Heat Transfer Radiation Calculator

Summary

This calculator applies the Stefan-Boltzmann law of thermal radiation: Q = εσA(T⁴ − T_surr⁴), where ε is emissivity (0–1), σ = 5.67×10⁻⁸ W/(m²·K⁴) is the Stefan-Boltzmann constant, A is the surface area, T is the absolute surface temperature, and T_surr is the surrounding (ambient) temperature, both in Kelvin. Select an emissivity preset or enter a custom value. Results are displayed in watts with a step-by-step substitution so you can verify every calculation.

How it works

1. Select a surface material preset — its typical emissivity fills automatically — or choose "Custom" and enter ε directly.
2. Enter the surface area (A) of the radiating object in m², cm², ft², or in².
3. Enter the surface temperature (T) and surrounding temperature (T_surr) in °C, °F, or K.
4. Click Calculate — the tool converts temperatures to Kelvin, evaluates Q = εσA(T⁴ − T_surr⁴), and displays the net radiated heat in watts.
5. Positive Q means the surface radiates net heat to the surroundings; negative Q means it absorbs more than it emits.
6. A step-by-step breakdown shows each substitution so you can trace the arithmetic.

Use cases

• Estimating heat loss from a hot pipe or furnace surface by radiation.
• Thermal engineering coursework on radiative heat transfer and emissivity.
• Comparing radiation from painted metal vs. bare polished metal.
• HVAC and building physics: quantifying radiant heat gain through a roof or wall.
• Electronics cooling: checking how much heat a PCB or heatsink radiates.
• Astronomical physics: applying the Stefan-Boltzmann law to stellar luminosity problems.
• Research and lab reports requiring unit-correct radiative flux calculations.

Frequently Asked Questions