In thermodynamics, the heat transfer coefficient or film coefficient, or film effectiveness, is the proportionality constant between the heat flux and the thermodynamic driving force for the flow of heat (i.e., the temperature difference, ). It is used in calculating the heat transfer, typically by convection or phase transition between a fluid and a solid. The heat transfer coefficient has SI units in watts per square meter kelvin (W/m2/K). The overall heat transfer rate for combined modes is usually expressed in terms of an overall conductance or heat transfer coefficient, . In that case, the heat transfer rate is: where (in SI units): : surface area where the heat transfer takes place (m2) : temperature of the surrounding fluid (K) : temperature of the solid surface (K) The general definition of the heat transfer coefficient is: where: : heat flux (W/m2); i.e., thermal power per unit area, : difference in temperature between the solid surface and surrounding fluid area (K) The heat transfer coefficient is the reciprocal of thermal insulance. This is used for building materials (R-value) and for clothing insulation. There are numerous methods for calculating the heat transfer coefficient in different heat transfer modes, different fluids, flow regimes, and under different thermohydraulic conditions. Often it can be estimated by dividing the thermal conductivity of the convection fluid by a length scale. The heat transfer coefficient is often calculated from the Nusselt number (a dimensionless number). There are also online calculators available specifically for Heat-transfer fluid applications. Experimental assessment of the heat transfer coefficient poses some challenges especially when small fluxes are to be measured (e.g. ). Composition A simple method for determining an overall heat transfer coefficient that is useful to find the heat transfer between simple elements such as walls in buildings or across heat exchangers is shown below. Note that this method only accounts for conduction within materials, it does not take into account heat transfer through methods such as radiation. The method is as follows: Where: = the overall heat transfer coefficient (W/(m2·K)) = the contact area for each fluid side (m2) (with and expressing either surface) = the thermal conductivity of the material (W/(m·K)) = the individual convection heat transfer coefficient for each fluid (W/(m2·K)) = the wall thickness (m). As the areas for each surface approach being equal the equation can be written as the transfer coefficient per unit area as shown below: