5:1 for large text.

Learn more

• Understanding Contrast (Minimum) - the WCAG success criterion this measures against
• WCAG