RGB to Grayscale Converter (ITU-R BT.601, BT.709 & Luminance

RGB to Grayscale, ITU-R Recommendations & Photopic Human Vision

Converting 3-channel RGB color into single-channel luminance (grayscale) accounts for human eye spectral sensitivity:

• **

• Universal Grayscale Formulas**:

• ITU-R BT.709 (Modern HDTV, sRGB, Web Standard):

Y = 0.2126 · R + 0.7152 · G + 0.0722 · B

• ITU-R BT.601 (Classic NTSC, PAL, JPEG):

Y = 0.2990 · R + 0.5870 · G + 0.1140 · B

• Simple Unweighted Average:

Y = (R + G + B) / (3)

• Photometric HSP Model:

P = √(0.299 R^2 + 0.587 G^2 + 0.114 B^2)

• **

• Why Green is Weighted Most Heavily (~71.5%)**:

• Human retinal cone cells are predominantly sensitive to mid-spectrum wavelengths (green-yellow light around 555 nm).

• Pure Green (0, 255, 0) appears significantly brighter (71.5\%) than pure Red (21.3\%) or pure Blue (7.2\%) to human perception.

How to Use This Calculator

Enter your Red Channel (R: 0 to 255) (e.g. 37 in R), Green Channel (G: 0 to 255) (e.g. 99 in G), Blue Channel (B: 0 to 255) (e.g. 235 in B), and Color Benchmark — choose from Custom RGB Values, Pure Green (0, 255, 0) → 58.7% BT.601 / 71.5% BT.709, Pure Red (255, 0, 0) → 29.9% BT.601 / 21.3% BT.

• Results update instantly as you type — adjust any value to compare scenarios.

The Formula

Formula Reference: RGB to Grayscale Converter (ITU-R BT.601, BT.709 & Luminance)

Input variables:

redChannelInput — Red Channel (R: 0 to 255)

greenChannelInput — Green Channel (G: 0 to 255)

blueChannelInput — Blue Channel (B: 0 to 255)

Computed outputs:

bt709HdtvGrayscaleHex — HDTV / sRGB Standard (ITU-R BT.709)

bt601NtscGrayscaleHex — SDTV / JPEG Standard (ITU-R BT.601)

averageGrayscaleHex — Unweighted Average Grayscale

relativeLuminancePercentage — Perceived Brightness (BT.709 %)

photometricHspBrightnessValue — Photometric HSP Brightness (0-255)

Mathematical relationships extracted from calculation logic:

y709 = Math.round(0.2126 r + 0.7152 g + 0.0722 * b)

y601 = Math.round(0.299 r + 0.587 g + 0.114 * b)

yAvg = Math.round((r + g + b) / 3)

hsp = Math.round(Math.sqrt(0.299 r r + 0.587 g g + 0.114 b b))

pct709 = safeDiv(y709, 255) * 100.0

Standard: NIST Handbook 44 and BIPM SI unit definitions

Worked Example

A computer vision engineer converting an RGB image matrix into an 8-bit grayscale array applies the BT.709 formula Y = 0.2126R + 0.7152G + 0.0722B, ensuring the resulting monochrome representation accurately mirrors human perceptual brightness.

Understanding Your Result

Unit conversion precision: round to the same number of significant figures as your original measurement. Converting 6 feet to meters gives 1.8288 m — but if your measurement was approximate (±1 inch), expressing more than 2 decimal places is false precision. For international trade and technical specifications, SI units (meters, kilograms, liters) are the standard. US customary units (feet, pounds, gallons) differ from Imperial UK units: 1 US gallon = 3.785 L vs. 1 UK gallon = 4.546 L.