About the Blackbody Radiation Calculator
Every hot object gives off thermal radiation. A perfect radiator, called a blackbody, sends out power that depends only on its temperature and area. This calculator uses the Stefan-Boltzmann law to find that power, scaled by the emissivity you choose, and uses Wien's law to find the wavelength at which the glow is strongest. Type the temperature in kelvin, the area and the emissivity and the blackbody radiation appears at once, with a few related figures underneath, and you can copy the result with one tap. The sums run in your browser, so nothing you enter is uploaded.
Updated October 2026
How to use the Blackbody Radiation Calculator
- 1Enter the temperature in kelvin, the area and the emissivity.
- 2Pick the units or options from the menus if the blackbody radiation tool offers them.
- 3Read the blackbody radiation in the large result box.
- 4Look at the extra rows for related figures.
- 5Press Copy result to keep the answer.
When you need the blackbody radiation
Physics students use it to model stars, furnaces and heaters. Engineers use it to estimate radiative heat loss.
Because the power depends on the fourth power of temperature, doubling the temperature makes the radiation sixteen times stronger. Most real surfaces have an emissivity between 0.05 (polished metal) and 0.95 (matt black paint).
How the blackbody radiation is worked out
Power = emissivity × σ × area × T⁴, with σ = 5.670374 × 10⁻⁸ W/m²·K⁴. Peak wavelength = 2,897,771.955 nm·K ÷ T. The surface of the Sun at 5,772 K radiates 62.9 megawatts from each square metre and peaks near 502 nm, which is blue-green light. A 1 m² surface at 300 K with emissivity 1 radiates 459 W.
Every answer on this page comes from that one rule, so you can check any result by hand with a calculator. The figures shown underneath, such as other units, are conversions of the same answer.
- Stefan-Boltzmann and Wien
- Per-square-metre figure
- Emissivity input
Mistakes to avoid with blackbody radiation
Temperatures must be in kelvin (add 273.15 to degrees Celsius). The tool gives the radiation emitted by a surface, not the net exchange, which also depends on the surroundings.
On phone and computer
On a phone, type into the boxes one after another and read the blackbody radiation underneath; Copy result puts it on the clipboard for a message or a note. On a computer the inputs sit on the left and the answer on the right, so you can change one figure and watch the blackbody radiation move. The blackbody radiation calculator works the same on both.
Checking your blackbody radiation by hand
A good habit is to test the blackbody radiation calculator with a case you already know. The surface of the Sun at 5,772 K radiates 62.9 megawatts from each square metre and peaks near 502 nm, which is blue-green light. A 1 m² surface at 300 K with emissivity 1 radiates 459 W.
If your own figures give a surprising answer, repeat the working yourself: Power = emissivity × σ × area × T⁴, with σ = 5.670374 × 10⁻⁸ W/m²·K⁴. Peak wavelength = 2,897,771.955 nm·K ÷ T. Small differences usually come from rounding, from mixing units, or from a typing slip in one of the inputs. Radiated heat is one way energy moves; touch and fluids move it in other ways. Compare it with the Heat Conduction Calculator, check size changes with the Thermal Expansion Calculator, and use the Wavelength Calculator when you need the light's wavelength for a given frequency.
Features
- Use kelvin
- Emitted, not net, power
- Fourth-power sensitivity
- Clear messages when the blackbody radiation inputs make no sense
- Copy the result with one tap
- Private: nothing is uploaded
Frequently asked questions
How do I calculate blackbody radiation?
Power = emissivity × σ × area × T⁴, with σ = 5.670374 × 10⁻⁸ W/m²·K⁴. Peak wavelength = 2,897,771.955 nm·K ÷ T. The surface of the Sun at 5,772 K radiates 62.9 megawatts from each square metre and peaks near 502 nm, which is blue-green light. A 1 m² surface at 300 K with emissivity 1 radiates 459 W.
What is the formula for blackbody radiation?
Power = emissivity × σ × area × T⁴, with σ = 5.670374 × 10⁻⁸ W/m²·K⁴. Peak wavelength = 2,897,771.955 nm·K ÷ T. The blackbody radiation calculator applies it exactly and rounds only the displayed answer to six significant figures.
What is emissivity?
How well a surface radiates compared with a perfect blackbody, from 0 to 1.
Why kelvin and not Celsius?
The law uses absolute temperature, which starts at zero.
What does Wien's law tell me?
The wavelength where the radiation is strongest; hotter objects peak at shorter wavelengths.
Is this blackbody radiation calculator free and private?
Yes. It is free, needs no account and runs in your browser, so the numbers you type are not uploaded or stored.