How to use this calculator
- Enter or choose emissivity (ε).
- Enter or choose surface area.
- Enter or choose surface temperature.
- Enter or choose surroundings temperature.
- The net radiant heat loss updates as you type. Use Copy result or Share to keep it.
Method
Q̇ = ε × σ × A × (T_s⁴ − T_sur⁴); h_rad = εσ(T_s + T_sur)(T_s² + T_sur²)
- ε
- emissivity (0–1)
- σ
- Stefan–Boltzmann constant, 5.670374419 × 10⁻⁸ W/(m²·K⁴)
- A
- surface area
- T_s, T_sur
- absolute temperatures in K
Worked example
- T_s = 333.15 K, T_sur = 293.15 K.
- Emitted = 0.9 × 5.670374419e-8 × 1.5 × 333.15⁴ = 942.99 W; absorbed = 0.9 × σ × 1.5 × 293.15⁴ = 565.33 W.
- Net = 942.99 − 565.33 = 377.65 W.
A 1.5 m² painted surface at 60 °C in a 20 °C room loses about 378 W by radiation.
What the result means
Because of the fourth power, radiation dominates at high temperatures. The radiative coefficient lets you add radiation to convection: total h ≈ h_conv + h_rad.
Assumptions and limits
- Grey, diffuse surface that is small compared with surroundings at a uniform temperature.
- Emissivity equals absorptivity.
- View factors between two finite surfaces are not included.
- Solar or other directed radiation is not included.
Questions
Why must temperatures be in kelvin?
The law depends on T⁴ measured from absolute zero; the calculator converts your °C values for you.
Does a shiny surface lose less heat?
Yes. Low-emissivity surfaces such as polished metal or low-e glass radiate far less, which is why survival blankets are shiny.