RGB to CIELAB Converter

RGB says little about what the eye actually sees: two colour pairs separated by the same numeric distance in RGB can look nothing alike. CIELAB was designed to be roughly uniform in perception, which is why colour difference maths and comparisons in graphics work start from it. Pick a colour in the swatch and the L, a and b coordinates appear underneath.

The route is the textbook one: channels are normalised, the sRGB curve is undone using the linear segment below 0.04045 and the 2.4 power above it, the sRGB to XYZ matrix is applied, and the result is divided by the two-degree D65 white point. Then comes the cube root with its linear segment at 0.008856 and the L, a and b formulas. The blue 3b82f6 gives L 55.6 a 17.6 b -64.4, and pure white gives L 100 with a and b at zero.

Only the RGB to Lab direction exists on this page, despite the name suggesting a round trip: there is no field for typing L, a and b in. Input comes from the native browser picker, so the value is always 8-bit-per-channel sRGB, with no Adobe RGB, Display P3 or ICC profile. The constants are the classic ones, slightly off from the exact CIE 216 over 24389 values, hence the approximate label. Delta E has to be worked out elsewhere.