Blackbody Peak Wavelength Calculator
Every warm object glows with thermal radiation, and the colour of that glow depends only on its temperature. Wien's displacement law gives the wavelength at which a blackbody emits most strongly, and it shows that hotter bodies peak at shorter, bluer wavelengths while cooler ones peak in the red and infrared. This calculator takes an absolute temperature in kelvin and returns the peak emission wavelength in nanometres and metres. It is used in astronomy to estimate star temperatures, in thermal imaging and in lighting design.
Wien's displacement law formula
lambda_peak = b / T
b = 2.897771955e-3 metre-kelvin (Wien's constant)
T = absolute temperature (kelvin)
wavelength in nm = lambda_peak * 1e9
The peak wavelength is inversely proportional to temperature. Wien's displacement constant is fixed by the SI definitions of the kelvin and the metre. Always use absolute temperature in kelvin.
Thermal radiation context
- Wien's displacement constant is 2.897771955 times ten to the minus three metre-kelvin.
- The Sun, at about 5,778 kelvin, peaks near 500 nanometres in the green.
- A human body near 310 kelvin peaks in the far infrared, around 9,400 nanometres.
- Hotter stars are bluer because their peak wavelength is shorter.
- Convert Celsius to kelvin by adding 273.15 before using this calculator.
Blackbody peak wavelength: frequently asked questions
What is Wien's displacement law?
Wien's displacement law states that the wavelength at which a blackbody emits most strongly is inversely proportional to its absolute temperature: lambda_peak = b / T. The constant b is Wien's displacement constant, 2.897771955 times ten to the minus three metre-kelvin. Hotter objects peak at shorter wavelengths.
What is Wien's displacement constant?
Wien's displacement constant b equals 2.897771955 times ten to the minus three metre-kelvin, a value fixed by the 2019 redefinition of SI base units. Dividing it by the absolute temperature in kelvin gives the peak wavelength of blackbody radiation in metres.
Why must I use absolute temperature?
Wien's law uses the thermodynamic temperature in kelvin, which starts at absolute zero. To convert from Celsius, add 273.15; from Fahrenheit, convert to Celsius first then add 273.15. Using degrees Celsius or Fahrenheit directly would give a meaningless result.
What is the peak wavelength of the Sun?
The Sun's surface is about 5,778 kelvin, so its peak emission wavelength is roughly 500 nanometres, in the green part of the visible spectrum. Our eyes evolved to be most sensitive near this peak, which is no coincidence given the Sun's central role in life on Earth.
Does a real object behave exactly like a blackbody?
No real object is a perfect blackbody, but many emit close to one. The closer the surface is to an ideal absorber and emitter, the better Wien's law predicts its peak wavelength. For rough estimates of stars, filament lamps and thermal cameras, the law works very well.
Official sources
- U.S. National Institute of Standards and Technology: Wien displacement law constant.
- NASA Science: The electromagnetic spectrum.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.