Stefan-Boltzmann law calculator
Enter emissivity, area and surface temperature to get the radiated power. Add the surroundings temperature to get the net heat lost by radiation.
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How to use it
- Enter emissivity ε (0 to 1), area, surface temperature, surroundings temperature (optional) and temperature unit.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
P = ε σ A T⁴, and net radiation P = ε σ A (T⁴ − Tₛ⁴), where σ = 5.670374419 × 10⁻⁸ W/(m²·K⁴).
Example
Inputs: Emissivity ε (0 to 1) = 1, Area = 1 m², Surface temperature = 300, Temperature unit = kelvin (K). Result: Radiated power: 459.3 W; Radiant exitance: 459.3 W/m².
Limits and caveats
- It assumes emissivity is the same at all wavelengths (a grey body); real emissivity depends on material and surface finish.
- Net radiation assumes the surroundings fully enclose the surface and behave as a black body.
Questions about this tool
What are typical emissivity values?
Roughly: human skin 0.98, matte paint 0.9 or more and polished aluminum about 0.05.
Why is temperature raised to the fourth power?
Radiated power grows very fast with temperature; doubling the absolute temperature multiplies it by 16.
Can I enter the temperature in Celsius?
Yes, choose Celsius in the unit list and it is converted to kelvin before calculating.