Heat Transfer Calculator

Heat transfer is the movement of thermal energy caused by a temperature difference. Engineers commonly separate it into conduction, convection, and radiation. This calculator provides a steady-state rate for each mode in watts using standard SI inputs. A positive result means heat flows from the listed hot or surface temperature toward the listed cold or surrounding temperature. Reversing those temperatures produces a negative value and indicates the opposite direction. Conduction moves energy through a stationary material. For a flat layer with constant thermal conductivity, Fourier's law gives the rate as conductivity multiplied by area and temperature difference, divided by layer thickness. Larger area, conductivity, or temperature difference increases the rate, while a thicker layer reduces it. This simplified model assumes one-dimensional flow, uniform properties, no internal heat generation, and negligible contact resistance. Convection transfers heat between a surface and a moving fluid such as air or water. Newton's law of cooling multiplies the convection coefficient by surface area and the surface-to-fluid temperature difference. The coefficient is not a basic material constant: it changes with fluid properties, speed, geometry, orientation, and whether flow is natural or forced. Select a coefficient derived from an appropriate correlation or measurement for meaningful results. Thermal radiation transfers energy through electromagnetic emission and does not require a fluid. The Stefan-Boltzmann relation uses emissivity, area, and the difference between the fourth powers of absolute surface and surrounding temperatures. Radiation temperatures must therefore be entered in kelvins. Emissivity ranges from zero to one; dark, nonmetallic surfaces are often high-emissivity, while polished metals can be much lower. Real assemblies may involve all three modes simultaneously, multiple material layers, changing properties, complex geometry, or transient temperatures. In those situations, calculate each applicable path carefully or use a thermal-resistance network and numerical model. The results here are idealized engineering estimates that are useful for classroom problems, early design comparisons, insulation checks, electronics cooling studies, and sanity checks on more detailed analysis.