Blackbody Radiation Calculator

How This Blackbody Calculator Works

This calculator helps you understand how hot objects glow and radiate energy. A blackbody is an ideal surface that absorbs all light that hits it and emits thermal radiation based only on its temperature. Real materials are not perfect, but blackbody physics is a powerful model used in astronomy, climate science, thermal engineering, and infrared imaging. With a few inputs, you can estimate the peak wavelength, radiated power, and total power output of a heated surface.

Two key relationships summarize blackbody behavior. Wien’s displacement law gives the wavelength where emission is strongest: λmax = b/T, with b ≈ 2.897771955×10⁻³ m·K. As temperature rises, the peak shifts to shorter wavelengths, which is why hotter objects look more blue-white and cooler ones look red or invisible to the eye. Stefan–Boltzmann’s law gives the total radiated power per unit area: q = ε σ T⁴. Here σ = 5.670374419×10⁻⁸ W·m⁻²·K⁻⁴ and ε is emissivity, a factor between 0 and 1 that describes how closely a surface behaves like a perfect blackbody. If you enter an emitting area A, the calculator reports total power with P = qA.

The optional spectrum plot uses Planck’s law to show how intensity changes with wavelength. The curve is normalized so you can compare shapes at different temperatures. This visualization helps connect temperature to color and infrared emission. For example, room-temperature objects peak in the infrared, a stove burner peaks in the red or near infrared, and the Sun peaks in visible light around green-yellow.

To use the calculator, start by entering the temperature in Kelvin or Celsius. If you only know the surface temperature in °C, the tool converts it to Kelvin automatically. Next, enter emissivity if the surface is not a perfect blackbody; matte paint, human skin, and many non-metallic materials are typically high (around 0.9), while shiny metals are lower. If you know the emitting area, add it to compute total power. The results include peak wavelength, radiative flux (power per square meter), and total power output.

Step by step: set temperature, set emissivity, optionally add area, then read the wavelength and power values. Use the spectrum plot to see where the emission lies in the electromagnetic spectrum. If you are comparing two temperatures, change only one field at a time so the effect is easy to see.

This is useful for estimating heat loss from a hot surface, interpreting infrared camera readings, comparing stars by temperature, or understanding why a filament bulb glows yellow while a hot arc lamp looks blue-white. It is also handy in physics homework when you need quick values for Wien’s law, Stefan–Boltzmann’s law, or blackbody radiation curves.

Tips: Temperature must be on an absolute scale (K). If you enter °C, we convert to K internally via T(K)=T(°C)+273.15. Emissivity ε ranges from 0–1; many real surfaces are 0.8–0.98. Outputs are SI: nm/µm/m for wavelength, W/m² for flux, and W for power.