How is the WCAG 2.1 contrast ratio calculated?

The WCAG 2.1 contrast ratio formula is (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, normalized between 0.0 (pure black) and 1.0 (pure white).

What are the WCAG Level AA and AAA requirements for body text?

WCAG 2.1 Level AA requires a contrast ratio of at least 4.5:1 for normal text (under 18pt regular or under 14pt bold). Level AAA requires a stricter contrast ratio of at least 7.0:1 for normal text.

Why does green contribute more to luminance than blue?

Under photopic daylight vision, the human eye is most sensitive to green wavelengths near 555 nm. In the standard sRGB luminance equation (Y = 0.2126·R + 0.7152·G + 0.0722·B), green contributes over 71.5% of perceived brightness, while blue contributes only 7.22%.

Accessibility & Optics LabRatio = (L₁ + 0.05) / (L₂ + 0.05)

WCAG 2.1 Contrast & Luminance Calculator

Verify text legibility and color compliance against W3C Level AA and AAA standards with real-time photometric luminance scoring.

WCAG 2.1 Sample Preview

17.24:1 Ratio

Large Heading Text (24px Bold)

Standard 16px body copy: Optical wave mechanics and photon energy quantization.

Foreground (Text) Color

Background Color

Laboratory Contrast Presets

WCAG 2.1 Compliance Matrix

Luminance: FG 0.009 / BG 0.972

Normal Text (<18pt / <24px)

AA (≥4.5:1) PASS

AAA (≥7.0:1) PASS

Large Text (≥18pt / ≥24px)

AA (≥3.0:1) PASS

AAA (≥4.5:1) PASS

UI Components & Icons

AA (≥3.0:1) PASS

Ratio ScoreAAA High

The Photometric Science of Visual Contrast

Visual accessibility is grounded in human neurobiology. When reading on electronic displays, the eye detects character boundaries through luminance differences between adjacent retinal receptive fields rather than color hue alone.

Photopic Luminance Equation

Relative luminance L measures physical radiant power weighted by the human spectral sensitivity curve V(λ):

L = 0.2126 · R_lin + 0.7152 · G_lin + 0.0722 · B_lin

Weber-Fechner Contrast Ratio

The constant +0.05 offset accounts for ambient flare light in typical viewing environments:

Ratio = (L_lighter + 0.05) / (L_darker + 0.05) : 1

lightcontrast-checker Specification-based recommendation

Datacolor SpyderX Pro

Fast monitor calibration with a lens-based color engine

Fast calibration workflow

Lens-based measurement

No ambient-light sensor

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Traian AnghelAuthor & Reviewer

Physics Teacher & Educational Technologist • Brăila, Romania

Last Reviewed: August 2026

Wave Lab calculators are constructed using rigorous peer-reviewed physics formulas from verified metrological repositories. Every equation, spectral conversion, and acoustic property is tested against NIST, CIE, and ISO datasets.

Verified Sources:NIST CODATACIE 1931 2° ObserverNASA Glenn Research