Science & engineering · Heat
Heat transfer rate calculator.
Pick conduction, convection or radiation and get the heat flow in watts.
Inputs
Heat flow
Enter values.
How do you calculate heat transfer by conduction, convection and radiation?
Conduction through a slab: Q/t = k A (T hot − T cold) / d. Convection: Q/t = h A ΔT, with h the heat transfer coefficient. Net radiation: σ ε A (T⁴ − Ts⁴), temperatures in kelvin (converted for you from the unit you choose). Results are in watts; multiply by time for energy.
Worked example. A Styrofoam ice box with k = 0.010 W/m·K, area 0.950 m² and walls 0.0250 m thick, with 35.0°C between inside and outside, gains 13.3 W, or about 1.15 MJ in a day (OpenStax Example 14.5).
You supply k and h; they depend on the material, finish and fluid flow, and h in particular is usually measured or estimated for the specific setup, so there is no single value. Conduction and convection temperatures are differences, so Celsius or kelvin inputs give the same answer.
Frequently asked questions
What is the formula for conduction heat transfer?
Q/t = kA(T2 − T1)/d: thermal conductivity times area times temperature difference, divided by thickness.
What is the convection heat transfer formula?
Q = hAΔT, where h is the heat transfer coefficient, A the surface area and ΔT the surface-to-fluid temperature difference.
Where do I find k and h?
OpenStax College Physics Table 14.3 lists conductivities near 0°C (for example air 0.023, glass wool 0.042, Styrofoam 0.010 W/m·K). h depends on the flow and must be measured or estimated for your case.
How is this different from thermal resistance?
The Thermal Resistance calculator expresses a layer as resistance in K per W. This tool gives heat flow directly for one mode at a time.
Sources
Checked
- OpenStax College Physics 2e, 14.5 Conduction
- OpenStax College Physics 2e, 14.7 Radiation
- Engineering LibreTexts: Thermodynamics (convection, conduction and radiation laws)
Conduction and radiation formulas and the worked example from OpenStax; convection formula Q = hAΔT from Engineering LibreTexts. Runs entirely in your browser. Nothing you type is sent or stored.
Scope
Education and estimating reference. One-dimensional steady heat flow only; no combined modes, edge effects or temperature-dependent properties.