Blackbody Radiation Calculator
How much a hot object radiates and at what color — the Stefan–Boltzmann law and Wien's displacement law together.
What a hot object glows like.
How the blackbody radiation calculator works
Every object radiates: P = εσAT⁴. The fourth power is brutal — doubling the absolute temperature multiplies the output sixteen times, which is why a filament at 2,800 K glows white while a radiator at 330 K does not glow at all.
Wien's law gives the color: the peak wavelength is 2,898 µm·K divided by the temperature. That is how the Sun's surface temperature was first measured.
Formula: P = εσAT⁴; λmax = 2898/T µm
Worked examples
| Inputs | Radiated power | Note |
|---|---|---|
| The Sun's surface, 5,778 K | 63,200,699.73 W | 63.2 MW/m², peaking at 501 nm |
| A tungsten filament, 2,800 K | 139.41 W | peaks in the infrared — most of the energy is wasted as heat |
| A person at 34 °C | 888.51 W | about 880 W radiated, 100 W net indoors |
FAQFrequently asked questions
Why does a hot object glow?
Because Wien's law shifts its peak emission into the visible as it heats. Below about 800 K the peak is entirely infrared and nothing is visible.
Why is the fourth power so important?
Because it makes radiative loss the dominant mechanism at high temperature. A furnace at 1,500 K radiates 130 times what one at 500 K does.
What is emissivity?
How well a surface radiates compared with an ideal blackbody. Matt black paint is about 0.95; polished aluminium is 0.05, which is why radiant barriers are shiny.
How was the Sun's temperature measured?
From its color. The spectrum peaks near 500 nm, and Wien's law gives 5,778 K.
Why do people radiate 880 W but not freeze?
Because the room radiates back. Net loss indoors is only about 100 W, which is roughly a person's resting metabolic output.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G, g₀, R, c
- NIST Special Publication 811 — Guide for the use of the International System of Units — unit conversions
- The Engineering ToolBox — material properties — specific heats, expansion coefficients, densities
- National Institute of Standards and Technology — the US measurement authority
Last checked: September 2026. Constants are the CODATA 2018 values; formulas are the standard textbook forms.