RGB Hex Converter

RGB and hexadecimal notation describe exactly the same thing, an additive colour built from three eight-bit channels. Eight bits per channel gives 256 levels each and 16,777,216 combinations in total, the range usually called truecolor. Hexadecimal is base 16, so one byte fits neatly into two characters from 0 to 9 and A to F, and each pair of a six-character code maps to one channel: FF is 255, 80 is 128, 00 is zero. Converting by hand is division by 16, where the quotient is the first hex digit and the remainder the second, so decimal 200 becomes C8. The luminance figure shown alongside uses the coefficients 0.2126 for red, 0.7152 for green and 0.0722 for blue, which come from the ITU-R BT.709 primaries and reflect the eye being far more sensitive to green than to blue. Contrast in WCAG terms is then the ratio of the lighter to the darker relative luminance with 0.05 added to each, and the thresholds to meet are 4.5 to 1 for body text and 3 to 1 for large text, meaning 18 point or 14 point bold. Several caveats apply. A strict WCAG relative luminance requires linearising each channel first, dividing by 255 and then applying the sRGB transfer function with its 0.04045 threshold and 2.4 exponent, before the weighted sum is taken; a weighted sum applied straight to gamma-encoded values is a fast approximation and will disagree with a conformance checker on mid-tone pairs. A hex triplet is also device-dependent unless a colour space is stated, and by long-standing web convention it is assumed to be sRGB, which is why the same code prints differently through a CMYK process. The complementary colour produced by subtracting each channel from 255 is an RGB inversion, not the perceptual complement an artist would name from a colour wheel. Three-digit shorthand only works when both digits of each pair match, so FF5533 collapses to F53 but 4285F4 cannot be shortened at all, and modern CSS Color Level 4 additionally accepts eight-digit codes carrying an alpha channel.

Hexadecimal became the standard shorthand for byte values through mainframe practice, with the IBM System/360 announced in 1964 doing much to fix the convention of expressing eight-bit quantities as two hex digits. Colour depth caught up gradually: early graphics adapters offered small palettes, and IBM VGA hardware in 1987 still drove an eighteen-bit digital-to-analogue converter, so a full 24-bit framebuffer with 256 levels per channel was initially a workstation feature before becoming universal. Once every channel occupied exactly one byte, the six-digit hex triplet was the obvious compact way to write a colour, and it entered web authoring through Netscape-era HTML attributes before being formally specified. CSS Level 1, published by the W3C in December 1996, defined both the three-digit and six-digit forms, and HTML 3.2 the following month carried the same notation alongside its sixteen named colours. The surrounding colour science was standardised in the same period. Hewlett-Packard and Microsoft proposed sRGB in 1996 as a default colour space for monitors and the internet, and it was published as IEC 61966-2-1 in 1999, which is why an unqualified hex code is read as sRGB today. Its primaries and white point were taken from ITU-R BT.709, the HDTV recommendation issued in 1990, and those same primaries supply the luminance weights used for accessibility contrast. The alternative cylindrical models arrived earlier from computer graphics research: Alvy Ray Smith described the HSV transformation in a 1978 SIGGRAPH paper as a more intuitive way for artists to pick colours than manipulating three additive channels, and HSL followed the same reasoning with lightness centred at 50 percent.