How to use this calculator
Pick a surface to fill in its emissivity, then enter the area and the surface and surroundings temperatures in any scale.
Formulas
Net exchange (small surface, large surroundings)Q = εσA(Ts⁴ − Tsur⁴) σ = 5.670 × 10⁻⁸ W/m²·K⁴
Radiation coefficienthr = εσ(Ts + Tsur)(Ts² + Tsur²)
Worked example
1 m² black-painted surface at 100 °C in a 20 °C room (the default)
- Ts = 373.15 K, Tsur = 293.15 K.
- Q = 0.95 × 5.670 × 10⁻⁸ × 1 × (373.15⁴ − 293.15⁴) = 647 W.
- hr ≈ 8.1 W/m²·K, comparable to natural convection in air.
Frequently asked questions
Why must temperatures be in kelvin?
The law depends on T⁴ of absolute temperature. Using Celsius would give completely wrong numbers.
What is emissivity?
How well a surface radiates compared with a perfect black body, from 0 to 1. Shiny metals reflect most radiation and emit little; most painted or oxidized surfaces are 0.8–0.95.