Stefan–Boltzmann Law Calculator

Every object above absolute zero continuously radiates thermal energy, and the Stefan–Boltzmann law gives the total power that leaves its surface: P = ε × σ × A × T⁴, where ε is emissivity (1 for a perfect 'blackbody' emitter, less for a real surface), A is surface area, T is absolute temperature in kelvin, and σ is the Stefan–Boltzmann constant, 5.670374419×10⁻⁸ W/(m²·K⁴) — a value now known exactly, since the 2019 SI redefinition derives it from other exactly-defined physical constants rather than measuring it. Solving the same equation for temperature gives the reverse direction, T = (P ÷ (ε × σ × A))^(1/4).

This is the companion calculation to Wien's displacement law (which finds where a blackbody's emission peaks, not how much it radiates in total): a default 1 m² blackbody at 300 K (room temperature) radiates about 459.3 W, and because power scales with the fourth power of temperature, doubling the absolute temperature multiplies radiated power by 16×, not 2×. Surface area gets this page's metric/imperial toggle, converted to square meters before the formula runs; temperature stays a plain kelvin input, since kelvin-to-Celsius/Fahrenheit is an offset conversion rather than a simple rescale.