Color Temperature Calculator

Reviewed by Pero Mikić, Physics & Technical Education (Gimnazija Sisak) • Last updated 2026-07-07

The Color Temperature Calculator calculates correlated colour temperature from chromaticity coordinates or spectral distribution, aiding lighting analysis and photometric comparisons.

Color Temperature Calculator Estimate light source color temperature from chromaticity coordinates or basic lamp type assumptions. Use exact CIE x,y if known for more accurate results.

Calculation Mode Choose how you want to estimate correlated color temperature.

CIE x coordinate Typical values range from about 0.25 to 0.45 for white light.

CIE y coordinate For standard daylight D65, y is approximately 0.3290.

Output Precision Controls rounding of the estimated color temperature.

Optional Notes Add context like usage, camera settings, or measurement device.

Example Presets Click a preset to populate inputs. Adjust as needed, then calculate.

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What Is a Color Temperature Calculator?

A color temperature calculator is a tool that estimates how “warm” or “cool” a light source appears. It reports correlated color temperature (CCT) in kelvins (K). Warm light has lower CCT values, like 2700 K. Cool light has higher CCT values, like 6500 K.

Unlike a true blackbody radiator, real lamps and screens are not perfect radiators. Their spectra differ from an ideal heater. CCT solves this by finding the closest point to your source on the Planckian locus. The locus is the curve of perfect blackbody colors across temperatures.

The calculator converts your input to a standard color space and computes CCT. It also returns optional metrics. These include Duv, the signed distance from the blackbody curve, and an uncertainty estimate when possible.

Color Temperature Calculator — Plan and estimate color temperature.

The Mechanics Behind Color Temperature

Color temperature links physics and perception. Physics provides spectra and blackbody radiation. Perception uses standardized color spaces that map spectra to visible color coordinates.

Blackbody basis: Planck’s law gives spectral radiance for temperature T. Its visible color traces the Planckian locus in chromaticity diagrams.
Chromaticity coordinates: We map a light source to coordinates, often CIE 1931 x,y or CIE 1976 u′,v′. These are dimensionless ratios.
Closest-match search: Correlated color temperature is the temperature of the blackbody point closest to your chromaticity.
Distance and sign: Duv measures offset from the locus. Positive Duv looks greenish; negative Duv looks magenta relative to the locus.
Observer functions: Calculations use standardized color matching functions, usually the CIE 1931 2° or 1964 10° observer.

The calculator performs these steps automatically. It transforms your input into chromaticity, finds the nearest blackbody point by a defined metric, and returns the temperature and distance. The variables and derivation are consistent with established CIE methods.

Color Temperature Formulas & Derivations

Different input types require different equations. The core idea is to derive chromaticity and then locate the nearest blackbody color. Below are the main formulas used in the computation and their roles.

Planck’s law (spectral radiance): B(λ,T) = 2hc²/λ⁵ · 1/(exp(hc/(λkT)) − 1). Variables: wavelength λ, temperature T, constants h, c, k.
XYZ from spectrum: X = ∫ S(λ) x̄(λ) dλ, Y = ∫ S(λ) ȳ(λ) dλ, Z = ∫ S(λ) z̄(λ) dλ. S is the spectral power distribution.
Chromaticities: x = X/(X+Y+Z), y = Y/(X+Y+Z). For 1976 UCS, u′ = 4X/(X+15Y+3Z), v′ = 9Y/(X+15Y+3Z).
McCamy’s approximate CCT (fast estimate): Let n = (x − 0.3320)/(y − 0.1858). Then CCT ≈ −449n³ + 3525n² − 6823.3n + 5520.33.
Robertson method (high accuracy): Interpolate along the Planckian locus in the CIE 1960 UCS, using iso-temperature line slopes to minimize perpendicular distance.
Duv (signed distance): Duv = sign · √[(u′ − u′_pl)² + (v′ − v′_pl)²], where subscript pl is the nearest point on the locus.

When only RGB values are available, the calculator converts RGB to XYZ using the assumed color space and white point. It then proceeds as above. The derivation ensures that your result is comparable across devices and spectra. The result includes CCT and optional Duv. For spectrum inputs, the method is exact to the chosen observer and sampling.

Inputs, Assumptions & Parameters

The calculator accepts several input formats. Pick the one you can measure or export. Each format is converted to a common color metric before CCT evaluation.

CIE x,y or u′,v′ chromaticity: Direct entry of coordinates is fastest. Choose the observer (2° or 10°).
Spectral power distribution (SPD): Provide S(λ) with wavelength units and step size. The tool integrates to XYZ.
RGB values: Enter RGB with the correct color space (sRGB, Adobe RGB, Display P3) and gamma. The tool maps to XYZ.
White point assumption: For RGB, specify the white point, often D65. This affects the chromaticity result.
Observer selection: Choose CIE 1931 2° or CIE 1964 10°. This changes x,y and thus CCT.
Smoothing/tolerance: Optional spectral smoothing and numerical tolerances control stability in noisy data.

Inputs should reflect realistic ranges. Chromaticities must lie within the valid diagram. SPDs should be sampled from 360–830 nm or your instrument’s range. RGB must be within 0–255 or 0–1. Edge cases include very spiky spectra, narrow-band LEDs, and chromaticities far from the locus. These may yield large Duv or uncertain CCT.

Step-by-Step: Use the Color Temperature Calculator

Here’s a concise overview before we dive into the key points:

Select your input type: chromaticity, spectrum, or RGB.
Set the observer standard (CIE 1931 2° is common).
If using RGB, choose the correct color space and white point.
Enter x,y or u′,v′, or upload the SPD file, or type RGB values.
Choose the computation method: fast (McCamy) or accurate (Robertson).
Run the calculation to obtain CCT, Duv, and uncertainty.

These points provide quick orientation—use them alongside the full explanations in this page.

Real-World Examples

Warm LED bulb in a living room. A handheld color meter reports CIE x=0.459, y=0.410. Compute n=(0.459−0.3320)/(0.410−0.1858)=0.566. Using McCamy’s formula, CCT ≈ 2704 K. Duv is slightly positive, indicating a faint green shift from the locus. The interpretation is a cozy, incandescent-like appearance.

What this means: The lamp is warm and inviting; skin tones will look pleasant, but whites may skew slightly green without correction.

Calibrating a phone display to D65. The target chromaticity is x=0.3127, y=0.3290. Compute n=(0.3127−0.3320)/(0.3290−0.1858)=−0.1348. CCT ≈ 6506 K by McCamy and ~6504 K by Robertson. Duv is near zero, so the white matches the blackbody curve closely. The result confirms a neutral, daylight white.

What this means: The display is tuned to standard daylight; photos and web content will look neutral across devices.

Accuracy & Limitations

CCT is a one-number summary of a complex spectrum. It is useful but incomplete. Two sources with identical CCT can look different because their spectra differ (metamers). Accuracy depends on input quality and method choice.

Approximation vs. exact: McCamy is fast but approximate. Robertson is more accurate near and along the locus.
Observer effects: 2° vs. 10° observers yield slightly different chromaticities and CCT values.
Off-locus sources: Some LEDs and fluorescents sit far from the locus. CCT becomes less predictive; Duv grows and appearance shifts.
Instrument noise: Noisy or sparsely sampled spectra increase uncertainty. Smoothing and proper sampling help.
RGB assumptions: Wrong color space or white point produces incorrect XYZ and misleading CCT results.

For critical work, prefer SPD-based calculations using the Robertson method and a well-calibrated instrument. Track Duv alongside CCT. When spectra are highly structured, consider additional metrics like CRI or TM-30 for a fuller picture.

Units & Conversions

Unit consistency ensures valid variables, derivations, and results. Kelvin measures temperature. Mired (micro reciprocal degree) is handy for small differences in warm ranges. Spectral math uses SI units, so wavelength and radiance must align.

Common units and conversions used in color temperature work
Quantity	Unit	Conversion
Color temperature	K (kelvin)	—
Mired (micro reciprocal degree)	µrd	mired = 1,000,000 / K; K = 1,000,000 / mired
Wavelength	nm ↔ m	1 nm = 1×10⁻⁹ m; 1 m = 1×10⁹ nm
Spectral radiance	W·sr⁻¹·m⁻²·nm⁻¹ ↔ W·sr⁻¹·m⁻³	B(λ) per nm = B(λ) per m × 10⁻⁹
RGB code values	8-bit ↔ normalized	R′ = R/255, G′ = G/255, B′ = B/255 (assumes 0–255 input)
Temperature offset	°C ↔ K	K = °C + 273.15 (thermodynamic temperature)

Use mired for practical photography adjustments, especially below 4000 K. Keep wavelength units consistent with your spectral data. When comparing results, confirm whether values came from 2° or 10° observers and ensure RGB was correctly normalized.

Troubleshooting

If results look odd, check assumptions first. Most discrepancies come from wrong color space settings, observer choices, or malformed spectra. A small input error can shift the final result by hundreds of kelvins.

RGB looks too cool or warm: Verify the color space and white point (e.g., sRGB, D65).
CCT jumps with small changes: Turn on spectral smoothing or increase wavelength sampling resolution.
Large Duv: Your source is far from the locus; use the accurate method and confirm chromaticity.
Out-of-gamut chromaticity: Recheck input formatting and value ranges.

Still unsure? Try entering a known reference like D65. If the calculator returns ~6500 K and near-zero Duv, your settings are consistent. Otherwise, revisit the observer and conversion parameters.

FAQ about Color Temperature Calculator

What is the difference between color temperature and CCT?

Color temperature refers to an ideal blackbody radiator at temperature T. CCT is the nearest blackbody temperature to a real source’s chromaticity.

Should I use McCamy or the Robertson method?

Use McCamy for quick estimates. Use Robertson for accuracy, off-locus sources, and formal reporting. They agree well near the locus.

Can I compute CCT from RGB values?

Yes, if you know the RGB color space, gamma, and white point. The calculator converts RGB to XYZ, then computes chromaticity and CCT.

What does Duv tell me?

Duv is the signed distance from the blackbody curve. Positive Duv looks greenish; negative Duv looks magenta. Zero means on the locus.

Key Terms in Color Temperature

Correlated Color Temperature (CCT)

The temperature of the blackbody whose chromaticity is closest to your source. Expressed in kelvins. Used to summarize perceived warmth or coolness.

Planckian Locus

The curve on a chromaticity diagram tracing blackbody colors as temperature changes. It is the reference for CCT calculations.

Chromaticity

A color description independent of brightness. Common coordinates are CIE x,y and CIE u′,v′. Derived from XYZ tristimulus values.

Duv

The signed shortest distance from a chromaticity point to the Planckian locus in the 1976 u′,v′ space. Indicates green or magenta tint.

Spectral Power Distribution (SPD)

The power of a light source as a function of wavelength. Integrating SPD with standard observer functions yields XYZ.

Standard Observer

A set of color matching functions defined by the CIE. The 2° and 10° observers represent average human visual responses.

McCamy Formula

An explicit cubic equation estimating CCT from CIE x,y. Fast and useful for many sources near the locus.

Robertson Method

An interpolation technique using iso-temperature line slopes in the CIE 1960 UCS. Provides accurate CCT and Duv near the locus.

References