# Stefan Boltzmann Law Calculator

Welcome to the Stefan-Boltzmann law calculator, a tool created to calculate the radiation heat transfer from a surface. The Stefan-Boltzmann law of radiation initially applies to a blackbody, but **this calculator allows the selection of different emissivities** so that you can use it in two ways:

- Blackbody calculator; and
- Radiation heat transfer calculator of any surface material.

Keep reading this article to learn more about Stefan-Boltzmann law and its formula.

## The Stefan-Boltzmann law of radiation

**Radiation** is the emission of energy in the form of waves or photons. Within the types of radiation, we can find **electromagnetic radiation**, in which energy is in the form of waves of the electromagnetic field. One of the most common ways of electromagnetic radiation is **thermal radiation**, which occurs because of the temperature of the particles of an object. Any object with a temperature above absolute zero (zero kelvin or Rankine degrees) will emit thermal radiation.

The Stefan-Boltzmann law calculates the **maximum** rate of thermal radiation (power radiated) a surface can emit, as a function of its temperature, using the following formula:

*P* = *σAT*⁴

, where:

— Rate of thermal radiation, in watts (**P****W**) or Btu per hour (**Btu/h**).— Surface area, in square meters (**A****m²**) or square feet (**ft²**).— Surface absolute temperature, in kelvin (**T****K**) or Rankine degrees (**°R**); and— Stefan–Boltzmann constant, whose value equals**σ****5.670 × 10⁻⁸ W/(m² K⁴)**or**0.1714 × 10⁻⁸ Btu/(h ft² R⁴)**.

The Stefan-Boltzmann law calculates the maximum rate of thermal radiation any surface can emit. The only surface that emits that amount is a **hypothetic** surface called a **blackbody**. Real surfaces will differ from the blackbody behavior and emit less thermal radiation. To calculate how much less, we must consider a material property called **emissivity**.

🙋 You must input the thermodynamic or absolute temperature in the formula. If it is in celsius degrees (**°C**) or degrees Fahrenheit (**°F**), you must convert it to kelvin or degrees Rankine. This Stefan Boltzmann law calculator converts it for you, but you can also do it using our temperature converter.

## The Stefan-Boltzmann law formula with emissivity

For a surface other than a blackbody, to calculate the rate of thermal radiation, we include the **emissivity** (* ε*) in the original Stefan-Boltzmann law:

*P* = *σεAT*⁴

The surface emissivity measures how close the surface is to the blackbody behavior, and its value goes in the range **0 ≤ ε ≤ 1**. For the theoretical blackbody,

**, and for other surfaces, the value is lower.**

*ε*= 1Apart from calculating the radiation heat transfer rate, this calculator provides the emissivity of the most common materials.