Blackbody Radiation Calculator

Calculate the spectral radiance of a blackbody in watts or photons per second using this blackbody radiation calculator.

Radiated power62.9386 MWσεAT⁴ at 5,772 K

Radiant flux6.294e+7 W/m²

Peak wavelength502.0395 nm502.04 nm — visible, by Wien's displacement law

Peak frequency597.1492 THz

Emissivity used1a perfect black body, the theoretical maximum

Net radiated power62.9382 MWσεA(T⁴ − T_amb⁴) — the surroundings radiate back, so this, not the gross figure, is what cools the object

Absorbed from the surroundings418.7659 W0.0007% of what it emits

At twice the absolute temperature1.007 GWSIXTEEN times the power — the fourth power is why radiation dominates at high temperature and is negligible at low

At half the absolute temperature3.9337 MW

Temperature for twice the power6,864.1 Konly 18.92% hotter

Against the Sun's surface1× the flux5,772 K, peaking at 502 nm

Against a tungsten filament18.0581× the flux2,800 K, peaking at 1,035 nm

Against a red-hot element1,109.9548× the flux1,000 K, peaking at 2,898 nm

Against human skin126,595.8834× the flux306 K, peaking at 9,470 nm

The formula

P = σεAT⁴; λ_max = b ÷ T

The fourth power changes everything

Radiated power goes as the fourth power of absolute temperature, which makes radiation negligible at room temperature and utterly dominant at high ones. Doubling the absolute temperature multiplies the output sixteenfold. It is why a filament at 2800 K is blindingly bright while the same filament cold radiates nothing you can see, and why conduction and convection matter for a warm radiator while radiation matters for a furnace.

Wien's law says the peak wavelength is inversely proportional to temperature. The Sun at 5772 K peaks around 500 nm, right in the middle of the visible band — which is not a coincidence, since eyes evolved under that spectrum. A tungsten filament at 2800 K peaks in the near infrared, which is why incandescent bulbs waste most of their energy as heat.

What cools an object is the net exchange. The surroundings radiate back, so the driving quantity is T⁴ − T_ambient⁴. For a warm object in a warm room those terms nearly cancel and radiative cooling is slow; for a hot object in a cold one the ambient term is negligible.

Emissivity scales the whole thing. Polished metal radiates only a few percent of a black body's output, which is why a vacuum flask is silvered and why thermal cameras misread shiny surfaces badly.