Color Temperature (Kelvin) to RGB Converter

About the Color Temperature (Kelvin) to RGB Converter

Type a color temperature — 2,700 K for a household bulb, 6,500 K for daylight — and see the color of that light as HEX, RGB and linear RGB. The color is computed from first principles: Planck’s law for a black body, summed against the official CIE 1931 2° colour-matching functions, or the CIE daylight locus for daylight from 4,000 K to 25,000 K. It is then scaled so that its brightest channel is 255, which is how light colors are usually given to apps, LEDs and 3D renderers.

It also works backwards: pick or paste any color and the converter estimates its correlated color temperature with McCamy’s formula, finds the exact value by the CIE definition, and gives Duv — how far the color is from the black-body line, and whether it is greenish or pinkish. A temperature strip, a table of common light sources and a CSV export are included.

Common uses

• Choosing light colors for 3D scenes, games, smart bulbs and LED strips from a Kelvin value.
• Making “warm” and “cool” UI themes or illustrations that match real light sources.
• Checking the white point of a design, photo or display color as a color temperature and tint.
• Teaching how black-body radiation and color temperature relate to what we see.

How the color is calculated

For a black body the tool evaluates Planck’s law, M(λ, T) ∝ λ⁻⁵ / (e^(c₂/λT) − 1) with c₂ = 1.4388 × 10⁻² m·K (CIE 015), at every 5 nm from 360 to 830 nm, and multiplies it by the CIE 1931 2° colour-matching functions x̄, ȳ and z̄ to get the tristimulus values X, Y, Z. The functions are the CIE’s own data table (CIE datatable, CC BY-SA 4.0). Checked against the CIE’s own definition of illuminant A (a black body at 2,848 K with the older c₂ = 1.435 × 10⁻² m·K), it gives A’s chromaticity, x 0.44757, y 0.40745, to within 0.00002; at 2,856 K with today’s c₂ it is within 0.00004. It also matches the published points of the black-body locus.

For CIE daylight it uses the daylight locus of CIE 015: x = −4.6070·10⁹/T³ + 2.9678·10⁶/T² + 0.09911·10³/T + 0.244063 from 4,000 to 7,000 K (with the constants −2.0064·10⁹, 1.9018·10⁶, 0.24748·10³ and 0.237040 above 7,000 K) and y = −3.000x² + 2.870x − 0.275.

The chromaticity is converted to linear sRGB with the IEC 61966-2-1 matrix for a D65 white (the exact form in CSS Color 4), scaled so the largest channel is 1, and encoded with the sRGB curve. There is no chromatic adaptation, so the colors are what each light looks like next to D65 white.

Lights outside the sRGB gamut

Below roughly 1,900 K a black body is a deeper orange-red than any sRGB color, and its blue channel would be negative. The tool then adds the smallest amount of white light (equal parts of linear red, green and blue — the sRGB white) that brings every channel to zero or more. That keeps the hue and lowers the saturation, and the result is marked “outside sRGB”. Wide-gamut displays could show these colors more saturated.

Color → Kelvin: McCamy, the CIE definition and Duv

McCamy’s cubic formula estimates the correlated color temperature (CCT) directly from the CIE 1931 chromaticity: n = (x − 0.3320) / (0.1858 − y) and CCT = 449n³ + 3525n² + 6823.3n + 5520.33 (McCamy, Color Research & Application 17(2)).

The CIE defines CCT as the temperature of the black body whose color is nearest in the CIE 1960 uv diagram. The tool finds that point by searching the black-body locus from 1,000 K to 100,000 K and reports Duv, the signed distance from the locus: positive above it (greenish), negative below (pinkish or magenta). CIE 015 advises against quoting a CCT when the color is more than 0.05 from the locus, and the tool says so.

On the black-body locus, McCamy’s estimate is within about 10 K of the CIE value from 2,856 to 6,504 K and within about 120 K from 2,000 to 10,000 K (this tool’s own comparison), but it falls hundreds of kelvin short above 12,000 K — so both values are shown.

Common light sources

The table lists typical values. The CIE illuminants are defined values: A (2,856 K, tungsten light), D50 (about 5,003 K, the graphic-arts viewing standard), D65 (about 6,504 K, the sRGB and HD-video white) and D75 (about 7,504 K). Lamp labels such as 2,700 K or 4,000 K are the nominal CCT printed on the pack; real LEDs and fluorescent lamps are not black bodies and can be visibly greener or pinker. Flames, sunlight and sky vary with conditions.

Limitations

• A color temperature describes only the chromaticity of a light, not its spectrum: two lamps of the same CCT can render colors very differently (see their CRI or TM-30 values).
• Colors are for an sRGB display with a D65 white. On a screen set to another white point, or after a camera’s white balance, the same light looks different.
• Below about 1,900 K the black-body color is outside sRGB and is shown desaturated.
• The CCT search covers 1,000 K to 100,000 K; colors beyond either end are reported as warmer or bluer than that.
• The exported CSV covers up to 10,000 temperatures per file.

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