Heat transfer calculator
Choose the way heat travels, enter the material or surface property, the area and the two temperatures. The calculator gives the heat flow in watts, the flux per square metre, the energy over a day in kWh and the thermal resistance.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose conduction, convection or radiation.
- Enter the property: k and the thickness for a wall, h for a surface in a fluid, or the emissivity for radiation.
- Enter the area and the hot and cold temperatures, in °C, °F or K.
- Conduction (Fourier): Q = k × A × ΔT ÷ d. Thermal resistance R = d ÷ (k × A).
- Convection (Newton): Q = h × A × ΔT. R = 1 ÷ (h × A).
- Radiation (Stefan–Boltzmann): Q = ε × σ × A × (T_hot⁴ − T_cold⁴), σ = 5.670374 × 10⁻⁸ W/(m²·K⁴).
- Heat flux q = Q ÷ A; energy per day = Q × 24 h.
Worked example
A 10 m² wall section insulated with 10 cm of material with k = 0.04 W/(m·K), 20 °C inside and 0 °C outside: Q = 0.04 × 10 × 20 ÷ 0.1 = 80 W, a flux of 8 W/m², 1.92 kWh a day, and R = 0.25 K/W.
A 1 m² surface at 100 °C with ε = 0.9 radiates to 20 °C surroundings: Q = 0.9 × 5.670374 × 10⁻⁸ × (373.15⁴ − 293.15⁴) = 612.55 W.
Tips
- Layers in series: add the thermal resistances of the layers, then Q = ΔT ÷ R_total.
- Steady state only. These formulas assume the temperatures do not change with time.
- Check k values in a datasheet for your material; they depend on density, moisture and temperature.
Frequently asked questions
What are the three modes of heat transfer?
Conduction passes heat through a solid or still fluid, convection carries it away with a moving fluid such as air or water, and radiation sends it as infrared light, even through a vacuum. Real cases often combine them.
What is Fourier's law of conduction?
The heat flow through a flat layer is Q = k × A × ΔT ÷ d, where k is the thermal conductivity in W/(m·K), A the area, ΔT the temperature difference and d the thickness. Doubling the thickness halves the heat loss.
What is the heat transfer coefficient h?
It describes how well a fluid takes heat from a surface, in W/(m²·K). It depends on the fluid, its speed and the geometry, so it is usually taken from tables or correlations. Still air gives low values, flowing water much higher ones.
Why does radiation need kelvin?
Radiated power goes with the fourth power of absolute temperature, Q = ε × σ × A × (T₁⁴ − T₂⁴), so the temperatures must be absolute. The calculator converts °C and °F for you.