Correlated Color Temperature Calculator and Guide

A correlated color temperature (CCT) calculator estimates the color temperature of a light source from its chromaticity coordinates, usually entered as CIE 1931 x and y values. Enter those two coordinates to get CCT in kelvins; the result describes the light’s apparent color, not its brightness or full color quality.

Formula

This calculator uses McCamy’s approximate CCT formula, a commonly used estimate for chromaticities near the Planckian locus:

n = (x − 0.3320) / (0.1858 − y)
CCT ≈ 449n³ + 3525n² + 6823.3n + 5520.33 K

• x and y are the CIE 1931 xy chromaticity coordinates. They are dimensionless values, not percentages.
• n is an intermediate, dimensionless value.
• CCT is the estimated correlated color temperature in kelvins (K).

The equation is an approximation, not a universal conversion for every possible color. It works best for white-light chromaticities close to the blackbody radiation curve. The calculator accepts coordinates from 0 to 1 as a basic input check, but that range alone does not guarantee that a coordinate is physically meaningful or suitable for this approximation.

Worked Example

Using the default inputs, x = 0.3127 and y = 0.3290:

1. Calculate n: (0.3127 − 0.3320) / (0.1858 − 0.3290) = −0.0193 / −0.1432 ≈ 0.1348.
2. Substitute n into the CCT equation: 449(0.1348³) + 3525(0.1348²) + 6823.3(0.1348) + 5520.33.
3. The estimate is about 6499 K, which rounds to 6499 K at the calculator’s displayed precision.

Because the equation is approximate and its constants are rounded, the result is best read as roughly 6500 K rather than as a laboratory-grade exact value.

What CCT Describes

Color temperature is a way of describing the color appearance of light by comparing it with the color emitted by an ideal heated radiator, called a blackbody. The comparison is expressed in kelvins. At lower CCTs, white light generally looks warmer, with more yellow or orange; at higher CCTs, it generally looks cooler or bluer.

Despite the wording, a CCT value is not a measurement of the physical temperature of an LED, lamp, room, or object. A lamp labeled 3000 K is not operating at a temperature of 3000 kelvins. CCT describes the light’s chromaticity as perceived relative to the blackbody reference.

Typical broad descriptions include warm white around 2700–3000 K, neutral or cool white around 3500–5000 K, and daylight-like light around 5000–6500 K. These are conventions, not strict universal boundaries. The same stated CCT can also look different depending on surrounding colors, viewing conditions, and the light source’s spectral output.

How to Get the Input Coordinates

The calculator needs chromaticity coordinates, not a lamp’s wattage, lumen output, or printed CCT label. Coordinates may come from a colorimeter or spectroradiometer, a lighting measurement report, or a source that publishes CIE 1931 xy data. Check that the values use the CIE 1931 xy system; coordinates from another color space cannot be entered directly without conversion.

• Enter x in the first field and y in the second field, as decimal values. For example, 0.3127 is not entered as 31.27.
• Use coordinates for the particular operating condition you care about. Dimming, temperature, and drive conditions can affect some sources’ chromaticity.
• Keep the coordinate pair together. An x value from one measurement and a y value from another do not describe a valid measured point.
• If the denominator in the n formula is zero, the calculation is undefined. Coordinates very near that condition can also produce unstable estimates.

Interpreting the Result and Its Limits

The output is an estimate in kelvins. A higher number generally corresponds to a bluer-looking white, and a lower number to a more amber-looking white. The result does not state whether the light is bright: brightness is commonly described using lumens or illuminance, such as lux. Nor does CCT report how accurately the source renders object colors. Color rendering is assessed with other measures, such as color rendering index (CRI) or the IES TM-30 system.

A CCT value also leaves out tint. Two lights can have similar CCTs but differ along the green-to-magenta direction. A chromaticity point’s distance and direction from the Planckian locus are often described with a measure such as Duv. For tasks where precise color matching matters, CCT alone is insufficient; use chromaticity and color-rendering data, and follow the relevant measurement method.

McCamy’s formula is convenient for quick estimates, but it can be inaccurate for chromaticities far from the Planckian locus, strongly tinted colors, or values outside its useful range. More rigorous methods determine the closest point on the Planckian locus in an appropriate chromaticity space and calculate the corresponding temperature. Results from different methods or instruments may therefore differ, especially for unusual spectra or borderline colors.

Common Mistakes and Better Comparisons

• Confusing CCT with physical temperature: the kelvin value describes light color, not how hot the lamp or room is.
• Comparing CCT as if it described the whole source: it does not capture brightness, tint, flicker, spectrum, or color rendering.
• Entering the wrong coordinate system: RGB values, CIE u′v′ coordinates, and xy coordinates are different quantities. Convert before using this formula.
• Overreading precision: an output like 6499 K should not be treated as a meaningful one-kelvin distinction. Measurement uncertainty and formula limits are generally more important.

When comparing two lamps, use CCT as one useful filter, then compare their chromaticity or Duv and color-rendering specifications if matching appearance matters. A nominal CCT printed on a product is often rounded and may represent a target or tolerance rather than the exact output of every unit.

Frequently Asked Questions

How do I calculate correlated color temperature?
Use measured CIE 1931 x,y chromaticity coordinates and apply a CCT estimation method. This calculator applies McCamy’s formula, which is convenient near the Planckian locus but is not exact for every color.

Can I calculate CCT from RGB or a hex color code?
Not directly with this calculator. RGB and hex values depend on a color space and often a display profile; they must be converted to an appropriate chromaticity representation first. Even then, a screen color is not necessarily a measurement of emitted light.

Is 6500 K the same as daylight?
It is commonly described as daylight-like, but daylight changes in color and spectrum with conditions. A 6500 K source does not necessarily reproduce natural daylight or render colors like it.

Does a higher CCT mean a brighter bulb?
No. CCT describes the appearance of the light’s color. Brightness or light output is described with other measurements, such as lumens for total output or lux for illuminance on a surface.

Why does the calculator result differ from a lamp’s label?
The label may be rounded, use a different estimation method, or describe a nominal target. Measurement conditions, instrument accuracy, and the source’s distance from the Planckian locus can also affect the result.

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in x | CIE x chromaticity | - | 0.3127 | 0 | 1
in y | CIE y chromaticity | - | 0.3290 | 0 | 1
out cct | Correlated color temperature | K | 449 * (n^3) + 3525 * (n^2) + 6823.3 * n + 5520.33 | 0
out n | McCamy n value | - | (x - 0.3320) / (0.1858 - y) | 4