Heat Transfer Calculator (Conduction, Convection, Radiation)

Heat moves by conduction through solids, by convection into a moving fluid and by radiation from warm surfaces. This calculator gives the heat flow rate in watts for each mechanism, plus the heat flux, the same rate in BTU/h and the energy over a period in kWh. For conduction it also reports the U-value, R-value and thermal resistance of the layer, and compares the same layer made of other materials.

How to use this tool

1. Pick conduction, convection or radiation.
2. Enter the area and the two temperatures, hot side or surface first, in your chosen scale.
3. For conduction, choose a material (or enter k) and the thickness. For convection enter h; for radiation, the emissivity.
4. Optionally change the period to see the energy in kWh.

Formula

Conduction: q = k·A·(T₁ − T₂) ÷ L · Convection: q = h·A·(T₁ − T₂) · Radiation: q = ε·σ·A·(T₁⁴ − T₂⁴)

k is thermal conductivity (W/m·K), L the layer thickness (m), h the convection coefficient (W/m²·K), ε the emissivity (0–1) and σ = 5.670374 × 10⁻⁸ W/m²·K⁴. Radiation uses absolute temperatures in kelvin; conduction and convection only need the difference. The presets are approximate textbook references, not specifications for a building product. The layer U-value excludes surface films and bridges; radiation assumes a small grey surface fully exposed to uniform large surroundings.

Example

A furnace wall 0.15 m thick with k = 1.7 W/m·K, an area of 0.6 m² and faces at 1,400 K and 1,150 K conducts q = 1.7 × 0.6 × 250 ÷ 0.15 = 1,700 W. The heat flux is about 2,833 W/m², and over 24 hours the wall loses 40.8 kWh.