Stefan–Boltzmann Law Calculator
Radiant power from a hot surface — emissive power, total watts from an area, net exchange with the surroundings and Wien’s peak wavelength, for any temperature and emissivity.
How it works
j = ε·σ·T⁴, σ = 5.670 × 10⁻⁸ W·m⁻²·K⁻⁴; P_net = ε·σ·A·(T⁴ − T_s⁴); λ_peak = 2898 µm·K ÷ T
Every object above absolute zero radiates, and the power climbs with the fourth power of temperature — so a surface at 1,500 K radiates 625 times more than at 300 K. Emissivity scales the ideal black-body value: polished metals are poor emitters (0.05), oxidised or painted surfaces excellent (0.9+). Net exchange subtracts what the surroundings send back, which is why radiation dominates heat loss for hot objects and is minor near room temperature. Wien’s law gives the colour: a 1,500 K object glows orange-red with its peak in the near infrared.
Worked example
A 1,500 K heating element, ε 0.9, 0.05 m²: 258 kW/m² → 12.9 kW radiated, 12.9 kW net against 300 K surroundings, peak emission at 1.93 µm.
Frequently asked questions
What emissivity should I use?
Black paint, brick, skin and most oxides: 0.9–0.98; aluminium foil 0.03–0.05; oxidised steel 0.7–0.9. Tables list values by surface finish.
Why is the human body not glowing?
It radiates about 100 W at 310 K but peaks at 9.4 µm, deep in the infrared — visible only to thermal cameras.
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This calculator is for educational purposes. Double-check important results before acting on them.