Question 1

What WCAG contrast ratio do I actually need for normal body text?

Accepted Answer

WCAG 2.1 Level AA (the legal standard in most jurisdictions for ADA, EN 301 549, and similar regulations): minimum 4.5:1 for text under 18pt (or under 14pt bold). Level AAA (enhanced, not legally required but recommended): minimum 7:1. For large text (18pt+ or 14pt+ bold): AA requires 3:1, AAA requires 4.5:1. For UI components and focus indicators: AA requires 3:1 against adjacent colors.

Question 2

How is the WCAG contrast ratio actually calculated?

Accepted Answer

Formula: Contrast Ratio = (L1 + 0.05) ÷ (L2 + 0.05), where L1 is the relative luminance of the lighter color and L2 is the relative luminance of the darker color. Relative luminance linearizes the sRGB channel values and weights them by human visual sensitivity: 0.2126×R + 0.7152×G + 0.0722×B. The 0.05 offset prevents division by zero for pure black.

Question 3

What is APCA and how is it different from WCAG 2.x contrast ratios?

Accepted Answer

APCA (Advanced Perceptual Contrast Algorithm) is proposed for WCAG 3.0 and uses a perceptual model that accounts for font size, weight, and the actual human visual system's response — which the current 4.5:1 ratio doesn't do. A pair of colors that passes WCAG 2.1 AA might fail APCA for small light text, and vice versa. APCA is not yet a legal standard but represents the direction accessibility is moving.

Question 4

Is 4.5:1 contrast ratio legally required in the US?

Accepted Answer

The ADA (Americans with Disabilities Act) doesn't specify color contrast directly, but DOJ guidance and court interpretations have broadly adopted WCAG 2.1 Level AA as the expected standard for web accessibility compliance. WCAG 2.1 AA at 4.5:1 is also the explicit standard in the EU (EN 301 549), UK (PSBAR), and many other jurisdictions.

Strict WCAG Contrast Math

Color Inputs

Live Preview Simulator

Large Text Preview

AA Level (Standard)

AAA Level (Strict)