What is a Color Contrast Checker?
A color contrast checker computes the contrast ratio between two colors and compares it to the WCAG accessibility thresholds. Contrast is what makes text legible: the ratio ranges from 1:1 (no contrast) to 21:1 (black on white), and the Web Content Accessibility Guidelines require at least 4.5:1 for normal text and 3:1 for large text at level AA.
Common Uses for Color Contrast Checker
- Check that body text is readable before shipping a design
- Find the lightest grey that still passes AA on a white background
- Verify button and form label contrast for an accessibility audit
Why contrast ratios matter
Around one in twelve men has some form of colour vision deficiency, and everyone loses contrast sensitivity with age or in bright sunlight. A ratio threshold turns 'readable enough' into a measurable number, which is why WCAG uses it — and why search engines and accessibility audits reward it.
Fixing a failed ratio
If text fails, the fastest fix is usually to move the lightness, not the hue: darken the text or lighten the background while keeping the color identity. The checker updates the ratio as you adjust, so you can stop at the first value that passes instead of overshooting to black.
Large text and UI elements
Large text (about 18pt, or 14pt bold) only needs 3:1, because bigger glyphs stay legible at lower contrast. Non-text cues such as input borders, icon buttons and focus indicators also need 3:1 under AA, and they are the most commonly failed check on otherwise compliant sites.