Radiation Heat Transfer Calculator

• •Use the Radiation Heat Transfer Calculator when solving homework or exam problems that require quick numerical verification of your hand calculations — instant feedback helps identify arithmetic errors before they propagate.
• •Use it during the early design phase to rapidly iterate on parameters and narrow down feasible configurations before committing time to detailed finite element simulations or full design packages.
• •Use it when reviewing a colleague's calculation or checking a vendor's data sheet for plausibility — a quick sanity check can prevent costly downstream errors.
• •Use it to generate reference data for a technical report or presentation without manual computation, ensuring consistent, reproducible numbers throughout the document.
• •Use it in the field when a quick estimate is needed and a full engineering software package is not available.

The Radiation Heat Transfer Calculator is a precision engineering calculation tool designed for students, engineers, and technical professionals. Calculate radiative heat transfer Q = εσA(T₁⁴ - T₂⁴) with common emissivity reference values All calculations are performed using established engineering formulas from the relevant scientific literature and standards. Inputs support both metric (SI) and imperial unit systems, with unit conversion handled automatically — simply select your preferred unit from the dropdown next to each field. Results are computed instantly in the browser without sending data to a server, ensuring both speed and privacy. This calculator is intended as a supplementary tool for learning and design exploration; always verify results against authoritative references for safety-critical applications.

Thermal radiation is energy transfer by electromagnetic waves from any body at absolute temperature above 0 K. The Stefan-Boltzmann law gives the total radiation emitted from a black body: Q/A = σ·T⁴, where σ = 5.67 × 10⁻⁸ W/(m²·K⁴) is the Stefan-Boltzmann constant and T is absolute temperature. For a real (gray) body with emissivity ε (0 < ε < 1), the emission is Q/A = ε·σ·T⁴. Net radiation exchange between two surfaces is Q = ε·σ·A·(T₁⁴ − T₂⁴), where T₁ is the hot surface and T₂ is the cold surroundings (assuming the geometry is such that one surface sees only the other). For more complex geometries with view factors F, the formula becomes Q = σ·A₁·F_{12}·(T₁⁴ − T₂⁴) for two black surfaces. The T⁴ temperature dependence means radiation heat transfer is very sensitive to temperature — a body at 600 K radiates 16× more energy than at 300 K. This is why radiation dominates heat transfer at high temperatures (flames, furnaces, incandescent surfaces) but is often negligible at room temperature compared to conduction and convection. Typical emissivities: polished metal 0.02-0.1, oxidized metal 0.5-0.8, painted surfaces 0.85-0.95, glass 0.85, water 0.95, snow 0.85, brick 0.93, carbon black 0.95-0.98. High emissivity means good radiator (and absorber, by Kirchhoff's law); low emissivity means good reflector. Radiation shields work by placing a low-emissivity surface between two hot bodies, reducing net radiation exchange. The calculator computes net radiative heat transfer between two surfaces with specified emissivities and temperatures.

• •Furnace and boiler heat transfer: at temperatures above 500°C, radiation is the dominant mode of heat transfer. Combustion gas radiation to furnace walls accounts for most of the heat transfer in fired heaters and boilers.
• •Solar collector performance: solar radiation heats the collector surface; re-radiation from the collector back to the sky is a major loss mechanism. Low-emissivity coatings (selective surfaces) absorb solar radiation but radiate less heat.
• •Building windows and radiant heating: radiation from warm floors, walls, and ceilings to occupants is a major contributor to thermal comfort. Radiant heating systems exploit this to provide comfort at lower air temperatures.
• •Spacecraft thermal design: in space, only radiation transfers heat (no convection or conduction to surroundings). Spacecraft surfaces with specific emissivity/absorptivity ratios maintain thermal balance against solar and Earth-emitted radiation.
• •Thermal imaging and infrared thermometers: these devices detect infrared radiation emitted by objects and compute their temperature using the Stefan-Boltzmann law or more complex multi-band models. Emissivity must be known or assumed for accurate temperature measurement.

• !Entering temperatures in Celsius or Fahrenheit. The T⁴ term demands ABSOLUTE temperature — using 600 instead of 873.15 K underestimates Q by more than half. Always convert to Kelvin (or Rankine) first.
• !Subtracting temperatures before raising to the fourth power, i.e. computing (T₁ − T₂)⁴ instead of T₁⁴ − T₂⁴. These are completely different; the law uses the difference of the fourth powers.
• !Using ε = 1 for every surface. Only an ideal black body has ε = 1; polished metals are 0.02–0.1 and will radiate 10–50× less than the black-body value at the same temperature.
• !Forgetting that net exchange can be negative: if T₂ > T₁ the surface ABSORBS heat (Q < 0). The sign of (T₁⁴ − T₂⁴) sets the direction.
• !Confusing the linearised coefficient h_rad (W/m²·K, used to fold radiation into a resistance network alongside convection) with the heat flux q (W/m²). They have different units and roles.