Stefan–Boltzmann

The Stefan-Boltzmann law says every object radiates thermal energy at a rate proportional to its surface area and the fourth power of its absolute temperature: P = εσAT⁴, where σ is the Stefan-Boltzmann constant (5.670374419×10⁻⁸ W/(m²·K⁴)) and ε is emissivity, a 0-to-1 factor describing how efficiently a real surface radiates compared to a perfect blackbody. This calculator plugs your temperature (in Kelvin), area (in square meters), and emissivity straight into that formula to return total radiated power in watts — no approximation, just the direct physics. Because temperature is raised to the fourth power, small changes matter enormously: doubling absolute temperature increases radiated power sixteenfold, which is why thermal radiation dominates so completely at stellar temperatures despite being a minor factor in everyday room-temperature heat transfer.

The defaults model a resting human body (310 K, about 1.8 m² of skin, emissivity 0.97, since skin radiates almost like a perfect blackbody regardless of visible skin color, which only affects reflectance in visible light, not infrared emissivity), but the same formula scales from a light bulb filament to a star's photosphere. One important limitation: this calculates gross power radiated outward only — it doesn't net out power absorbed from the surrounding environment, so it will overstate real-world heat loss for anything sitting in a warm room or in sunlight, where two-way radiative exchange applies. Temperature must be entered in Kelvin, not Celsius or Fahrenheit — a common input mistake, since the T⁴ dependence makes using the wrong scale produce wildly wrong results rather than a small error.