How this calculator works
Total hemispherical emission scales with absolute temperature to the fourth power and with emitting area. A grey surface multiplies blackbody power by emissivity between zero and one. These are emitted watts; heat gained from surroundings is not subtracted. For a small grey surface in a large black enclosure, net radiation would instead use the difference of fourth powers. Surface presets are illustrative ASHRAE table values, not universal material constants; ice uses the midpoint of 0.96 to 0.97. Enter measured total emissivity for precision work. The constant is derived from exact CODATA 2018 h, kB and c.
Formula
P = epsilon sigma A T⁴; sigma = 2pi⁵kB⁴/(15h³c²) = 5.670374419...e-8 W/(m² K⁴). T is absolute temperature in kelvins.
Worked example
A 1 m2 ideal blackbody at 300 K emits about 459.300328 W. Raising it to 600 K multiplies emission by 16, giving about 7348.805247 W.
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Frequently asked questions
Can I enter Celsius?
Yes. The engine converts Celsius or Fahrenheit to kelvins before applying the fourth power.
Is this net heat loss?
It is emitted power. Absorbed radiation from the surroundings must be included separately for a net exchange calculation.
Why do material tables disagree?
Emissivity changes with surface finish, oxidation, wavelength and temperature. Presets are illustrative.