WCAG 2.0 Level AA:
- Contrast ratio of 4.5:1 for normal text, or
- 3:1 for large text (18 pt or 14 pt bold)
WCAG 2.0 Level AAA:
- Contrast ratio of 7:1 for normal text, or
- 4.5:1 for large text
Example Color Contrast Checker
There are several free contrast checkers available. The one pictured below is Contrast Analyser from The Paciello Group.